Thermal Effects of Residential-Installed Type NM-B Cable Encased in Spray-Foam Insulation Outlined

According to a bulletin recently released by NEMA, a report from the University of Toronto in 1985 illustrates the potential heating effects of Type NM-B Cable when encased in spray foam insulation. In reviewing the test results for the 1985 University of Toronto study, the maximum temperatures reached for the 14 AWG and 12 AWG conductors were 48.8°C and 53.9°C, respectively. Both of these temperatures are far below the 90°C insulation rating of the conductors of listed Type NM-B Cable.

For more detailed information, specifically on how the 2017 NEC deals with this ampacity issue, read the entire bulletin. However, NEMA summarized key take-away points as follows:

• The National Electrical Code does not prohibit installing Type NM-B cable in spray foam insulation.
• The NEC contains requirements for derating the conductors when bundled together. These, and all other Code requirements, must be followed.
• The manufacturers of Type NM-B cable allow encasing the cable in foam insulation.
• The University of Toronto study indicates that the conductors will not be subjected to objectionable temperatures even under very severe conditions.
• Type NM-B cable is routinely installed within heavily insulated walls, ceilings, and floors with no reported detrimental effects.

For more on this subject, read “What Happens When Nonmetallic Sheathed Cables Come in Contact with Spray-Foam Insulation?” In this piece, electrical inspectors from Washington State discuss questions and share best practices related to electrical installers using polyurethane spray foam insulation in residential construction. Specifically, many are unsure whether nonmetallic-sheathed cable is allowed to be in contact with, or encased in, spray-foam insulation.