Panelboard Application Guide

• Introduction
• General information
• Glossary
• Manufacturer’s identification
• Catalog designation
• Electrical rating
• Voltage rating
• Current rating
• Short-circuit current ratings
• Suitable for use as service equipment
• Cabinets and enclosures
• Enclosure types
• Copper or aluminum wiring
• Temperature rating of installed conductors
• Field installed units or equipment
• Modular panelboards
• Identification of phase arrangement and 3-phase, 4-wire delta system
• Factory bonded neutrals
• Equipment grounding terminal bar
• Ground-fault protection of equipment
• Maximum size fuseholders or circuit breakers
• Panelboards with provisions for watt-hour meters
• Circuit breaker trip indicator
• Wiring terminals
• Panelboards with provisions for cord connection
• Main or main disconnect
• Wire bending space
• Accessible only to qualified persons
• Investigated for use in optional standby systems
• Investigated for use with interconnected parallel electric power production sources (NEC Article 705 application)
• Taps

The evidence of UL Certification (Listing) is the Certification (Listing) Mark on the product. The Certification (Listing) Mark for panelboards includes the name and/or symbol of UL Solutions, together with the word “Certified” or “Listed,” a control number, and one of the following product names as appropriate: “Panelboard,” “Enclosed Panelboard” or “Marine, Enclosed Panelboard For Use on Vessels Over 65 Feet.” The product name may include the wording “Suitable for Use as Service Equipment” where appropriate. The product name “Enclosed Panelboard” covers both the panel and enclosure with which it is provided.

The product names for modular panelboards are “Panelboard Module” and “Panelboard Accessory Module.”

The basic Standard used to investigate products in these categories is UL 67, the Standard for Panelboards. In addition, each accessory module in a modular panelboard system is investigated in accordance with the applicable UL Standard.

Panelboard markings may be molded, die-stamped, paint-stenciled, stamped, etched in metal that is permanently secured, or printed on a label secured by adhesive and located so that it will not be covered when the units are installed. Some markings may be located on a wiring diagram in a pocket within the panelboard.

• Ampacity – The current in amperes a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
• Bonding – The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.
• Bonding jumper – A reliable conductor to establish ensure the required electrical conductivity between metal parts required to be electrically connected.
• Bonding screw – A screw that is used as a bonding jumper.
• Cabinet – An enclosure designed for either surface mounting or flush mounting that is provided with a frame, mat or trim in which a swinging door or doors are or can be hung.
• Cartridge fuse – A fuse consisting of a current-responsive element inside a fuse body with contacts on both ends.
• Circuit breaker – A device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.
• Continuous duty – Operation at a substantially constant load for an indefinitely long time.
• Current-limiting device (AC) – An overcurrent protective device that, when interrupting currents in its current-limiting range, will reduce the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having a comparable impedance.
• Current rating – The designated maximum direct or alternating current in rms A at rated frequency that a device can carry continuously under specified conditions.
• Cutout box – An enclosure designed for surface mounting that has swinging doors or covers secured directly to and telescoping with the walls of the box proper.
• Device – A unit of an electrical system that is intended to carry or control, but not utilize, electrical energy.
• Enclosed panelboard – An assembly of buses and connections, overcurrent devices, and control apparatus with or without switches, or other equipment, installed in a suitable cabinet, cutout box or enclosure suitable for a panelboard application.
• Enclosed recreational vehicle (RV) panelboard – An enclosed panelboard intended to be installed in a recreational vehicle (RV) in accordance with Article 551 of the National Electrical Code® (NEC), ANSI/NFPA 70.
• Enclosure – A surrounding case constructed to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection to the enclosed equipment against specified environmental conditions.
• Filler plate – A plate intended to close an opening that would otherwise be closed by the subsequent installation of a circuit breaker or other device.
• Flush-mounted (type) – A device designed to be set into and secured to a flat surface, with a minimal front projection.
• Frame size – A term applied to a group of molded case circuit breakers of similar physical configuration. Frame size is expressed in amperes and corresponds to the largest ampere rating available in the group. The same frame size designation may be applied to more than one group of circuit breakers.
• Fuse – A non-resettable protective device that opens a circuit during specified overcurrent conditions by means of a current responsive element or elements.
• Fuse clips – The contacts of the fuseholder that support the fuse and connect the fuse terminals with the circuit.
• Fusible switch – A switch in which one or more poles have a fuse in series in a composite unit.
• Fuseholder – An assembly of a base, fuse clips and necessary insulation for the mounting and connecting of a fuse into a circuit.
• Ground-fault protection of equipment – A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.
• Grounded conductor – A system or circuit conductor that is intentionally grounded.
• I²t (ampere squared seconds) – An expression related to the circuit energy as a result of current flow. The “I²” stands for the square of the effective (rms) let-through current and the “t” stands for the time of current flow in seconds. “I²t” is a common expression for the circuit energy between the initiation of the fault current and the clearing of the circuit.
• Interrupting rating – The highest current at rated voltage that a device is intended to interrupt under standard test conditions.
• Knockout – A portion of the wall of an enclosure so fashioned that it is capable of being readily removed by a hammer, screwdriver and pliers at the time of installation in order to provide an opening or hole for the attachment of a raceway, cable or fitting.
• Lighting and appliance branch-circuit panelboard – A lighting and appliance branch-circuit panelboard is one having more than 10% of its overcurrent devices protecting lighting and appliance branch circuits. Such circuits have a connection to the neutral of the panelboard and overcurrent protection of 30 A or less in one or more conductors.
• Mains (main terminals) – The terminals, or main device, provided for the connection of the main incoming line conductors.
• Neutral (assembly); solid neutral – An assembly consisting of enough terminals to provide for the connection of the grounded (neutral) line and load conductors. When used as a component of service equipment, the neutral also includes the following: (a) a means for making the required bonding connection between the neutral and the enclosure; and (b) a terminal for the grounding electrode conductor.
• Neutral conductor – A conductor that is connected to the midpoint of a three-wire single-phase system, the center point of a wye-connected 3-phase system, or the midpoint of one side of a delta-connected 3-phase system. Note: The neutral conductor is the grounded conductor.
• Overcurrent protective device – An individual fuse or circuit breaker pole.
• Panelboard – A single panel or a group of panel units designed for assembly in the form of a single panel; includes buses, automatic overcurrent devices, and may be equipped with switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall or partition and accessible only from the front.
• Plug fuse – A screw-in type fuse for use in an Edison base type fuseholder.
• Pressure wire connector – A reusable connector into which the conductor (wire) is secured by mechanical pressure applied by an integral screw, cone or other mechanical parts.
• Pullout switch – A switch, enclosed or nonenclosed, that is operated to open a circuit by manually separating the movable contact from the stationary contact and is operated to close a circuit by manually reconnecting the movable contact and the stationary contact.
• Recreational (RV) panelboard – A panelboard installed in a recreational vehicle (RV) in accordance with Article 551 of the NEC.
• Service – The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.
• Service equipment – The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s), and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.
• Short-circuit-current rating – The maximum rms available current to which a device can be connected. The rating is expressed in amperes and volts.
• Switch – A device, manually operated, unless otherwise designated, for opening and closing or for changing the connection of a circuit.
• Symmetrical current – Alternating current having no offset or transient component and therefore having a wave form essentially symmetrical about the zero axis. Symmetrical current is expressed in terms of rms A.

