Romex is a Southwire brand name that has become the generic term for NM-B — nonmetallic-sheathed cable, the flat plastic-jacketed cable used for almost all residential branch circuits in the United States. Its conductors are 90°C rated, but NEC 334.80 requires you to use the 60°C column of Table 310.16 for ampacity, which is why 12 AWG Romex is a 20 A circuit and 6 AWG Romex will not serve a 60 A load.
| Property | Value |
|---|---|
| Voltage rating | 600 V |
| Temperature rating | 90°C conductors |
| Ampacity column | 60°C per NEC 334.80 |
| Insulation | THHN-type conductors inside a PVC outer sheath |
| Conductor | Copper, solid through 10 AWG, stranded 8 AWG and larger |
| Sizes | 14 AWG through 2 AWG |
| NEC article | Article 334 |
| Locations | Dry, indoor, protected locations only |
| Direct burial | Never permitted — not even inside conduit |
| Common configurations | 14-2, 14-3, 12-2, 12-3, 10-2, 10-3, 8-3, 6-3 |
| Size | Ampacity |
|---|---|
| 14 AWG | 15 A |
| 12 AWG | 20 A |
| 10 AWG | 30 A |
| 8 AWG | 40 A |
| 6 AWG | 55 A |
| 4 AWG | 70 A |
| 3 AWG | 85 A |
| 2 AWG | 95 A |
| 1 AWG | 110 A |
| 1/0 AWG | 125 A |
| 2/0 AWG | 145 A |
| 3/0 AWG | 165 A |
| 4/0 AWG | 195 A |
Since 2001, NM-B manufacturers have colour-coded the outer sheath by conductor size. It is an industry convention rather than a code requirement, but it is followed universally in the US and it is the fastest way to identify cable in a panel or attic.
White — 14 AWG, 15 A circuits. Yellow — 12 AWG, 20 A. Orange — 10 AWG, 30 A. Black — 8 AWG and 6 AWG, 40 A to 55 A. Grey — never NM-B; grey is UF-B, the direct-burial cable.
The number after the dash is the count of insulated current-carrying conductors, and the ground is extra. So 12-2 is a black hot, a white neutral, and a bare ground — three wires total. 12-3 adds a red conductor for three-way switching, multi-wire branch circuits, or 240 V loads that need a neutral.
NEC 334.12 lists the prohibitions, and they catch people out constantly:
Wet or damp locations. That includes underground, in a buried conduit, in a masonry block wall exposed to moisture, and outdoors. Putting Romex inside conduit does not make it legal outdoors — conduit is not a dry location.
Exposed to physical damage. It must be protected where run across the face of framing in a garage or basement below 8 feet.
Commercial and multifamily buildings above three floors. NEC 334.10 limits NM-B to one- and two-family dwellings, multifamily dwellings, and other structures of Types III, IV and V construction. Type I and II construction — steel and concrete commercial buildings — requires conduit.
Embedded in concrete, or as service-entrance cable. Never.
This is the practical consequence of the 60°C rule. In the 60°C column, 6 AWG copper is 55 A. A 50 A range circuit is fine on 6-3 NM-B. A 60 A sub-panel feeder is not — you need 4 AWG NM-B (70 A at 60°C), or you switch to THHN in conduit, where the 75°C column gives 6 AWG a 65 A rating.
The reason for the derating is heat. NM-B has three or four conductors packed inside a tight PVC sheath with no air gap, usually stapled to wood framing and often buried in insulation. The 60°C column is the code's allowance for that.