What Is the Minimum Wire Size for a 30 Amp Branch Circuit?

A user asked what minimum conductor size a 30 amp branch circuit requires. The direct answer under NEC 2023 is 10 AWG copper. If you are pulling aluminum, the minimum is 8 AWG.

Why 10 AWG copper is the floor

NEC 240.4(D)(7) caps the overcurrent protection for 10 AWG copper at 30 amps regardless of what the ampacity table might otherwise allow. That small conductor rule exists because 14, 12, and 10 AWG conductors serve everyday receptacle and appliance loads where a conservative limit keeps insulation temperature in check. For aluminum and copper clad aluminum, 240.4(D)(6) caps 10 AWG at 25 amps, which is why an aluminum 30 amp circuit steps up to 8 AWG.

Termination temperature still applies

Table 310.16 lists 10 AWG copper at 30 amps in the 60 degree C column and 35 amps at 75 degrees C. Residential 30 amp circuits wired in NM cable are stuck with the 60 degree C ampacity per 334.80, so 10/2 or 10/3 NM works out exactly at 30 amps. In conduit with THWN-2 you have thermal headroom, but 240.4(D) still stops you from protecting 10 AWG above 30 amps, so the breaker stays at 30 either way.

Common 30 amp circuits and their catches

Electric dryers take 10/3 with ground, since 250.140 has required a separate equipment grounding conductor for new work since 1996. A 30 amp RV or welder receptacle wants attention to voltage drop; past roughly 75 feet at full load, stepping up to 8 AWG copper keeps the drop near the 3 percent recommendation in the informational note to 210.19. Water heaters on 30 amp circuits are sized from 125 percent of the nameplate per 422.13, which is why a 4500 watt element lands comfortably on 10 AWG. And if the run passes through a hot attic, the correction factors in 310.15(B) can eat the margin on 10 AWG, so check the math before you staple it in.

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