If there is a question on the design or construction of a panelboard, the identification of the organization responsible for the product is important. This is one of the basic markings required by Section 110.21 of the NEC.

For manufacturers who produce panelboards at more than one factory, UL Solutions also requires a distinctive marking to identify the factory at which the panelboard was made. This information is generally found on the UL certification (Listing) label. It enables the manufacturer to pinpoint problems and take immediate action.

UL Solutions requires the manufacturer’s identification be visible without disturbing interior parts and factory or field installed wiring. Whether the marking appears on an inside wall of the enclosure or on the side of a barrier, the manufacturer’s identification must be located near the front edge of the box or barrier.

Panelboards are marked with a catalog number, a general type designation or other distinctive marking identifying the particular panelboard construction. Additional designations are provided on modular panelboards.

UL Solutions requires that the basic electrical rating markings be visible without disturbing wiring or other interior parts. Electrical rating information includes voltage and ampere ratings. For alternating current ratings, the information includes the number of phases, if other than single phase, and the frequency, if other than 50 or 60 hertz.

The basic voltage rating markings must be visible without disturbing wiring and other interior parts. A panelboard designed and intended for use only on a supply circuit involving two different potentials (for example, 120/240 volts, three-wire; or 208Y/120 volts, 3-phase, 4-wire) is so marked.

In many cases, however, the basic voltage rating marking — for example, 480 volts, 3-phase — indicates that the panelboard is suitable for various supply systems (such as 208Y/120 volts, 3-phase, 4-wire; 120/240 volts, 3-wire; 240/120 volts, 3-phase, 4-wire delta, etc.). These voltage ratings may be shown on a wiring diagram affixed to the panelboard or its enclosure.

A single-phase, 3-wire panelboard is not permitted to be marked with a 120/240 volt, 3-phase, 4-wire delta rating. 3-wire panelboards should not be used for this system. The use of a delta breaker to adapt a 3-wire panelboard to the system has been prohibited by Section 408.36(C) of the NEC.

Although delta breakers could be used properly in 3-wire, split-bus panelboards, they were being misused in 3-wire panelboards with a single main disconnect.

Misusing delta breakers in this manner allows voltage to backfeed through the delta breaker load when the panelboard main disconnect is opened. This allows voltage to be present on the main bars when none is expected.

The current rating of a panelboard is the maximum continuous current that can be supplied through the main terminals.

Unless the assembly, including the overcurrent device(s), are marked for use at 100% of their current rating, overcurrent protection devices should not be loaded continuously to more than 80%of their rating if nuisance opening of the overcurrent device is to be avoided.

The current rating of a panelboard may be supplemented by one or more reduced ratings, each applicable under specified conditions.

For example, a manufacturer may wish to provide terminals suitable for both copper or aluminum wire but space in the panelboard may not be sufficient for terminals and wire bending space. In this case, the ampere rating is reduced to compensate for the size of aluminum wire that can be used.

Sometimes a panelboard may be provided with a main circuit breaker with a current rating less than the marked rating of the panelboard. In this case, the marked current rating is followed by the words “Maximum — See main circuit breaker rating.” This does not apply to panelboards having a main fused switch. Such panelboards are not provided with fuses when stocked. Lower-rated fuses within the same case size, however, can be installed later.

If the ampacities of the ungrounded (main) bars and the grounded (neutral) busbars are not identical, the current rating markings of the panelboard are required to show the ampacity of each busbar.

While it is unusual for the phase bars to be of different ampacities, the neutral can be a reduced size according to Section 220.61 of the NEC.

