Using Type NM Cable in the Attic: An important heating issue was fixed in the 1984 NEC

Type NM-B cable installed in a box
Published On
Apr 15, 2022

Over the long history of the National Electrical Code, there have been numerous changes to how we use different wiring systems. Those electrical industry long-timers may have historical knowledge of the reasons for various changes or been personally involved in making them.

I have been involved in the NEC process on a local and a national basis since the 1978 Code cycle. I started as a member of the City of Phoenix NEC Review Board that evaluated the 1978 Code for possible local amendments, usually based on the elevated temperature issues found in the desert during the summer. This review process involved many hours and days of studying the Code to determine whether the local conditions required changes and, if so, what the changes would be and how the local code could be changed using proper Code language.

This in-depth study was incredibly challenging for me, but it provided a better understanding of the local and national electrical issues. Starting in the 1990 edition of the NEC , I was on Code -Making Panel (CMP) 20, then went to work for NFPA for nine years doing the 1993, 1996 and 1999 NEC , and then spent the next 20 years on CMPs 1, 3, 4, 7 and 13, and the NEC Correlating Committee from 2005 through 2017.

The in-depth background and historical reasons behind most NEC changes have helped better my understanding of the issues in previous and recent NEC editions. For example, many of today’s electricians, electrical contractors and electrical engineers are not aware of NM and NMC cables’ history as related to limited ampacity requirements.

First, it is important to understand the definition of NM and NMC cable. NM cable is defined as a nonmetallic-sheathed cable assembly with an overall covering or sheath that is flame-retardant and moisture-resistant. NMC has an overall sheath that is flame-retardant and moisture-, fungus- and corrosion-resistant. NMC cable is not currently manufactured, but could be dual-rated NMC and UF (underground feeder and branch circuit) cable.

To understand the application of 334.80, dealing with the limited ampacity of NM and NMC in recent editions of the NEC , you would need to go back to the 1981 Code and before to understand the changes in the 1984 NEC .

Prior to this edition, NM and NMC cable could have conductor insulation on the individual conductors rated at 60°C (140°F), 75°C (167°F) or 90°C (194°F) for use in different ambient temperatures. Cables with conductors with 75°C insulation were marked with letter designations NM-A and NMC-A. Cables with a 90°C were marked with letter designations of NM-B and NMC-B.

In the attic

In most cases prior to 1984, electricians used NM cable with TW (60°C) insulation on the internal conductors. Over the course of many years, NM cable with 60°C insulated conductors would be installed for overhead lighting fixture wiring with the cable encased in attic insulation and supplying two or four 60W incandescent lamps in the fixture. The high heat of the incandescent lamps and lack of heat dissipation due to attic insulation caused degradation of the 60°C insulation on the conductors within the box. In many cases, lighting fixtures listed for use only with 60W incandescent lamps would be replaced with 100W incandescent lamps, causing even more heat to these insulated conductors in the overhead box. The insulation on the conductors would become brittle and, in some cases, turn to charcoal. Something had to be done to correct this problem.

In 1984, NM and NMC cable were required to have insulated conductors rated at 90°C, but the conductors’ ampacity was limited to the ampacity in the 60°C column in Table 310.16. By using 90°C insulated conductors in the cable assembly and limiting the ampacity to the current, the cable can be used in attic insulation with incandescent lamps and will not be subjected to the heating issue addressed in the previous paragraph.

Understanding this heating issue and the resulting remedy, as provided in the 1984 NEC , will further explain the text found in the installation instructions of most lighting fixtures (luminaires), even though installers don’t always read the instructions.

These instructions usually state, “Where installing lighting fixtures in systems wired prior to 1984, consult a licensed electrician or licensed electrical contractor” so the appropriate wiring system can be installed.

Sometimes electricians don’t read instructions, but safety is a key issue here, and instructions are worth following.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.