What size wire do I need for a 40 amp breaker?

8 AWG copper is the smallest conductor our model estimates at or above 40 A using a 75 °C insulation rating; 6 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 6 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 40 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 40 A circuit steps up to 6 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 40 amp circuit run?

On 8 AWG copper the 3% voltage-drop guideline is reached near 57 ft one-way at 120 V and 115 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 40 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 40 A of continuous current becomes 50 A of design current and lands on a 50 A device with conductors sized to match.

What is typically on a 40 amp circuit?

Commonly electric ranges, larger cooktops, EV chargers at 32 A. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 40 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 40 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 40 amp circuit needs 8 AWG copper with 75 °C terminations, or 6 AWG aluminium. 6 AWG copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 115 ft one-way on 240 V and 57 ft on 120 V.

Formula estimate

Copper @ 75 °C

8 AWG

Copper @ 60 °C

6 AWG

Aluminium @ 75 °C

6 AWG

3% drop @ 240 V

115ft

Conductor options for 40 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 40 A
Copper, 60 °C	6 AWG	50 A	+10 A
Copper, 75 °C	8 AWG	44 A	+4 A
Copper, 90 °C	8 AWG	49 A	+9 A
Aluminium, 75 °C	6 AWG	46 A	+6 A
Aluminium, 90 °C	6 AWG	52 A	+12 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 8 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	57 ft	95 ft	5.21%
240 V 1Ø	115 ft	191 ft	2.6%
208 V 3Ø	115 ft	192 ft	2.6%
480 V 3Ø	266 ft	443 ft	1.13%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 8 AWG is 16,510 circular mils and 0.6282 Ω per 1000 ft at 20 °C.

Wire size by run length for 40 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	8 AWG	8 AWG	8 AWG
75 ft	6 AWG	8 AWG	8 AWG
100 ft	4 AWG	8 AWG	8 AWG
150 ft	3 AWG	6 AWG	8 AWG
200 ft	2 AWG	4 AWG	8 AWG
250 ft	1 AWG	4 AWG	8 AWG
300 ft	1/0 AWG	3 AWG	6 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 40 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	8 AWG	6 AWG	6 AWG
40 °C	6 AWG	6 AWG	4 AWG
50 °C	6 AWG	4 AWG	4 AWG
60 °C	4 AWG	3 AWG	2 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 40 A.

Where 40 A circuits are actually used

40 A is the classic electric-range rating on smaller cooktops, plus heat-pump air handlers with strip heat, hot tub feeds, and 32 A Level 2 EV chargers. It is also the point where aluminium becomes economically interesting for feeders in some markets.

Two different sizing treatments commonly meet at this rating. Ranges get demand-factor treatment, so a 40 A range circuit can serve an appliance whose nameplate sum is well above 40 A. EV chargers get the opposite treatment as continuous loads sized at 125%, which is exactly why a 32 A charger needs a 40 A circuit and cannot share it with anything else.

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. 40 A continuous becomes 50 A of design current, which lands on a 50 A overcurrent device and 6 AWG copper.

Frequently asked

What size wire do I need for a 40 amp breaker?: 8 AWG copper is the smallest conductor our model estimates at or above 40 A using a 75 °C insulation rating; 6 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 6 AWG. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 40 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 40 A circuit steps up to 6 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 40 amp circuit run?: On 8 AWG copper the 3% voltage-drop guideline is reached near 57 ft one-way at 120 V and 115 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 40 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 40 A of continuous current becomes 50 A of design current and lands on a 50 A device with conductors sized to match.
What is typically on a 40 amp circuit?: Commonly electric ranges, larger cooktops, EV chargers at 32 A. The load type also drives the receptacle and disconnect ratings, not just the conductor size.