Because neutrals are often fabricated from connector bars with unusual shapes, in most cases it is not possible to judge ampacity from physical dimensions. UL Solutions conducts a temperature test on the assembly to determine ampacity.

If a panelboard employs a snap switch rated 30 amperes or less in any branch circuit, it cannot be rated more than 200 amperes unless there is a supply-side overcurrent protection at 200 amperes or less within the panelboard. This requirement assumes that panelboards rated 200 amperes or less will be installed with overcurrent protection in accordance with Section 408.36(A) of the NEC.

Section 408.36(A) of the NEC was adopted years ago when snap switch panelboards were common and short-circuit problems were caused by small electrical spacings between live parts and the ground within snap switches. It should be noted that this section does not apply to snap switches rated over 30 amperes or to switches or circuit breakers that have larger electrical spacings and are suitable for use as service disconnects.

A load-side circuit breaker may be a branch, sub-main or an integral main used on the load side of a remote main. A line-side circuit breaker or fused switch may be a sub-main, integral main or a remote main. This series combination interrupting rating shall not exceed that of the line-side circuit breaker or fused switch.

There are other markings that identify special conditions when a short-circuit current rating is applicable. These markings must be followed whenever overcurrent devices are added or replaced.

These are the basic requirements that a panelboard rated 1000 volts or less must meet in order to be used as service equipment:

A. Service disconnecting means must be provided.

B. Each service disconnect provided must have a switching feature that disconnects all conductors from the service-entrance conductors and that is suitable for use as a service disconnect. There is one exception: the neutral service conductor can be disconnected by removing the wires from the pressure wire connectors on the service neutral bus as noted in Section 230.75 of the NEC.

Disconnects and overcurrent protection which are located on the supply side of the service disconnecting means may be located behind a deadfront or screwed-on cover if:

1. The circuit being controlled is installed as part of the Certified (Listed) panelboard.
2. The circuit being controlled in contained within the panelboard enclosure.
3. The panelboard is marked, adjacent to the main disconnect(s) to alert the user that the main(s) does not disconnect control and instrument circuits.

In general, snap, toggle or similar switches, are not acceptable because their internal electrical spacings are too small.

Circuit breakers, either molded case, fused or in combination with ground-fault circuit interrupters, are suitable for use as service disconnects. Other devices that are used to protect individual circuits, circuits within equipment or appliances, or circuit protectors without on and off features are not suitable for use as service disconnects.

The removal of a plug or cartridge fuse from its fuseholder, while serving to de-energize the circuit, does not provide service disconnection. Panelboard switches, pullout switches and some industrial control switches are suitable as service disconnects. Note that pullout switches, while they serve as a fuse puller, do have switchblades and contact jaws and are tested as switches.

C. Overcurrent protection suitable as branch or feeder protection must be provided for service conductors. Miscellaneous, miniature and micro fuses, thermal cutouts, relays and other supplementary overcurrent protection are not acceptable; however, “special purpose fuses” that are suitably rated for use as branch circuit, feeder, or service overcurrent protection may be used if:

1. The fuses are non-interchangeable with fuses of incompatible ratings, and
2. The panelboard is marked for use with the specific special purpose fuses.

D. The number of service disconnects and overcurrent devices must conform to the NEC, Section 230.71.

E. As required in Section 230.95 of the NEC, panelboards rated for use on solidly grounded wye electrical services of more than 150 volts to ground must provide ground-fault protection for each service disconnect rated 1000 amperes, or more. An exception is covered under Item 53.

F. There must be provision for connecting a grounded service conductor and a grounding-electrode conductor. If there is a neutral bus, a means to bond the panelboard enclosure or mounting pan to the neutral bus is required unless the bus is mounted in electrical contact with the enclosure or pan.

In general, the grounding-electrode connection in service equipment is required to be made to the grounded service conductor at the neutral bar. However, Section 250.24(A)(4) of the NEC permits this connection to be made to the equipment grounding terminal bar, provided the main bonding jumper is a wire or a busbar and is installed from the neutral bar to the equipment grounding terminal bar. If in a panelboard suitable for use as service equipment, the main bonding jumper wire or busbar is provided for field installation, instructions are marked on the panelboard for proper installation of the jumper.

A panelboard with the neutral insulated from the enclosure may be marked “Suitable for use as service equipment when not more than six main disconnecting means are provided” when the following conditions are met:

A. There must be at least one combination of switching units that can be mounted to occupy all available space for switching units; and, whether by using handle ties or similar devices, not more than six main disconnects will result (including factory-installed disconnects).

B. With this combination of switching units, no more than six overcurrent-protective devices will be connected to each ungrounded service conductor.

Note that a panelboard marked “Suitable for use as service equipment when not more than six main disconnecting means are provided” may permit some combinations of switching units varying in ampere ratings and physical size that would exceed the six disconnect rule on a completely filled panelboard. The six disconnect rule can be exceeded if handle ties are not installed where needed.

Panelboards marked as noted above and used as service equipment must have the neutral bonded to the enclosure as required by Section 408.3(C) of the NEC. These panelboards are provided with means to accomplish this bonding. When the panelboard is not used as service equipment, the neutral bonding means must not be installed. This would violate Section 250.24(A)(5) of the NEC and would constitute a fire hazard as noted in Item 50 of this marking guide.

A panelboard with the neutral factory-bonded to the enclosure is marked “Suitable only for use as service equipment. Install no more than six main disconnecting means.”

Some panelboards may have the required number of handles and service overcurrent devices, when the maximum number of the smallest units are installed and used without handles ties or similar devices. These panelboards may have the shorter marking “Suitable for use as service equipment” or “Suitable only for use as service equipment.”

