A 50 A circuit feeds ranges, welders, EV chargers and subpanels. The correct conductor depends on the wire type, the temperature rating of the terminals and the length of the run. This guide uses the ampacities in NEC Table 310.16.
Step 1: Start with the load
The breaker must cover the load. For a continuous load (three hours or more), the breaker and conductors must be sized at 125% of it (NEC 210.19(A)(1) and 210.20(A)). A 40 A continuous load therefore needs a 50 A breaker, because 40 × 1.25 = 50.
Step 2: Pick the conductor from Table 310.16
Copper conductors are rated 8 AWG = 40/50/55 A and 6 AWG = 55/65/75 A in the 60/75/90°C columns. Most equipment terminals are rated 75°C, so 8 AWG THHN copper in conduit (50 A at 75°C) can serve a 50 A breaker. Non-metallic cable (NM-B) is limited to the 60°C column by NEC 334.80, which means it needs 6 AWG copper (55 A).
- 8 AWG copper THHN in conduit, 75°C: 50 A
- 6 AWG copper NM-B, 60°C: 55 A
- 6 AWG aluminum in conduit, 75°C: 40 A — not sufficient for a 50 A circuit under this terminal-temperature assumption
- 4 AWG aluminum, 60°C: 55 A
Aluminum conductors require terminals listed for aluminum. The terminal temperature rating can limit the usable ampacity: do not treat the 90°C column as the final allowable ampacity when equipment terminals are rated 60°C or 75°C.
Step 3: Apply corrections
High ambient temperatures and more than three current-carrying conductors in one raceway reduce ampacity. For six current-carrying conductors in a raceway, the common adjustment factor is 80% under the applicable NEC rule. Use the conductor’s 90°C ampacity for the correction calculation when the insulation is rated 90°C, then compare the corrected result with the equipment terminal limit. For 8 AWG copper THHN, 55 A × 0.80 = 44 A, which is insufficient for a 50 A circuit. For 6 AWG copper THHN, 75 A × 0.80 = 60 A; this is below the 65 A value in the 75°C column. This example assumes the cited table values and no additional correction factors; all applicable limits and code requirements still need to be checked.
Step 4: Check voltage drop
At 240 V, a 50 A load over 100 ft one-way on 6 AWG copper drops 2 × 12.9 × 50 × 100 ÷ 26,240 = 4.9 V, which is 2.0%. On 8 AWG the drop is 7.8 V, or 3.3%, so long runs often justify the larger wire.
Common mistakes
- Using the 90°C column without checking the terminal rating.
- Forgetting the equipment grounding conductor; Table 250.122 calls for 10 AWG copper with a 60 A device.
- Using a small cable such as 10/3 on a 50 A breaker.
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