NM and NM-B are both types of nonmetallic-sheathed electrical cable used in residential wiring, and the key difference is the temperature rating of the insulation around the individual conductors inside. Original NM cable had insulation rated to 60°C (140°F), while NM-B cable has insulation rated to 90°C (194°F). In practical terms, NM-B replaced NM decades ago, and it’s the only version you’ll find on store shelves today.
Why the “B” Was Added
The 1984 National Electrical Code required the temperature rating of nonmetallic-sheathed cable to jump from 60°C to 90°C. The reason was straightforward: homes were being insulated more heavily with thermal insulation in walls and attics, and electrical cables buried in that insulation needed to handle higher ambient temperatures without degrading. The upgraded cable got the “-B” suffix to distinguish it from the older 60°C version.
If you’re working in a home built before the mid-1980s, there’s a chance the existing wiring is original NM with 60°C insulation. Any new cable you buy and install will be NM-B.
What’s Different Inside the Cable
The construction of NM-B is similar to the older NM cable in its basic layout: two or three insulated copper conductors plus a bare copper grounding conductor, all bundled inside a PVC outer jacket. The critical upgrade is the insulation material on the individual conductors. NM-B uses PVC insulation rated as THHN, which is the same high-temperature insulation type used on individual building wires. This is what gives NM-B its 90°C rating.
Inside the cable, a paper wrap serves a few purposes. Paper around the bare grounding conductor keeps it separated from the insulated conductors. A second paper layer wraps the entire assembly before the outer jacket is applied, holding everything in place during manufacturing and allowing the wires to slide smoothly inside the sheath during installation. One thing to be aware of: if the cut ends of NM-B cable sit in water, that internal paper can absorb moisture and wick it into the cable assembly.
How to Identify NM-B Cable
Every NM-B cable has its type printed directly on the outer jacket. A typical marking reads something like “NM-B 14-2 W/G 600V.” Here’s what each part means:
- NM-B: Nonmetallic-sheathed cable with 90°C insulation
- 14-2: Contains two insulated conductors, each 14 AWG
- W/G: Includes an equipment grounding conductor (the bare copper wire)
- 600V: Rated for up to 600 volts
So a cable labeled “14-2 W/G” actually has three wires total: two insulated circuit conductors and one bare ground. If you come across older cable in a wall that’s simply marked “NM” without the “-B,” that’s the pre-1984 version with 60°C insulation.
The 90°C Rating Has a Catch
NM-B insulation can withstand temperatures up to 90°C, but that doesn’t mean you can load the cable as if it were a standalone 90°C-rated wire. The National Electrical Code generally requires NM-B to be used at the 60°C ampacity column for determining how much current the cable can safely carry. The higher-rated insulation provides a safety margin, not permission to push more current through the wire. This margin is exactly what makes NM-B safe when it’s surrounded by blown-in insulation in an attic or packed into an insulated wall cavity, situations that trap heat around the cable.
Where NM-B Can and Cannot Be Used
NM-B is designed for dry, indoor, residential applications. It’s the standard cable for outlets, switches, and lighting circuits inside homes. You’ll find it running through wall cavities, across attic joists, and through holes drilled in floor framing.
It is not permitted in wet locations, outdoors where exposed to the elements, or in environments with corrosive substances like chlorine. It also cannot be embedded in concrete or used in commercial buildings above three floors. For outdoor or wet-location wiring, you’d need a cable type specifically rated for moisture exposure, such as UF (underground feeder) cable.
Does the Difference Matter for Your Project?
If you’re buying new cable for a residential wiring project, the NM vs. NM-B distinction is mostly academic. All nonmetallic-sheathed cable sold today is NM-B. You won’t accidentally buy the old 60°C version because it hasn’t been manufactured in decades. Where it matters is if you’re evaluating existing wiring in an older home. Pre-1984 NM cable with 60°C insulation is more vulnerable to heat damage, particularly in areas where insulation has been added after the original wiring was installed. The cable itself doesn’t need to be replaced solely because of its age, but it’s worth noting during any electrical inspection or renovation planning.