A panelboard intended for service equipment use must have the marking “Service Disconnects” near the switch or circuit breaker handles. If this is not done in the factory, pressure-sensitive labels must be provided. This marking identifies the service disconnects when branch disconnects are also present. This is required by Section 230.70(B) of the NEC.

Panelboards are installed in cabinets, cutout boxes or within compartments of other equipment, such as deadfront switchboards. Some panelboards are shipped from the factory in an enclosure designed for their use. When they are, the manufacturer is permitted to place the UL certification (Listing) Mark “Enclosed Panelboard” with or without additional modifying phrases on the assembly or use a “Panelboard” certification (Listing) Mark with or without additional modifying phrases with an “Electric Cabinet Box” certification (Listing) Mark and an “Electric Cabinet Front” certification (Listing) Mark.

Except for the panelboards intended for service equipment use discussed in Item 27, it is the responsibility of the installer to match a panelboard with an enclosure that is suitable in size and construction. The enclosure must meet the requirements of the NEC, including wiring space, wire bending space and environmental conditions.

Because of the importance of grounding and bonding at service locations, UL Solutions requires that a panelboard marked as suitable for use as service equipment be identified with a particular box. Unless the panelboard cannot readily be removed from the box in which it is shipped from the factory, UL Solutions also requires that the panelboard marking identify the box or boxes with which it is intended to be used. Panelboards intended to be used in certain enclosures (i.e., for a Short-Circuit-Current Rating greater than 10,000 amperes rms symmetrical) are either marked to identify the enclosure by manufacturer’s name and catalog numbers or may identify suitable enclosure by specifying minimum overall dimensions of length, height and width.

The suitability of an enclosure for environmental conditions for which it has been investigated is indicated by an enclosure type designation. One or more of the type designations indicated on the following pages are marked inside or outside the panelboard enclosure. This marking helps inspection authorities to judge whether an enclosure is suitable for a specific environment as mentioned in Section 110.3(A)(1) of the NEC. Enclosure type designations are coordinated with requirements in Section 110.28 of the NEC.

See Item 11 for Enclosure Types.

An enclosed panelboard marked with an enclosure designation of Type 3, 3S, 4, 4X, 6 or 6P may additionally be marked “Raintight” or “Rainproof.” An enclosed panelboard marked with an enclosure designation of Type 3R may additionally be marked “Rainproof.”

Some enclosed panelboards have a semi-flush enclosure that has a flange extending from the sidewalls. This type of enclosure is intended to be mounted such that the front portion of the enclosure projects out of the wall and the rear portion extends within the wall in which it is mounted. These panelboards are marked with instructions regarding the proper overlap or flashing to be provided in the installation.

An enclosed panelboard intended for use in a Recreational vehicle is marked “Enclosed RV Panelboard” or equivalent.

For some panelboards, it is intended that unused openings in the enclosure be closed by filler plates. These panelboards are marked with the catalog number of the filler plates to be used, and the manufacturer is required to have the plates available. Use of the filler plates facilitates compliance with Sections 110.12(A) and 408.7 of the NEC, which requires unused openings in cabinets, equipment housings, etc., to be effectively closed.

Panelboards intended for use with aluminum wire require special consideration. First, panelboard wire connectors must be identified for use with aluminum wire. Second, the size of the enclosure must be increased because aluminum wire is larger than copper wire of the same ampacity. This requires more cross-sectional area for the wiring gutters and more wire bending space at terminals and where wires enter the enclosure. Third, the larger wiring terminals may make it necessary to check through-air electrical spacings between adjacent terminals of opposite polarity.

Because of these considerations, UL Solutions requires the wiring diagram or nameplate to be marked to indicate the use of copper and/or aluminum wire if the symbol “AL” appears on any part that is intended for use in the panelboard. It may be necessary to remove a cover, front or trim to see the marking.

If the panelboard has not been evaluated for use with aluminum wire, the marking will read “Use Copper Wire Only.”

If the wiring terminals and other factors have been evaluated for use with copper and aluminum wire, the panelboard is required to be marked “Use Copper or Aluminum Wire.”

If only some terminals have been evaluated for use with aluminum and copper wire with the remainder acceptable for use with copper wire only, the panelboard is required to be marked “Use copper wire only, except at terminals …” Variations of this marking are also permitted if the terminals that have been evaluated for use with aluminum wire are identified.

In general, the testing and construction of panelboards are based on the use of 60°C ampacities for wire size Nos. 14–1 AWG and 75°C ampacities for wire size Nos. 1/0 AWG and larger, taken from Table 310.16 of the NEC, with no adjustment made for correction factors. Panelboards are marked to indicate temperature ratings and sizes of conductors that can be used.

If the equipment is normally intended for wire sizes within the range 14-1 AWG but is marked 75°C only or 60/75°C, it means that the 75°C wire may be used at full 75°C ampacity.

Higher temperature rated conductors than specified may be used if the size is based on the preceding statements. When the connection is made to a circuit breaker or switch unit within the equipment, such a unit must also be marked for the temperature rating of the conductor.

Single-phase, 3-wire panelboards may be marked for use with reduced wire sizes as indicated in Section 310.12 and Table 310.12 of the NEC, when installed as residential service equipment.

