What size wire do I need for a 20 amp breaker?

12 AWG copper is the smallest conductor our model estimates at or above 20 A using a 75 °C insulation rating; 12 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 10 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 20 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 20 A circuit steps up to 10 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 20 amp circuit run?

On 12 AWG copper the 3% voltage-drop guideline is reached near 45 ft one-way at 120 V and 91 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 20 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 20 A of continuous current becomes 25 A of design current and lands on a 25 A device with conductors sized to match.

What is typically on a 20 amp circuit?

Commonly kitchen and bathroom receptacles, garage outlets, small window AC. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 20 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 20 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 20 amp circuit needs 12 AWG copper with 75 °C terminations, or 10 AWG aluminium. 12 AWG copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 91 ft one-way on 240 V and 45 ft on 120 V.

Formula estimate

Copper @ 75 °C

12 AWG

Copper @ 60 °C

12 AWG

Aluminium @ 75 °C

10 AWG

3% drop @ 240 V

91ft

Conductor options for 20 A

Estimated ampacity of each candidate size, so you can see the margin you are buying.

Material / rating	Minimum size	Estimated ampacity	Margin over 20 A
Copper, 60 °C	12 AWG	20 A	+0 A
Copper, 75 °C	12 AWG	24 A	+4 A
Copper, 90 °C	14 AWG	20 A	+0 A
Aluminium, 75 °C	10 AWG	25 A	+5 A
Aluminium, 90 °C	12 AWG	21 A	+1 A

Ampacity estimated as I = 0.0670 × cmil^0.6505, scaled for insulation rating and material.

Source & method: Conductor areas come from the ASTM B258 relationship d = 5 × 92^((36 − n) / 39). Ampacity is a fitted physical model, not a reproduction of any code table, and runs within roughly ±8% of published values.

Maximum run length on 12 AWG copper

One-way distance before voltage drop passes the 3% and 5% guidelines.

System	3% limit	5% limit	Drop @ 100 ft
120 V 1Ø	45 ft	75 ft	6.58%
240 V 1Ø	91 ft	151 ft	3.29%
208 V 3Ø	91 ft	151 ft	3.29%
480 V 3Ø	210 ft	350 ft	1.43%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 12 AWG is 6,530 circular mils and 1.5882 Ω per 1000 ft at 20 °C.

Hot attics, conduit runs and bundling

What a 20 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	12 AWG	10 AWG	10 AWG
40 °C	12 AWG	10 AWG	8 AWG
50 °C	10 AWG	8 AWG	8 AWG
60 °C	8 AWG	8 AWG	6 AWG

Source & method: Ambient correction F = √((Tc − Ta) / (Tc − 30)) with Tc = 75 °C; bundling adjustment uses the standard stepped schedule (1.00 / 0.80 / 0.70). The conductor shown is the smallest copper size whose corrected ampacity still carries 20 A.

Continuous loads

A continuous load — one expected to run three hours or more, like commercial lighting or an EV charger — is designed at 125% of the load current. 20 A continuous becomes 25 A of design current, which lands on a 25 A overcurrent device and 10 AWG copper.

Frequently asked

What size wire do I need for a 20 amp breaker?: 12 AWG copper is the smallest conductor our model estimates at or above 20 A using a 75 °C insulation rating; 12 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 10 AWG. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 20 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 20 A circuit steps up to 10 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 20 amp circuit run?: On 12 AWG copper the 3% voltage-drop guideline is reached near 45 ft one-way at 120 V and 91 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 20 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 20 A of continuous current becomes 25 A of design current and lands on a 25 A device with conductors sized to match.
What is typically on a 20 amp circuit?: Commonly kitchen and bathroom receptacles, garage outlets, small window AC. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

Wire and breakers for a 20 A circuit

12 AWG copper wire

Matched to the 20 A sizing on this page.

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