What to do with water-immersed Type NM-B nonmetallic-sheathed cable?
By Austin Wetherell & Chuck Mello
Type NM-B nonmetallic-sheathed cable (commonly called "Romex®" in the industry) is Listed by Underwriters Laboratories Inc. for use in accordance with the National Electrical Code® (NEC®) under the product category "Nonmetallic-sheathed Cable (PWVX)." Guide Information for this category can be found in UL's Online Certifications Directory and on page 249 of UL's 2008 Guide Information for Electrical Equipment - The White Book.
Decades ago, the outer jacket of this cable changed from an impregnated, braided covering to polyvinyl chloride (PVC). In the mid-1980s, the internal conductor insulation went from a 60 C to a 90 C rating, and the required marking was changed from "Type NM" to "Type NM-B."
Section 334.10(A)(1) of the NEC states that Type NM cable shall be permitted for use in "normally dry locations." Article 100 of the NEC defines a dry location as:
Location, Dry. A location not normally subject to dampness or wetness. A location classified as dry may be temporarily subject to dampness or wetness, as in the case of a building under construction.
The older, braided jacketed version of this cable has less resistance to water ingress than the newer, PVC jacketed version, and if subject to immersion, such as from flooding, the suitability for continued use is unknown. Any cable of this type that has been subjected to flooding should be replaced without question.
In general, cables with PVC insulation and jacket can withstand immersion in clean water for a short period of time without being damaged as long as the ends are not immersed. For example, a building may be subject to rainfall while under construction, and water may come in contact with the outer jacket of nonmetallic-sheathed cable. This is not prohibited by the NEC. The UL Standard for Safety for Nonmetallic-Sheathed Cables, ANSI/UL 719, requires Dielectric Voltage-Withstand Tests after 24-hour water immersion of cable samples (with ends in free air). If the ends of the cable are immersed for any period of time, however, the internal paper wrapping around the bare equipment-grounding conductor will absorb and transfer the water into the cable assembly. The water may then start degrading the insulation or possibly corrode the conductors. If the cable comes into contact with contaminated water, the contaminants may also act on the insulation or conductors. Over time, failures can occur.
In a flooding situation, there is no way of knowing how long the cables were immersed in water, or what types of potentially corrosive substances may have been in the water that flooded the cables. As was widely reported after Hurricane Katrina, raw sewage and chemicals were known to be in the floodwaters afflicting the Gulf Coast region of the United States. Nonmetallic-sheathed cable has not been investigated by UL for this type of exposure. Therefore, it is not possible for UL to state that cable in a particular installation is acceptable for continued use after having been subjected to the flooding.
The safest approach is to replace any nonmetallic-sheathed cable that was immersed in water for any period of time during the flooding.
For more information on Type NM-B nonmetallic-sheathed cable, contact Tom Cybula in Melville, N.Y., by phone at 631.271.6200, ext. 28259; or by e-mail at Thomas.H.Cybula@us.ul.com.
