Wire Size for 40 Amp Circuit
Find the copper or aluminum wire for a 40 amp breaker and check the voltage drop for your run length.
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Assumes NEC 2023 Table 310.16: copper or aluminum, up to 3 current-carrying conductors in a raceway or cable, 30 °C ambient, 60 °C terminals for a 40 A circuit (110.14(C)), 3 % drop target; ground wire not included. Runs in your browser. Verify with your local code and a licensed electrician before you install.
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How to use the 40 Amp Wire Size Calculator
- Keep 40 in “Breaker rating (A)”, or type the rating printed on your breaker.
- Enter the nameplate or calculated current in “Load current (A)”. Leave it empty and the wire is sized for the full 40 A breaker rating. A 32 A load is the usual size for a 40 A EV charger or hot tub.
- Tick “Continuous load (3 hours or more)” for an EV charger, a heat pump or a spa heater. The design current then becomes 125 % of the load, and the tool warns when that is above the breaker.
- Pick the “Supply type” and enter “Voltage (V)”: 240 for most 40 A residential circuits, or the line-to-line voltage for three phase.
- Type the “One-way length” in ft or m, measured along the cable route from the panel to the load.
- Choose Copper or Aluminum under “Conductor” and the “Insulation temperature rating” printed on the cable, for example 60 °C for NM-B or 90 °C for THHN.
- Leave “Allowed voltage drop (%)” at 3 unless your project sets another limit.
- Open the advanced settings only when they apply: “Terminal temperature rating”, “Ambient temperature” and “Current-carrying conductors in the raceway or cable”.
- Read “Recommended wire size” and “Deciding limit”, then the steps and warnings below the result.
The example buttons load an EV charger, a small range, a heat pump, an aluminum garage subpanel and a hot tub. “Copy result”, “Copy link” and “Print / save as PDF” keep the answer, and Reset restores the starting values.
What size wire do I need for a 40 amp breaker?
With the terminal setting left on Auto, the answer is 8 AWG copper or 6 AWG aluminum. Copper 8 AWG has exactly 40 A in the 60 °C column of NEC Table 310.16, which matches the breaker. Aluminum 8 AWG has only 35 A there, so aluminum moves up to 6 AWG. A long run can still push either size up.
The calculator checks in this order:
- The conductor must be protected by the breaker, so its allowed ampacity has to be at least 40 A (NEC 240.4 and 310.16).
- The insulation column gives the starting value; the ambient factor (310.15(B)(1)) and the factor for more than three current-carrying conductors (310.15(C)(1)) reduce it.
- The terminal column caps the result: circuits of 100 A or less use the 60 °C column unless the equipment is listed for 75 °C (110.14(C)(1)).
- Voltage drop at your load current is checked last, and the larger of the two sizes wins.
How far can you run 40 amp wire?
At a 32 A load on 240 V, 8 AWG copper reaches about 144 ft one way before the drop passes 3 %. At a full 40 A load it reaches about 115 ft. Past those distances the tool moves up a size because of voltage drop, not ampacity.
| Copper, 240 V, 3 % target | One-way length | Result |
|---|---|---|
| 32 A load | 100 ft | 8 AWG, 2.08 % drop |
| 32 A load | 200 ft | 6 AWG, 2.61 % drop (8 AWG would drop 4.2 %) |
| 40 A load | 100 ft | 8 AWG, 2.60 % drop |
| 40 A load | 150 ft | 6 AWG, 2.45 % drop |
At 120 V the same 50 ft drops 2.6 % instead of 1.3 % with a 40 A load, so voltage matters for distance but not for the ampacity answer.
Does a 40 amp EV charger or hot tub need bigger wire?
A charger or spa that draws 32 A continuously needs 32 × 125 % = 40 A, which fits a 40 A breaker, and the wire answer stays 8 AWG copper at 60 ft (1.25 % drop). The ground wire is separate: the equipment grounding conductor for a 40 A breaker is 10 AWG copper or 8 AWG aluminum, found with the equipment grounding conductor calculator.
Worked examples
| Input | Result |
|---|---|
| 40 A breaker, 32 A load, 240 V, 50 ft, copper 75 °C | 8 AWG (1.04 %) |
| 40 A breaker, 40 A load, 240 V, 30 ft, copper 75 °C | 8 AWG (0.779 %) |
| 40 A breaker, 32 A load, 240 V, 40 ft, copper 75 °C | 8 AWG (0.831 %) |
| 40 A breaker, 32 A load, 240 V, 150 ft, aluminum 75 °C | 4 AWG (2.03 %) |
| 40 A breaker, 32 A load, 240 V, 40 ft, copper 60 °C | 8 AWG (0.831 %) |
Calculated by the tool above; open it with the same values to see each step.
Questions, briefly answered
What size wire for a 40 amp breaker?
8 AWG copper or 6 AWG aluminum with the Auto terminal setting. Both reach 40 A in the 60 °C column, which is the column that applies to circuits of 100 A or less.
Is 10 gauge wire enough for 40 amps?
No. 10 AWG copper carries 30 A in the 60 °C column and 35 A in the 75 °C column, so it is below a 40 A breaker. NEC 240.4(D) also limits 10 AWG copper to a 30 A breaker.
Can I use 8 AWG aluminum on a 40 amp breaker?
Not with 60 °C terminals: aluminum 8 AWG has 35 A there. It reaches 40 A only in the 75 °C column, which counts only when the breaker and the load equipment are listed for 75 °C; the “75 °C” terminal setting shows it.
What wire size for a 40 amp 240 volt circuit?
The same 8 AWG copper or 6 AWG aluminum. Voltage changes the drop, not the ampacity: 8 AWG copper at 32 A over 100 ft drops 2.08 % on 240 V and twice as much on 120 V.
How far can I run 8 gauge wire on a 40 amp circuit?
About 144 ft one way at a 32 A load and 115 ft at 40 A, both at 240 V with a 3 % target. The row “Longest run of the recommended size” updates with your load.
What size wire for a 40 amp EV charger?
An EV charger rated 32 A is a continuous load, 32 A × 125 % = 40 A, so a 40 A breaker with 8 AWG copper works for runs up to about 144 ft at 3 % drop.
Can I run 40 amps on 12 gauge wire?
No. NEC 240.4(D) limits 12 AWG copper to a 20 A breaker and 12 AWG carries 25 A in the 75 °C column.
Is 3% voltage drop a requirement?
No. The 3 % for a branch circuit and 5 % in total are advice from informational notes in the NEC. You can set any target in “Allowed voltage drop (%)”.