What Size Wire for 400 Amps? — Complete NEC Guide
For a 400 amp circuit, you need 500 kcmil copper wire or 750 kcmil aluminum wire per NEC Table 310.16. Use a 400A breaker. Here is the complete sizing guide with distances, costs, and professional tips.
Common Uses for 400 Amp Circuits
Large residential estates, commercial buildings, multi-family residential. These applications require a dedicated 400A breaker and 500 kcmil copper wire (or 750 kcmil 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 400 amps, the minimum conductor size at 75°C insulation (the most common rating for modern wire) is 500 kcmil for copper and 750 kcmil for aluminum.
Maximum Wire Run Distance
Wire gauge determines not just ampacity but also voltage drop over distance. For a 400 amp load:
At 240V: Consult engineer — parallel runs typical. 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 400 amp wire size requirement comes from NEC Table 310.16. 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.
400A Service Configuration
A 400A residential service typically uses two parallel sets of 3/0 AWG aluminum conductors, or a single set of 500-750 kcmil. CT metering is required. Many utilities require engineering drawings and coordination meetings for services this size. The main disconnect is often a separate device from the panel.
Conduit vs. Romex Installation
For residential work, Parallel 3/0 AWG sets or single large conductors is the standard choice for concealed wiring in walls and attics. For exposed runs in garages, basements, or outdoors, use 500 kcmil copper or parallel sets in EMT or PVC conduit. In commercial and industrial settings, individual 500 kcmil copper or parallel sets 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 500 kcmil may be less than 400A, requiring you to upsize. Use our Derating Calculator to check.
Wire Cost Estimate
500 kcmil copper wire costs approximately $20-35 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.