What size wire do I need for a 15 amp breaker?

14 AWG copper is the smallest conductor our model estimates at or above 15 A using a 75 °C insulation rating; 14 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 12 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 15 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 15 A circuit steps up to 12 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 15 amp circuit run?

On 14 AWG copper the 3% voltage-drop guideline is reached near 38 ft one-way at 120 V and 76 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 15 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 15 A of continuous current becomes 18.8 A of design current and lands on a 20 A device with conductors sized to match.

What is typically on a 15 amp circuit?

Commonly lighting circuits, general receptacles, smoke detectors. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 15 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 15 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 15 amp circuit needs 14 AWG copper with 75 °C terminations, or 12 AWG aluminium. 14 AWG copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 76 ft one-way on 240 V and 38 ft on 120 V.

Formula estimate

Copper @ 75 °C

14 AWG

Copper @ 60 °C

14 AWG

Aluminium @ 75 °C

12 AWG

3% drop @ 240 V

76ft

Conductor options for 15 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 15 A
Copper, 60 °C	14 AWG	15 A	+0 A
Copper, 75 °C	14 AWG	18 A	+3 A
Copper, 90 °C	16 AWG	15 A	+0 A
Aluminium, 75 °C	12 AWG	19 A	+4 A
Aluminium, 90 °C	14 AWG	16 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 14 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	38 ft	63 ft	7.85%
240 V 1Ø	76 ft	127 ft	3.93%
208 V 3Ø	76 ft	127 ft	3.92%
480 V 3Ø	176 ft	294 ft	1.7%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 14 AWG is 4,107 circular mils and 2.5252 Ω per 1000 ft at 20 °C.

Wire size by run length for 15 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	12 AWG	14 AWG	14 AWG
75 ft	10 AWG	14 AWG	14 AWG
100 ft	8 AWG	12 AWG	14 AWG
150 ft	8 AWG	10 AWG	14 AWG
200 ft	6 AWG	8 AWG	12 AWG
250 ft	4 AWG	8 AWG	12 AWG
300 ft	4 AWG	8 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 15 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	14 AWG	12 AWG	12 AWG
40 °C	14 AWG	12 AWG	10 AWG
50 °C	12 AWG	10 AWG	10 AWG
60 °C	10 AWG	8 AWG	8 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 15 A.

Where 15 A circuits are actually used

15 A is the lighting and general-receptacle rating: bedroom and living-area outlets, hard-wired luminaires, smoke alarm circuits, doorbell transformers, furnace control circuits. In new residential work it survives mainly on lighting, because receptacle circuits in kitchens, bathrooms and laundry are required to be 20 A.

The rating is capped by the conductor, not the load. 14 AWG copper is limited to a 15 A overcurrent device by the small-conductor rule no matter what the ampacity calculation returns, so there is no legitimate way to squeeze more out of a 15 A circuit than re-pulling it. The common field mistake is the reverse — a 20 A breaker left on a 14 AWG lighting home run after a panel change.

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. 15 A continuous becomes 18.8 A of design current, which lands on a 20 A overcurrent device and 12 AWG copper.

Frequently asked

What size wire do I need for a 15 amp breaker?: 14 AWG copper is the smallest conductor our model estimates at or above 15 A using a 75 °C insulation rating; 14 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 12 AWG. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 15 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 15 A circuit steps up to 12 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 15 amp circuit run?: On 14 AWG copper the 3% voltage-drop guideline is reached near 38 ft one-way at 120 V and 76 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 15 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 15 A of continuous current becomes 18.8 A of design current and lands on a 20 A device with conductors sized to match.
What is typically on a 15 amp circuit?: Commonly lighting circuits, general receptacles, smoke detectors. The load type also drives the receptacle and disconnect ratings, not just the conductor size.