40 Amp Wire Size: What Gauge Wire Do You Need?
NEC-based wire gauge for a 40 ampbreaker — copper and aluminum sizes, ground wire, conduit, and when to upsize for voltage drop.
For a 40 amp circuit, use #8 AWG copper or #6 AWG aluminum wire (THWN/THHN rated at 75°C).
Based on NEC Table 310.16 (75°C column) and, where noted, Table 310.12 for dwelling services. Confirm termination ratings and local amendments before installing.
40 Amp Wire Size at a Glance
| Copper wire size | #8 AWG |
|---|---|
| Aluminum wire size | #6 AWG |
| Breaker size | 40 amp |
| Ground wire | #10 AWG copper |
| Typical conduit | 3/4" EMT (three #8 THHN conductors) |
Verify conduit fill for your exact conductor count and insulation type with the conduit fill calculator.
Common Uses for a 40 amp Circuit
- Electric ranges and cooktops (40A, 240V)
- Level 2 EV chargers set to 32A continuous output
- Heat pumps and central air condensers
- Larger electric water heaters
- Hot tubs with smaller heaters
When to Upsize for Voltage Drop
At 240V with a full 40 amp load, #8 copper keeps voltage drop under 3% out to roughly 85-95 feet. For longer runs — a detached garage EV charger is the classic case — upsize to #6 copper, which extends the range to about 140 feet. Continuous EV charging makes voltage drop worth checking carefully.
The NEC recommends (in informational notes to 210.19 and 215.2) keeping voltage drop under 3% on a branch circuit or feeder and 5% total. Ampacity tables ignore distance entirely, so on any run much past 100 feet, check the numbers with the voltage drop calculator before buying wire.
NEC References and Code Notes
- #8 AWG copper is rated 50A at 75°C in Table 310.16, so it comfortably covers a 40 amp breaker. The 75°C column applies to common THWN/THHN terminations on breakers rated 75°C.
- #6 AWG aluminum is rated 50A at 75°C, covering 40 amps with margin.
- EV chargers are continuous loads: NEC 625.41 requires the circuit to be sized at 125% of the charger output. A 32A charger needs a 40A circuit, and a 40A charger needs a 50A circuit — do not put a 40A-output charger on a 40A breaker.
- The equipment grounding conductor for a 40 amp circuit is #10 AWG copper per Table 250.122.
See the NEC code reference for related articles. Always confirm requirements with your local authority having jurisdiction.
Frequently Asked Questions
Can I use 8 gauge wire on a 40 amp breaker?
Yes. #8 AWG copper THWN/THHN is rated 50 amps at 75°C in NEC Table 310.16, so it is fully compliant on a 40 amp breaker. Note that #8 NM-B (Romex) is limited to its 60°C rating of 40 amps — still acceptable for a 40A circuit, but with no headroom.
What size wire for a 40 amp EV charger circuit?
A 40 amp circuit (for an EV charger with 32A continuous output) needs #8 AWG copper or #6 AWG aluminum. But if your charger actually outputs 40 amps continuously, NEC 625.41 requires a 50 amp circuit with #6 AWG copper. Size the circuit at 125% of the charger output current, not the breaker you happen to have.
What size wire do I need for a 40 amp electric range?
Use #8 AWG copper (commonly 8/3 NM-B with ground) on a 40 amp double-pole breaker, run as a 4-wire circuit to a NEMA 14-50R or direct connection. Ranges 12kW and under are typically fine on a 40A circuit per the NEC Table 220.55 demand factors.
Is 6 gauge aluminum OK for 40 amps?
Yes. #6 AWG aluminum is rated 50 amps at 75°C, so it covers a 40 amp circuit with margin. Use conductors rated for the application (XHHW-2 or SER cable are common), apply antioxidant compound where required, and torque lugs to spec — aluminum terminations are where problems start, not the wire itself.
Related Tools and Wire Size Guides
Wire Sizing Calculator
Calculate the exact wire gauge for any load, voltage, and distance per NEC tables.
Calculate Wire Size →Voltage Drop Calculator
Check whether your run length forces a larger conductor than ampacity alone requires.
Calculate Voltage Drop →Wire Ampacity Chart
Full NEC Table 310.16 ampacities at 60°C, 75°C, and 90°C for copper and aluminum.
View Ampacity Chart →Breaker Size Calculator
Find the right breaker for a given load, including the 125% continuous-load rule.
Size a Breaker →50 Amp Wire Size: What Gauge Wire Do You Need?
For a 50 amp circuit, use #6 AWG copper or #4 AWG aluminum wire (THWN/THHN rated at 75°C).
Read the Guide →60 Amp Wire Size: What Gauge Wire Do You Need?
For a 60 amp circuit, use #6 AWG copper (rated 65A at 75°C) or #4 AWG aluminum wire.
Read the Guide →100 Amp Wire Size: What Gauge Wire Do You Need?
For a 100 amp feeder or circuit, use #3 AWG copper or #1 AWG aluminum wire (THWN/THHN at 75°C).
Read the Guide →