What Size Wire for a 40 Amp Circuit Breaker? (NEC Table 310.16)

Complete Copper vs. Aluminum AWG wire size, continuous load limit (32A max), ground wire gauge, and conduit sizing for 40A circuits (Electric ranges, cooktops & 32A continuous Level 2 EV chargers).

Direct NEC Table 310.16 Answer for 40 Amps

8 AWG

Minimum Copper Wire (75°C)

THHN / THWN-2 / XHHW in Conduit

6 AWG

Minimum Aluminum Wire (75°C)

SER / XHHW-2 Feeder Cable

32 Amps

Max Continuous Load (80% Rule)

For loads running 3+ hours (EVs, lighting)

3/4" EMT

Recommended Conduit Size

Fits 3x 8 AWG + Ground under 40% fill

Common Applications for a 40A Circuit

Electric ranges, cooktops & 32A continuous Level 2 EV chargers. Remember that under the NEC 125% continuous-load rule (NEC 210.19 & 210.20), a standard 40A breaker can only supply up to 32A of continuous load (any load that runs for 3 hours or more without stopping, such as an EV charger or commercial lighting).

When to Upsize for Voltage Drop (Over 100 Feet)

If your one-way wire run exceeds 100 feet at 240V (or 50 feet at 120V), step up one wire gauge from 8 AWG to keep total line voltage drop under the NEC 3% branch-circuit recommendation. Use our Voltage Drop Calculator to check your exact footage.

2. NEC Wire Size & Ampacity Calculator (Table 310.16)

Load Current (Amps)

Conductor Metal

Terminal Temp Rating

Conductors in Raceway

Continuous Load (runs 3+ hours continuously — apply NEC 125% rule)

What Size Wire for a 40 Amp Circuit Breaker? (NEC Table 310.16)

Direct NEC Table 310.16 Answer for 40 Amps

Common Applications for a 40A Circuit

When to Upsize for Voltage Drop (Over 100 Feet)

2. NEC Wire Size & Ampacity Calculator (Table 310.16)

Recommended Wire, Breakers & Termination Tools