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What size wire for 20 amps?

12 AWG copper — at 75 °C terminals, NEC Table 310.16. Aluminum: 10 AWG.

A 20 A circuit needs 12 AWG copper, rated 25 A at 75 °C with a 20 A breaker cap under 240.4(D). NM-B cable at the 60 °C column gives the same answer. 14 AWG reaches 20 A on raw ampacity alone, but its breaker is capped at 15 A, so it cannot be used.

Safety first. This is a starting point before derating, not a load calculation. Wire sizing affects fire and shock safety — verify against the current NEC and have circuits designed or reviewed by a licensed electrician for your jurisdiction.

20 amp wire size by material and terminal rating

Minimum conductor size for a 20 amp circuit by material and terminal temperature rating.
Material 60 °C (NM-B) 75 °C (typical) 90 °C
copper 12 AWG (20 A) 12 AWG (25 A) 12 AWG (30 A)
aluminum 10 AWG (25 A) 10 AWG (30 A) 10 AWG (35 A)

Sizes are the smallest conductor whose ampacity and NEC 240.4(D) breaker cap both reach 20 A, before any derating. Use the 75 °C column unless every terminal, breaker and device in the circuit is rated otherwise (NEC 110.14(C)).

If the load runs continuously

A load operating for three hours or more is a continuous load, and NEC 210.19(A)(1) requires the conductor to be sized at 125% of it — 20 A becomes 25 A for sizing purposes. That moves this circuit to 10 AWG copper , one or more sizes above the 12 AWG the plain table suggests. EV chargers, electric heat and commercial lighting are the loads this catches most often.

How far can a 20 amp circuit run?

The table above only prevents overheating. On a long run the limit is voltage arriving at the far end, and it bites well before ampacity does. These are the longest one-way runs for 12 AWG copper at 20 A within the 3% drop recommended by the NEC 210.19(A) informational note — and what going up to 10 AWG buys you.

Maximum one-way run length for a 20 amp circuit by system voltage and conductor size.
System Typical use 12 AWG 10 AWG
120 V standard receptacle and lighting circuits 57 ft 90 ft
240 V ranges, dryers, EV chargers, feeders 113 ft 180 ft
12 V DC automotive, marine, solar and battery runs 6 ft 9 ft

Single-phase, one-way distance, copper at 20 °C. Aluminum reaches its limit at roughly 60% of these distances. The 3% target is a recommendation, not a code requirement.

What runs on a 20 amp circuit

The catch at 20 amps

20 A is the size where the ampacity table and the breaker cap collide: 14 AWG copper reaches 20 A at 75 °C on paper, but 240.4(D) caps it at 15 A — so the answer is 12 AWG, not 14.

Working the other way round? See how many amps 12 AWG carries , including its diameter, resistance and full run-length table.