Panel builder tip archives
When UL field representatives and engineers conduct inspections, they often find reoccurring panel building errors. These panel building tips have been compiled to help you avoid these common pitfalls and save time and money. New tips will be added as they are suggested, so check this page periodically.
Control circuit wire sizing
Many panel builders undersize control circuit wires. Make sure your wire size is based on the rating of the fuse upstream. To do this:
- Fuse smaller wires separately
- Use larger conductors
- Use smaller fuses upstream
Submit ventilated enclosures
In general, ventilated enclosures should be submitted to UL for evaluation and description in the UL procedure. Even UL Recognized fan/ventilation kits have conditions of acceptability -- conditions that must be met when used in a UL Listed control panel.
Watch power supply ampere ratings
A power supply secondary can only be loaded at more than 50 percent of its ampere rating when it has passed the temperature test rating in accordance with UL 508, and is fully enclosed and provided with means for connection to conduit.
In addition, each secondary can also be fused at 50 percent of it rated load. If fused, secondary conductors should be sized according to the fuses protecting them.
Intrinsically safe/ hazardous locations panel components -- installation requirements vary
The scope of UL 508 does not cover intrinsically safe or hazardous locations panels. These are covered in two separate UL Standards. However, UL508A does not prohibit panel builders from using UL Listed intrinsic safety barriers or other hazardous locations components (e.g., PLCs used in hazardous locations). If these components are used, the panel and all connected circuits must be installed:
- In an ordinary, non-hazardous location
- With no references to class and division or zones on the panel or in literature
- To meet additional requirements for space, separation of circuit wiring, and special markings if the load side of the intrinsic safety barriers extends into a hazardous location.
UL has a separate hazardous locations panel builder program. Find out more about it on our hazardous locations panel program pages.
Maintain environmental ratings for enclosure mounted devices
Cutting holes for conduit, components or for mounting components directly to the enclosure can affect the environmental rating of the enclosure. It requires an evaluation to determine the correct rating for the modified enclosure. Even popping rivets through the enclosure to hold the nameplate affects the environmental "type" rating. There are two exceptions to this:
- Type 1 rated enclosures do not require re-testing
- Situations where all of the components are UL Listed or Recognized with marked type ratings equal to or better than the rating of the enclosure
Save all component packaging
This sounds a bit picky, but can help save you money. The reason the UL panel program is flexible is largely because UL Listed and/or Recognized Components can be used. Traceability for each component (this includes wire) is key to determining conditions of acceptability and compliance with UL 508A.
Many panel builders either throw away boxes, information sheets and other packaging materials or take off original manufacturer markings on the panel itself. When this information is unavailable at the time of inspection, the field representative must call the supplier or UL engineering office to verify listings. This can take an hour or more on average.
It is best to save one copy of all the different component cartons, information sheets and installation instructions for Listed and Recognized Components.
Ensure markings remain visible
Fuse replacement markings and field wiring terminal markings must be provided and visible after installation. These are the most commonly overlooked markings. Others include panel schematics and the motor overload heater (replacement) table. Overload relay manufacturers should provide a crack and peel label with this information in every box-finding it can require sifting through all the packaging. Also, please keep in mind that the marking must be properly printed and in the right location. Being visible after installation is a key factor.
Calculate recommended fuse sizes for transformers
It is best to read the section in the NEC on transformer fusing thoroughly to determine the proper fuse size and type. Control circuitry and power circuitry is often confused. It is also important to read the definitions in UL508A. Keep in mind that although some manufacturers provide a table of recommended fuse sizes for protection of transformers, we have found that some tables do not meet the requirements of UL508A. It is best to do the calculations yourself. This is what the UL field inspector will do during your inspection.
Proceed with caution on motor circuit protectors, mag only breakers, MCPs
Use of Recognized instantaneous-trip breakers (MCPs) in place of branch circuit protection is a problem. These breakers do not have thermal overload trip protection (it will never trip at the ampere rating on the handle) as in a conventional UL Listed breaker. Because of this, these breakers can only be used as branch circuit protection for a motor circuit when purchased as part of a UL Listed combination motor controller. Otherwise, the breaker must be investigated for use with the specific vendor and part number of contractor and overload relay.
Avoid misusing fuses and supplementary protectors for branch protection
When branch protection is required we often find that UL Listed miscellaneous fuses are used instead of UL Listed Class J, K, R, or CC fuses. Miscellaneous fuses are only Listed for supplementary circuit protection. Supplementary devices are ignored in 508A when branch circuit protection requirements are determined. Don't forget about fuse holders either, they must be Listed for the proper fuse class as well!
Protect VFDs in accordance with Markings
Variable frequency drives may or may not be provided with a motor overload function and branch circuit protection.
