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.

Wire size by run length for 20 A

Ampacity sets the floor; distance is what actually forces the upsize on long runs. Smallest copper conductor that carries the load and still holds 3% drop.

One-way run	120 V 1Ø	240 V 1Ø	480 V 3Ø
50 ft	10 AWG	12 AWG	12 AWG
75 ft	8 AWG	12 AWG	12 AWG
100 ft	8 AWG	10 AWG	12 AWG
150 ft	6 AWG	8 AWG	12 AWG
200 ft	4 AWG	8 AWG	12 AWG
250 ft	4 AWG	6 AWG	10 AWG
300 ft	3 AWG	6 AWG	10 AWG

Sized on both criteria at once: estimated 75 C ampacity and 3% voltage drop at the stated one-way distance.

Source & method: A dash means no listed size holds 3% at that distance and voltage — split the load, raise the system voltage, or move the panel closer.

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.

Where 20 A circuits are actually used

20 A is the workhorse general-purpose branch circuit: kitchen and dining small-appliance circuits, bathroom receptacle circuits, laundry receptacles, garage and workshop outlets, and most commercial receptacle work. It is also the largest rating permitted on a general-purpose 15/20 A receptacle circuit, which is why so much residential rough-in is 20 A even where the connected load is trivial.

The two rules that bite at this rating are receptacle rating and multi-wire branch circuits. A single receptacle on an individual 20 A circuit must itself be rated 20 A; duplex receptacles on a multi-outlet 20 A circuit may be 15 A rated. If the circuit is a shared-neutral multi-wire branch circuit, both ungrounded conductors must be disconnected simultaneously by a handle tie or two-pole breaker.

Failure mode at 20 A is almost never the conductor melting — it is nuisance tripping from too many outlets on one circuit, or voltage drop on a long garage or outbuilding run that dims lights and shortens motor life. Load counting matters more than conductor size here.

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.