A panelboard not having facilities for the normal size wire may have an ampere rating that is based solely on use in accordance with the requirement. In this case, the panelboard must be marked to indicate that the rating is applicable only if the panelboard is installed as single-phase, 3-wire residential service equipment. For example, a panelboard rated “200 ampere maximum — see main circuit rating” could be designed for an enclosure that provides 6 inches of wire bending space suitable for 2/0 AWG in accordance with Table 312.6(B) of the NEC. The panelboard would then have to be marked to indicate that the 200 ampere rating applies only if the panelboard is installed as single-phase, 3-wire residential service equipment. With a 175-ampere or smaller main breaker installed, the panelboard could be installed elsewhere since bending space would be adequate for the wire sizes required by Table 310.16.

A panelboard to which a unit, such as a circuit breaker, switch or the like, may be added in the field is required to be marked to identify the units that can be added. Units made by different manufacturers or of a different style are not identical in all details and therefore may not be interchangeable.

Exceptions exist for:

A. Certified (Classified) molded-case circuit breakers rated 15 to 60 A, 120/240 V ac, that have been investigated and found suitable for use in place of other certified (Listed) circuit breakers in specific certified (Listed) panelboards. These breakers are limited for use with panelboards rated 225 A or less, 120/240 V ac. The circuit breakers are certified (Classified) for use in specific panelboards in accordance with the details described on the circuit breaker, or in the publication provided therewith. These breakers are suitable for use in equipment connected to circuits having a maximum available system short-circuit current of 10 kA.

B. Certified (Classified) surge-protective devices (SPDs) rated 120/240 V ac that have been investigated and found suitable for use in specific certified (Listed) panelboards. These SPDs are limited for use with panelboards rated 120/240 V ac, for use in specific panelboards in accordance with details described on the SPD, or in the publication provided therewith. These SPDs are suitable for use in equipment connected to circuits having a maximum available system short-circuit current of 10 kA. Classified SPDs will be marked with one of the following Type designations:

1. Type 1 – Permanently connected SPDs intended for installation between the secondary of the service transformer and the line side of the service equipment overcurrent device, as well as the load side, including watt-hour meter socket enclosures and intended to be installed without an external overcurrent protective device.
2. Type 2 – Permanently connected SPDs intended for installation on the load side of the service equipment overcurrent device; including SPDs located at the branch panel.

Plug-in clips and blades must be matched if poor connections and overheating are to be avoided. Additionally, over-surface and through-air electrical spacings, between live parts of opposite polarity and to grounded metal, often depend on the proper mating of units and the bases into which they are plugged or bolted.

Panelboards are usually provided with the required main line and neutral terminals. The overcurrent protection units are furnished with required load terminals. However, if the pressure wire connectors are not provided on the panelboard when shipped, the panelboard is required to be marked stating which pressure wire connectors or component terminal kits are acceptable for use with the panelboard.

A main terminal kit consisting of individual wire connectors or an assembly of terminals, bus connectors and means for bolting or plugging, is required to be marked with the manufacturer’s identification and catalog designation. If this is not done, the carton is required to be marked. A separate feed-through terminal kit requires similar marking and, if a separate enclosure is required for its use, this too must be marked and provided with instructions for its use.

If a panelboard is intended to be used in a certain box or boxes and neutral terminals are mounted in that box, both the panelboard and the box are required to be marked. These markings must indicate that each shall be used with the other unless the panelboard and box are shipped together from the factory. Some column type panelboards have the neutrals mounted in a separate junction box. In this case, correlating markings on each device are required.

Panelboards may have multiple ratings when intended for use on systems that include a neutral. For these panelboards, the neutral may be factory or field-installed where marking on the neutral assembly and panelboard correlate and installation instructions are provided. The neutral may be omitted when no line to neutral loads are served.

A panelboard that has space for the installation of additional branch-circuit switches, circuit breakers or fuseholders may be shipped from the factory without the necessary branch-circuit busbars. In this case, the panelboard must be marked to indicate the catalog number or the equivalent of the busbar kit that is to be used when the unit is installed.

A panelboard supplied with branch-circuit bars for adding a branch-circuit unit is required to be marked on a wiring diagram, on the branch-circuit busbar or in some other location. This marking indicates the ampacity of the busbar, unless its ampacity is equal to or greater than the maximum current rating of any unit to be connected to the panelboard.

Markings on panelboards that employ plug-in units require the use of a hold-down kit when the units are back-fed and field installed supply conductors are terminated on the plug-in unit. The marking indicates: “Back-fed _____ requires hold-down kit Cat. No._____ “ or the equivalent. An identification of the applicable back-fed unit is specified in the first blank — for example, circuit breaker, fused switch or terminal kit — and the catalog number of the required hold-down kit is specified in the second blank.

A modular panelboard system includes the following types of modules: an enclosed panelboard or a column-type panelboard, and one or more accessory modules such as termination boxes, enclosed switches, or circuit breaker enclosures. Each module of the system has one or more openings in one or more sides of the enclosure for busbar connections, or terminals for field wiring connections to other related modules. Typical applications for these modular systems include apartment houses and strip malls. Panelboard modules used in these modular panelboard systems are labeled “Panelboard Module” and all other system modules are labeled “Panelboard Accessory Module.”

A panelboard module to which another panelboard accessory module — such as a termination box, enclosed switch, circuit breaker enclosure or the like — may be added in the field is required to be marked to identify the panelboard accessory modules that can be added unless the entire modular panelboard system is marked with a common series designation. In this case, the series designation is marked on the panelboard module and each panelboard accessory module.

