WireSizing
Wire gauge reference & calculator

What Wire Size for 40 Amps?

Copper Wire
8 AWG
3.2636 mm · 8.3656 mm²
Aluminum Wire
6 AWG
4.1154 mm · 13.3018 mm²
Breaker: 40A · Based on NEC Table 310.16 (75°C insulation)

Common Uses for 40A Circuits

Electric ranges, large AC units

8 AWG Copper — Key Specs

Diameter3.2636 mm (0.12849")
Area8.3656 mm²
Resistance at 20°C2.0608 Ω/km
Weight74.955 kg/km
Ampacity (75°C)40 A

Important: For long wire runs, you may need to upsize the wire to compensate for voltage drop. NEC recommends keeping voltage drop under 3% for branch circuits. Use our voltage drop calculator to check.

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8 AWG full specifications → Full AWG chart → Wire size calculator → Ampacity tables →