What Size Wire for 40 Amps? — Complete NEC Guide
For a 40 amp circuit, you need 8 AWG copper wire or 6 AWG aluminum wire per NEC Table 310.16. Use a 40A breaker. Here is the complete sizing guide with distances, costs, and professional tips.
Common Uses for 40 Amp Circuits
Electric ranges/cooktops, large AC units, hot tubs (some), EV chargers (Level 2 at 32A continuous). These applications require a dedicated 40A breaker and 8 AWG copper wire (or 6 AWG aluminum for longer runs or cost savings).
The wire size is determined by NEC Table 310.16, which lists the maximum allowable ampacity for each conductor size based on insulation temperature rating and conductor material. For 40 amps, the minimum conductor size at 75°C insulation (the most common rating for modern wire) is 8 AWG for copper and 6 AWG for aluminum.
Maximum Wire Run Distance
Wire gauge determines not just ampacity but also voltage drop over distance. For a 40 amp load:
At 240V: 75 ft with 8 AWG. At 120V: N/A.
If your wire run exceeds these distances, you must upsize the wire by one or two gauges to keep voltage drop under the NEC-recommended 3% for branch circuits. Use our Voltage Drop Calculator to check your specific run.
NEC Code Reference
The 40 amp wire size requirement comes from NEC Table 310.16, 210.19(A)(1). This table provides ampacity ratings for insulated conductors rated 0 through 2000 volts, 60°C through 90°C, not more than three current-carrying conductors in raceway, cable, or earth. For ambient temperatures above 30°C (86°F), derating factors from NEC Table 310.15(B)(1) must be applied.
Electric Range vs Cooktop
A standalone electric range typically needs a 40A or 50A circuit. If you have a separate cooktop and wall oven, each may need its own circuit — check manufacturer requirements. NEC 220.55 allows demand factors for multiple cooking appliances.
Conduit vs. Romex Installation
For residential work, NM-B 8/3 with ground or SE cable is the standard choice for concealed wiring in walls and attics. For exposed runs in garages, basements, or outdoors, use THHN 8 AWG in EMT or PVC conduit. In commercial and industrial settings, individual THHN 8 AWG conductors in conduit are standard.
When running in conduit with more than 3 current-carrying conductors, ampacity must be derated per NEC 310.15(C). This means the effective ampacity of 8 AWG may be less than 40A, requiring you to upsize. Use our Derating Calculator to check.
Wire Cost Estimate
8 AWG copper wire costs approximately $0.80-1.20 per foot for NM-B (Romex) cable. For a typical 50-foot run, expect to spend $25-90 on wire alone, plus breaker ($5-15), box, connectors, and labor if hiring an electrician. Aluminum wire costs roughly 40-60% less than copper for equivalent ampacity but requires larger gauge sizes.