Section 408.3(E) of the NEC specifies the required phase arrangement for 3-phase buses. This Section also notes that the B-phase shall be that having the higher voltage to ground on a 4-wire delta system. This section does allow other arrangements for addition to existing installations so long as the arrangement is marked. Section 110.15 of the NEC requires markings to identify the B-phase as the higher voltage to ground on a 4-wire delta connected system when the midpoint of one phase is grounded.

Accordingly, UL Solutions requires that panelboards with other than an A-B-C bar arrangement be marked to indicate the busbar arrangement. Also, UL Solutions requires that panelboards intended for a 240/120 volt, 3-phase, 4-wire, delta system be marked to identify the different busbars with reference to the voltage between them. However, if a panelboard is intended for use only on this system, the main busbar having the higher voltage to ground may be identified by an orange marking or by tagging. Such a panelboard must be marked to indicate the necessary voltage rating of the device for each branch-circuit position.

B-phase is 208 volts to ground while the A- and C-phases are only 120 volts to ground. Some circuit breakers, like single-pole breakers for use with handle ties rated 120-240 volts, should not be connected to the phase that is 208 volts to ground. Also, holders for plug fuses should not be connected where the voltage to ground exceeds 150 volts.

Generally, the B-phase is used only in conjunction with either the A- or C-phase for a 240-volt single-phase branch circuit or with both the A- and C-phase for a 3-phase branch circuit. Circuit breakers or cartridge fuses rated for straight 240-volt systems are suitable for this use.

The NEC requirements in Section 408.3(E) do not cover 3-phase panelboards having two buses and a neutral and intended for use on a 240-volt, 3-phase, 3-wire grounded B phase system. In these panelboards, the neutral is connected to the grounded B-phase. UL Solutions requires a phase arrangement of A-, C-, with the neutral as the B-phase.

Some panelboards are intended only for service equipment use on an AC system requiring grounding of the system (see Items 18-21 under “Suitable for Use as Service Equipment”). These panelboards may have the enclosure bonded to the neutral at the factory. This eliminates the need for a neutral insulating support base.

It is difficult to check for unintentional grounds on the installed building wiring when the neutral is mounted directly on the enclosure. Therefore, some manufacturers provide an insulating liner under the neutral to permit use of a megger or similar resistance measurement instrument. However, this does not provide the electrical spacings required for the neutral if the panelboard is used away from the service as a feeder or branch-circuit panelboard. These panelboards are required to be marked “Bonded Neutral — Remove bonding device for test purposes only” or an equivalent marking.

Most installers recognize the importance of bonding the neutral to the enclosure at the service. However, many do not realize that it is just as important to omit the bonding and provide a fully insulated neutral when the panelboard is used in nonservice applications.

If neutrals are bonded at distribution points on the load side of the service disconnecting means, the neutral currents take parallel paths through neutral conductors and the grounding conductor (which may include metal raceways). If neutral conductors open, the full neutral current flows over the grounding conductor system (which may include metal raceways). As a result of this loss of the neutral connection, steel raceway joints and box connections overheat, creating a potential fire hazard.

In accordance with Section 230.95 of the NEC, a panelboard marked for use as service equipment for 3-phase, 4-wire solidly grounded wye-connected services rated in excess of 150 volts to ground but not exceeding 1000 volts phase-to-phase shall be provided with ground-fault protection for each service disconnecting means rated 1000 amperes or more.

Exception No.1 for Section 230.95 of the NEC indicates that this does not apply to service disconnects for a continuous industrial process, where a non-orderly shutdown introduces additional or increased fire and shock hazards.

In accordance with this Exception, UL Solutions permits a certified (Listed) panelboard marked for service equipment use and rated for use on solidly grounded wye-connected electrical services of more than 150 volts to ground to omit ground-fault protection if the panelboard is marked “Suitable For Use As Service Equipment Only When Supplying A Continuous Industrial Process.” This shortened wording is not intended to circumvent the need for a judgment. Inspectors concerned about the hazards of a non-orderly shutdown decide whether or not ground-fault protection is needed.

In some panelboards, only a shunt trip service disconnect is provided. In this case, the marking on the panelboards gives the manufacturer’s name and the catalog number of the ground-fault protection equipment with instructions covering its interconnections.

Panelboards provided with ground-fault protection are required to be marked to indicate the circuit-main, feeder or branch circuit that is so protected. If a marking on the ground-fault sensing or relaying equipment is not visible from the front of the panelboard with its cover removed, a separate marking, such as on the wiring diagram, is required.

If a transformer providing control voltage for ground-fault protection is connected to the line side of the main disconnect, this disconnect may be identified as the “main.” In this case, the panelboard is required to be marked “Danger — this main does not disconnect control and instrument circuits” adjacent to the main disconnect.

In a panelboard with ground-fault protection, the part of the neutral bus used for load terminations is required to be marked “WARNING — Do not connect grounding conductors to these or any other neutral terminals, to do so will defeat ground-fault protection.” This marking must be placed on or adjacent to the neutral.

If the ampacity of a branch bar or wire is less than the maximum current rating of any fuse accommodated by a fuseholder it supplies, or if it is less than the current rating of any trip unit (including rating plugs) of an interchangeable trip circuit breaker that it supplies, UL Solutions requires a clear and permanent marking, plainly visible when the fuse or trip unit is being replaced. This prevents the use of a fuse, trip unit or rating plug having more ampacity than a busbar or wire.

A panelboard with branch-circuit bars that permit adding a branch-circuit unit, circuit breaker, switch or fuseholder requires markings on the wiring diagram, the branch-circuit busbars or some other location. Markings indicate the ampacity of the bus. This marking is not required if the ampacity of the busbar is not less than (a) the maximum current rating of any unit to be connected to it; or (b) the current rating of the panelboard.

