What Size Wire for a 20 Amp Circuit?

Use 12 AWG copper for a 20 amp breaker, or 10 AWG if you use aluminum. 14 AWG copper is not allowed on a 20 amp breaker.

The answer

12 AWG copper

10 AWG aluminum (75 °C terminals)

Wire size for a 20 amp breaker
Case Wire size Table ampacity Area
Copper · 75 °C THHN in conduit, most modern equipment 12 AWG 25 A 3.3 mm²
Copper · 60 °C NM / Romex cable, older terminals 12 AWG 20 A 3.3 mm²
Aluminum · 75 °C Needs connectors and terminals listed for aluminum 10 AWG 30 A 5.3 mm²

Calculate for your exact distance

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How the answer is worked out

The breaker is 20 A. The smallest copper wire with a 75 °C table ampacity at or above 20 A is 12 AWG, rated 25 A, with a cross-section of 3.3 mm².

For 60 °C terminals or NM cable the smallest copper size is 12 AWG, rated 20 A in the 60 °C column.

Aluminum uses its own column. The smallest aluminum size at 75 °C for this rating is 10 AWG, rated 30 A. Aluminum conductors need connectors and terminals listed for aluminum.

NEC 240.4(D) also limits the breaker for small conductors: 14 AWG copper is capped at 15 A, 12 AWG copper at 20 A, 10 AWG copper at 30 A, 12 AWG aluminum at 15 A and 10 AWG aluminum at 25 A. The sizes in the table respect those caps, so a wire is never paired with a breaker larger than its limit.

Why not 14 AWG on a 20 amp breaker

14 AWG copper has a table ampacity of 20 A at 75 °C, but NEC 240.4(D) limits it to a 15 amp breaker. 12 AWG copper is the smallest wire that may be paired with a 20 amp breaker.

Where 20 amp circuits are used

Twenty-amp circuits are common for kitchen small-appliance receptacles, bathrooms, laundry areas, garages and workshops. A single 120 V, 20 A circuit can feed a good deal of equipment, but if the load runs for three hours or more (a space heater, for example) the continuous load should stay at 16 A or less.

Long runs and voltage drop

A 20 A load at the end of a long run loses voltage in the wire. At 120 V, a 3% drop is only 3.6 V, so 12 AWG copper passes that limit at full load after roughly 45–55 ft, depending on wire temperature. Step up to 10 AWG for long garage or outbuilding feeds. The distance table below shows the exact limits.

How far can you run it? (3% voltage drop)

The distance table uses the same resistance-only model as the calculator, with a 3% limit and the wire at 75 °C. Aluminum has about 1.6 times the resistance of copper for the same size, so an aluminum run of the same length drops more voltage.

Assumes the full 20 A load on a single-phase 120 V circuit, a 75 °C conductor, copper or aluminum, a 3% voltage drop limit and the one-way length in the table. Aluminum shows a dash where that size cannot carry the full current.

Maximum one-way length for 3% drop at 20 A, 120 V, 75 °C conductor
Wire size Copper Aluminum
12 AWG 45.7 ft [13.9 m] 27.7 ft [8.5 m]
10 AWG 72.6 ft [22.1 m] 44.1 ft [13.4 m]
8 AWG 115.5 ft [35.2 m] 70.1 ft [21.4 m]
6 AWG 183.6 ft [56.0 m] 111.5 ft [34.0 m]

Need a specific distance? See: What Size Wire for 20 Amps at 100 Feet?

Before you buy wire

The table is a starting point. Work through this list before you order conductors, and keep the answer that matches your real installation.

This is general information based on NEC tables, not a permit or an installation plan. Local amendments, the equipment nameplate and an inspector decide what is acceptable. Read the disclaimer.

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