What Size Wire for 300 Amps? — Complete NEC Guide
For a 300 amp circuit, you need 350 kcmil copper wire or 500 kcmil aluminum wire per NEC Table 310.16. Use a 300A breaker. Here is the complete sizing guide with distances, costs, and professional tips.
Common Uses for 300 Amp Circuits
Large residential (5000+ sqft), small commercial, multiple sub-panels. These applications require a dedicated 300A breaker and 350 kcmil copper wire (or 500 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 300 amps, the minimum conductor size at 75°C insulation (the most common rating for modern wire) is 350 kcmil for copper and 500 kcmil for aluminum.
Maximum Wire Run Distance
Wire gauge determines not just ampacity but also voltage drop over distance. For a 300 amp load:
At 240V: Consult engineer. 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 300 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.
When Do You Need 300A+?
Homes over 5,000 sqft with all-electric appliances, multiple HVAC zones, pool equipment, and EV chargers may require 320A or 400A service. This typically requires a CT (current transformer) metering setup rather than a standard residential meter base.
Conduit vs. Romex Installation
For residential work, Parallel runs often required is the standard choice for concealed wiring in walls and attics. For exposed runs in garages, basements, or outdoors, use 350 kcmil copper or 500 kcmil aluminum in EMT or PVC conduit. In commercial and industrial settings, individual 350 kcmil copper or 500 kcmil aluminum 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 350 kcmil may be less than 300A, requiring you to upsize. Use our Derating Calculator to check.
Wire Cost Estimate
350 kcmil copper wire costs approximately $12-20 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.