Separate meter sockets are required to be marked with a continuous ampere rating. In some cases, meter sockets also may have a maximum use (intermittent) ampere rating of not more than 125% of the continuous ampere rating. Similar markings are required for any meter mounting base in a panelboard. The continuous ampere rating may be less than the circuit that contains the meter mounting base. This means that, for example, a 125-ampere panelboard can have a meter mounting base rated “125 Amps (100 Amps Continuous).” Some inspectors may judge that a continuous duty meter socket is not needed because of a panelboard’s load diversity. Continuous duty sockets can be required when load and environmental conditions would cause overheating in panelboards.

If the socket jaws of meter mounting bases are mounted on terminals intended for field wiring, the panelboard is required to be marked to indicate the maximum torque to be applied to these terminals.

If the handle of a circuit breaker, or a simple extension of that handle, assumes other than the off position when the breaker trips, the trip position of the handle is required to be indicated. The method for resetting the breaker is also a required panelboard marking.

Marking the tripped position is not required for a separate, external operating handle that is not part of the circuit breaker. Such a handle may remain in the on position when the breaker trips. These constructions may be encountered in panelboards certified (Listed) for use in hazardous locations where operating springs in the circuit breaker mechanism do not provide sufficient tension to operate external handles.

Section 110.14 of the NEC states that terminals for more than one conductor must be identified. In all cases, marking is required to identify the combinations and sizes of conductors for each terminal. Marking is not required where conductors in parallel are secured by a single wiring terminal having individual holes and set screws for each conductor.

Where conductors in parallel are secured at a terminal (one connector with one or more barrels or multiple individual wire connectors) of an enclosed panelboard and where the connectors will accommodate more than one combination of conductors (size and number) that will have the required ampacity, the wiring diagram will state the number and size of wires for which the terminal is acceptable unless the wiring space is suitable for all combinations of conductors that have the required ampacity.

Tightening torque is marked for all panelboard terminals (other than wire binding screws) except when installed units have their own marking.

UL Solutions requires that an individual terminal be provided for the connection of each branch-circuit neutral conductor and, with one exception, that the number of individual terminals be not less than 75% of the total number of individual fuseholder or circuit breaker poles capable of being installed in the panelboard. Under the exception, the number of terminals may be reduced to 50% if the panelboard is marked to indicate the maximum number of circuits and the need to use multipole branch-circuit units to limit the number of terminals to a specified number.

Interconnection of panelboards using inlets is desirable in installations where portable generators are used as alternate sources of power. Panelboards may also connect to loads using outlets for cord connection.

Note that use of portable generators inside or in confined spaces is extremely dangerous as carbon monoxide poisoning can quickly lead to death. Panelboards with an integral inlet are rated for outdoor use in wet locations. The inlets on these panelboards are rated for the environmental condition with the cord connector installed as well as with the connector removed. Refer to Section 30 for additional details regarding connection to optional standby systems.

Panelboards with inlets are provided with branch-circuit protection rated not greater than the rating of the inlet for the circuit supplied through the inlet.

Panelboards with inlets are marked in accordance with Section 702.7(C) of the NEC to indicate the type of derived system that the panelboard is intended to interconnect:

• Panelboards that do not switch the neutral conductor shall be marked with the following: “WARNING – For Connection of a Nonseparately Derived (Floating Neutral) System Only”
• Panelboards that switch the neutral conductor shall be marked with the following: “WARNING – For Connection of a Separately Derived (Bonded Neutral) System Only”

Panelboards provided with single pole separable connectors (input or output connections) that are not mechanically interlocked are marked:

• To indicate that connections are to be made by qualified personnel only.
• To indicate the proper order of connection and disconnection:
  
  WARNING – Risk of Electric Shock
  Plug connection should be in the following order:
  1. Equipment grounding conductor connectors.
  2. Grounded circuit conductor connectors.
  3. Ungrounded conductor connectors.
• Disconnection should be in the reverse order.

Except for a panelboard that has the “Service Disconnect(s)” identified, a switch or circuit breaker that controls all load circuits from the panelboard, other than a feed-through circuit, is required to be marked “Main” and no other switching device is permitted to have this marking. If two or more switches or circuit breakers control all load circuits, other than a feed-through circuit, they are required to be marked “Main Disconnect” either on or adjacent to each unit if there are other switches or circuit breakers in the panelboard.

A panelboard constructed in accordance with Exception No. 3 of Section 408.55(A) of the NEC is required to be marked by means of a diagram that shows and specifies the method of wiring that shall be used to accomplish the 90° bend.

Minimum wire bending space requirements for panelboards rated for use with aluminum conductors are based on the use of compact stranded conductors made from AA-8000 series electrical grade aluminum alloy. See NEC Section 310.3.

Section 240.40 of the NEC requires a disconnecting means on the supply side of cartridge fuses where the fuses are accessible to other than qualified persons. Section 408.38 requires all panelboards to be deadfront unless they are accessible only to qualified persons. UL Solutions requires such panelboards to be marked “This panelboard shall be located where accessible only to qualified persons.”

Panelboards may be constructed with interlocked switching devices or designed for use with interlock kits that have been investigated for use in optional standby systems in accordance with Article 702 of the NEC. Panelboards shipped with factory installed interlocked switching devices that have been investigated for use in optional standby systems are marked “Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70”; panelboards available for use with field installable assemblies that have been investigated for use in optional standby systems are marked “Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70 when provided with interlock kit Cat. No. _____.”

