Romex (NM-B) Cable: Ampacity and Common Sizes

Romex is the trade name most electricians and DIYers reach for when wiring a house. The cable itself is non-metallic sheathed cable, designated NM-B, and it covers the vast majority of branch circuits in residential construction. Understanding its ampacity limits and the way the size labels work saves time and prevents costly mistakes.

What NM-B Actually Means

The "NM" stands for non-metallic sheathed cable. The "B" suffix indicates the conductors inside are rated for 90°C, which sounds generous. The outer plastic jacket, however, is only rated for 60°C, and NEC Table 310.15(B)(16) requires that the ampacity of NM cable be taken from the 60°C column, not the 90°C column.

This distinction trips up a lot of people. The wire inside could theoretically carry more current at 90°C, but the jacket degrades at lower temperatures, so the whole assembly is derated to match the weakest link. In practice, this means 12 AWG NM-B is rated at 20 A, not the 25 A you might find for 12 AWG THHN in conduit. Always use the 60°C ampacity for any NM-B circuit. Verify these figures against the current edition of the NEC and consult a licensed electrician for your specific installation.

Decoding the Cable Label: 14-2, 12-3, and Everything In Between

The label printed on the jacket follows a simple pattern: gauge-conductors. "12-2" means 12 AWG conductors with two insulated wires. "14-3" means 14 AWG conductors with three insulated wires.

What the number does NOT include is the bare copper ground wire. That wire is always present in NM-B but is counted separately. So a 12-2 cable contains three wires total: a black (hot), a white (neutral), and a bare copper ground. A 12-3 cable contains four wires: black, white, red (second hot), and bare copper ground.

The three-conductor cables (12-3, 14-3) are common for multi-wire branch circuits, 240 V circuits that also need a neutral (like a dryer or range receptacle), and three-way switch loops. For a simple 120 V outlet circuit, you almost always reach for the two-conductor version.

For a broader look at how wire gauge affects current capacity, see the AWG wire size chart.

NM-B Ampacity Table and Typical Breaker Pairings

The breaker size must match the ampacity of the wire, not the load you expect to run. Undersized wire on an oversized breaker is a fire hazard because the breaker will not trip before the wire overheats.

A few things worth noting at the practical level. Most residential lighting circuits run 14 AWG on 15 A breakers, which is fine as long as the total load stays within the 80% continuous-use limit (12 A sustained). Kitchen counter receptacles are required by code to be 20 A circuits, so 12-2 is the minimum. A dedicated 240 V/30 A circuit for a clothes dryer typically uses 10-3 NM-B, with the red and black wires serving as the two hots, and white as neutral.

For the underlying theory behind these numbers, ampacity explained covers conductor physics in more detail.

The Wet Location Prohibition

NM-B has a firm rule: it cannot be used in wet or damp locations. The plastic jacket is not waterproof, and moisture intrusion leads to insulation breakdown and corrosion over time. This rules out direct burial, installation in conduit that could collect condensation, outdoor exposed runs, or any location classified as wet or damp by the NEC.

For outdoor circuits, underground feeders (UF-B cable), or conductors in weatherproof conduit, are the appropriate choices. If you are running a circuit to a detached garage or garden structure, NM-B stops at the exterior wall of the house. From there, you need a different wiring method.

Worked Example: Sizing a 20 A Kitchen Circuit

Suppose you need a dedicated 20 A small appliance circuit for a kitchen counter. The NEC requires at least two 20 A small appliance branch circuits for kitchen counter receptacles.

Starting from the breaker requirement: the circuit needs a 20 A breaker. Working backward from the 60°C ampacity table, 12 AWG NM-B carries 20 A. So you pull 12-2 NM-B from the panel to the first receptacle, daisy-chain to additional outlets along the counter, and land it on a 20 A breaker. The receptacles themselves must be 20 A rated (look for the T-slot on one of the vertical slots).

If instead you grabbed 14-2 because it was on the shelf, the wire would be limited to 15 A, and the 20 A breaker would allow the circuit to carry more current than the wire can safely handle. That is the mismatch to avoid.

For circuits involving 240 V, how to size a cable for a 240 V circuit walks through the full process.

Installation Limits and Physical Protection

NM-B is approved for dry, indoor, protected locations. A few installation specifics matter for a safe job. The cable must be supported within 12 inches of every box and at intervals not exceeding 4.5 feet. It needs physical protection (conduit or a nail plate) where it passes through studs or plates within the first 1.25 inches of the surface. Stapling it flat against framing is acceptable; running it loose across a crawl space ceiling generally is not.

The jacket must enter the box and be secured by a connector or clamp. Stripping the jacket before it enters the box is a common mistake that leaves conductors exposed inside the wall cavity.

For a systematic method covering all of these decisions together, how to size a cable step by step provides a complete framework.

Frequently asked questions

Can I use NM-B cable in conduit?

You can run NM-B through conduit indoors in dry locations, but it rarely makes sense. The cable's bundled jacket adds bulk, conduit fill calculations still apply, and many installers just pull individual THHN conductors instead. The bigger issue is that NM-B in conduit outdoors or in wet locations is still not permitted because the jacket rating does not change based on the conduit around it.

Why does 12 AWG NM-B only get 20 A when 12 AWG THHN gets 25 A?

The conductors in NM-B are physically similar to THHN, but the NEC requires NM cable ampacity to be taken from the 60°C column in Table 310.15(B)(16) regardless of the conductor's own temperature rating. The outer jacket limits the thermal envelope of the whole cable. THHN in conduit benefits from the 75°C or 90°C column because the jacket and conduit assembly handles heat differently.

What is the maximum length for a 12-2 NM-B circuit at 20 A?

Length itself is not capped by a specific rule, but voltage drop becomes a practical limit. At 120 V, most engineers target 3% maximum voltage drop for branch circuits, which works out to roughly 50 feet one-way for a fully loaded 20 A circuit on 12 AWG. At 100 feet one-way, you would want to step up to 10 AWG to keep drop within acceptable limits. The AWG wire size chart includes voltage drop reference data to help with longer runs.

Is Romex the only brand of NM-B cable?

Romex is a brand name owned by Southwire, and it has become genericized the same way "Sheetrock" refers to drywall. Other manufacturers produce NM-B cable to the same UL listing and NEC requirements. The electrical properties are equivalent as long as the cable is listed and labeled correctly. The brand name on the jacket does not affect ampacity or code compliance.