NM-B wire size by circuit: 15 to 60 amp breakers

NM-B (the cable most people call Romex, a Southwire trade name) is sized by the breaker it sits behind. The rule that trips people up is that NM-B is limited to the 60 °C ampacity column, even though the conductors inside are rated 90 °C.

The rule: 60 °C column for NM-B

NEC 334.80 says the ampacity of Type NM cable is the 60 °C value from Table 310.16. The 90 °C conductor rating can be used as the starting point for derating (for example, several cables bundled together or run through a hot attic), but the final ampacity cannot exceed the 60 °C value.

On top of that, NEC 240.4(D) caps the overcurrent protection for small copper conductors: 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG, whatever the table says.

NM-B size by breaker

These are the copper values from the 60 °C column of NEC Table 310.16, with the 240.4(D) limits applied to the three smallest sizes.

The 60 °C column continues past 6 AWG (4 AWG at 70 A, 3 AWG at 85 A, 2 AWG at 95 A), but NM-B that large is uncommon; most people switch to SER or conductors in conduit at that point.

About 6 AWG: a 6 AWG NM-B cable is good for 55 A, and 55 A is not a standard breaker size (NEC 240.6(A)). NEC 240.4(B) lets you round up to the next standard breaker size (60 A) when the ampacity does not match a standard size, as long as the circuit is not a multi-outlet branch circuit feeding receptacles and the rating is 800 A or less. Confirm this against the load and your local rules before you use it.

The jacket colors are an industry convention adopted by the major manufacturers, not a code requirement. They make inspection quicker, but always read the printed legend.

2 conductor or 3 conductor

The number after the slash is the count of insulated conductors, not counting the bare ground.

• 14/2, 12/2, 10/2: one hot, one neutral and a ground. Use it for 120 V circuits and for 240 V loads that need no neutral, such as most water heaters and baseboard heaters.
• 14/3, 12/3, 10/3: two hots (black and red), a neutral and a ground. Use it for 120/240 V appliances such as dryers and ranges, three-way switch travelers and multiwire branch circuits.

When you have to go bigger

Size up when any of these apply:

• Long runs. Voltage drop is not a code requirement for most branch circuits, but NEC 210.19(A) Informational Note suggests keeping it to 3 percent on a branch circuit. A 20 A load 100 ft away on 12 AWG copper at 120 V loses about 6.6 percent. Our voltage drop calculator does the math, and this worked example walks through it by hand.
• Bundled cables. Cables bundled without spacing for more than 24 in. (310.15(C)(1)), or more than two NM cables through the same bored hole that is then sealed with caulk, foam or insulation (334.80), need an ampacity adjustment.
• Cable in an attic hotter than 30 °C takes the ambient temperature correction of 310.15(B).
• A continuous load, one that runs three hours or more, is sized at 125 percent under 210.19(A)(1) and 210.20(A).

Where NM-B is not allowed

NM-B is for dry locations only. It cannot go in wet or damp locations, outdoors, underground or in conduit below grade, and it is not permitted in some types of building construction (NEC 334.10 and 334.12). For outdoor and buried circuits use UF-B; see UF-B vs NM-B.

Shop NM-B

• 14/2 NM-B and 14/3 NM-B
• 12/2 NM-B and 12/3 NM-B
• 10/2 NM-B and 10/3 NM-B
• 8/3 NM-B and 6/3 NM-B
• All sizes in copper building wire

Verify with your local code. The values above come from the NEC, and most states adopt it with amendments. Your local inspector's reading of the edition in force is what counts. This guide is general information, not an engineering or code determination for your project.