What size wire for 50 amps? Why NM-B uses the 60 °C column
Short answer: For a 50 A circuit in NM-B (Romex) cable, use 6 AWG copper. 8 AWG copper is only allowed when it is individual THHN wire in conduit with 75 °C terminations at both ends. The reason: NEC 334.80 limits NM cable to its 60 °C ampacity, and 8 AWG copper is rated 40 A at 60 °C but 50 A at 75 °C.
Three ampacities for one wire
NEC Table 310.16 gives each conductor size three ampacities, by the temperature its insulation and terminations can tolerate. For 8 AWG copper: 40 A at 60 °C, 50 A at 75 °C, 55 A at 90 °C. The column you are allowed to use is set by the weakest link: the cable type and the terminals at each end.
- NM-B cable: conductors are 90 °C rated, but NEC 334.80 says the cable's ampacity is that of the 60 °C column. The 90 °C rating is only for derating calculations.
- UF-B cable: also 60 °C (340.80).
- THHN/THWN-2 in conduit: 75 °C if the breaker and the equipment terminals are rated 75 °C, which most modern ones are. Circuits of 100 A or less default to 60 °C when terminals aren't marked (110.14(C)(1)(a)).
Wire size for common circuits
| Breaker | Copper NM-B (60 °C) | Copper THHN (75 °C) | Aluminum SE/NM (60 °C) |
|---|---|---|---|
| 30 A | 10 AWG (30 A) | 10 AWG (35 A) | 8 AWG (35 A) |
| 40 A | 8 AWG (40 A) | 8 AWG (50 A) | 6 AWG (40 A) |
| 50 A | 6 AWG (55 A) | 8 AWG (50 A) | 4 AWG (55 A) |
| 60 A | 4 AWG (70 A) | 6 AWG (65 A) | 3 AWG (65 A) |
| 70 A | 4 AWG (70 A) | 4 AWG (85 A) | 2 AWG (75 A) |
| 100 A | 1 AWG (110 A) | 3 AWG (100 A) | 1/0 AWG (100 A) |
A 60 A circuit on 6 AWG NM-B (55 A) is allowed by NEC 240.4(B), which permits the next standard breaker size above the wire's ampacity, provided the actual load is no more than 55 A. A 48 A EV charger is a 60 A load after the 125% rule, so it needs 4 AWG in NM-B. See the EV charger calculator.
Where people go wrong
- Reading the 75 °C column for Romex because "the wire is 90 °C rated".
- Using a range or welder outlet chart written for conduit, then running NM-B.
- Forgetting that a hot attic or more than three current-carrying conductors bundled together lower the ampacity further.