Panelboards may be intended for interconnection with one or more electric power production sources operating in parallel with a primary source(s) of electricity, in accordance with Article 705 of the NEC. Panelboards with this marking have provisions for connection(s) as noted in either (a) and/or (b):

a. Supply-side connection – Panelboards intended for use in accordance with Section 705.11 of Article 705 of the National Electrical Code, NFPA 70, shall be permitted to have provisions for interconnecting parallel power sources on the supply side of the service disconnecting means.

b. Load-side connection – Panelboards for use in accordance with Section 705.12 of Article 705 of the National Electrical Code, NFPA 70, shall be permitted to have one or more load-side disconnects for the interconnection of parallel power sources.

Markings associated with this application are provided as noted below:

Disconnect markings:

1. Each disconnect or provision for connection for interconnected parallel power production source(s) will be marked “Parallel Energy Source Disconnect” or “Parallel Energy Source Tap” or the equivalent, or be provided with a space for the source(s) to be labeled in the field.
2. A marking shall be placed near each Parallel Energy Source Disconnect to be visible when the terminals are visible to warn the user that both the line and load terminals may be energized when the breaker is in the open (Off) position.
3. Panelboards with the load side disconnect(s) intended for connection to parallel power sources positioned at the opposite end from the main input in accordance are marked with the following, or equivalent:
   
   WARNING
   POWER SOURCE OUTPUT CONNECTION
   DO NOT RELOCATE THIS
   OVERCURRENT DEVICE

Panelboard markings:

1. “Suitable for use with interconnected parallel electric power production sources,” “Suitable for use in accordance with Article 705 of the National Electrical Code, NFPA 70” or the equivalent.

2. “Turning Off Parallel Energy Source Disconnect does not deenergize this panel. Turn off power from all sources supplying this equipment before working inside” or equivalent wording.

3. For panelboards intended for load side connections to other power production sources (item b above), the panelboard will be marked with the maximum ampere rating for all overcurrent devices intended for connection to electric power production sources operating in parallel with a primary source(s).

4. Panelboards intended to have any non-primary sources controlled by a Power Control System (PCS — refer to NEC 705.13) will be provide with the following markings, as appropriate:
    A. Panelboards with a PCS integrated into the design shall be marked “Alternate power sources are controlled by a Power Control System (PCS). The setting of the PCS shall be considered the power source output circuit current” or equivalent.
    B. Panelboards provided with components intended for use as part of a PCS will be marked “This Panelboard is Intended to be Installed as Part of a System Where Alternate Power Sources are controlled by a Power Control System (PCS). Refer to Installation Instructions for additional details regarding the complete installation of the PCS. The setting of the PCS shall be considered the power source output circuit current” or equivalent.
    C. Panelboards provided with instructions to install a PCS will be marked “When used to interconnect parallel electric power production sources, Power Control System (PCS) Model ______, Manufactured by _________ may be used to control the output of alternate sources” or equivalent.

Some panelboards may have terminals or provisions for terminals, marked as taps, located on the supply side of the service disconnecting means. The suitability of these terminals as taps connected on the supply side of the service disconnect is intended to be determined in accordance with NEC Sections 230.46, 230.82 and 705.11.

Panelboards are not certified (Listed) to have their busbars tapped in the field unless there are existing holes in the busbars marked with the word “Tap” adjacent to the holes in the factory. Other holes in the busbar that are not marked with the word “Tap” are intended for the connection of overcurrent devices, other devices as identified by the product markings and in the installation instructions, or other uses identified by the manufacturer. When the electrical equipment certification (Listing) does not include product markings or instructions for tapping busbars, this situation should be treated like any other field modification of certified (Listed) equipment.

Some certified (Listed) power equipment may have installation instructions with specific directions on tapping the busbars. If this is the case, this equipment can be field modified, following those manufacturer’s instructions, in accordance with NEC Section 110.3(B). The product category guide information for Panelboards (QEUY) in Product iQ identifies the required markings for field-installed equipment that have been evaluated by UL Solutions. For additional information, please see the guide information for Panelboards (QEUY).

Drilling or enlarging holes in busbars can increase the current density and reduce current carrying capacity. Some equipment is constructed with fully rated busbars, which have a typical current density of 1000 A per square inch of cross-sectional area for copper and 750 A per square inch of cross-sectional area for aluminum. However, some equipment use busbars at a higher current density and have temperature testing conducted to determine compliance with UL Solutions' requirements.

Removing material can result in higher operating temperatures, and additional holes can potentially weaken the busbar, which adversely affects the short-circuit rating of the equipment required by NEC Section 110.10. Both sufficient wiring space and wire bending space need to be provided for the conductors and the wire connector at the tap connection. In measuring the wiring space, code authorities need to consider the possibility of the connectors rotating, which may result in reduction of the spacing between uninsulated live parts of opposite polarity and uninsulated live parts and ground.

Wire connectors (lugs) need to be certified (Listed) for the purpose and have the proper ratings for specific application, and the mounting hardware for wire connectors needs to be properly selected and attached with the correct torque. The potential reduction of required spacings from the wire connectors or fasteners to the enclosure or other busbars also needs to be evaluated. In completing the modification, all foreign material — such as cutting oil, burrs and metal shavings — needs to be removed from the equipment enclosure. Temporarily removed materials such as insulating barriers need to be returned to their original positions and secured. These are just a few of the concerns and items that must be inspected, checked and reviewed where such modifications are made to this type of equipment.