What Size Wire for 100 Amps? — Complete NEC Guide
For a 100 amp circuit, you need 3 AWG copper wire or 1 AWG aluminum wire per NEC Table 310.16. Use a 100A breaker. Here is the complete sizing guide with distances, costs, and professional tips.
Common Uses for 100 Amp Circuits
Sub-panels (standard), older home main service, detached garage/workshop panels, small commercial. These applications require a dedicated 100A breaker and 3 AWG copper wire (or 1 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 100 amps, the minimum conductor size at 75°C insulation (the most common rating for modern wire) is 3 AWG for copper and 1 AWG for aluminum.
Maximum Wire Run Distance
Wire gauge determines not just ampacity but also voltage drop over distance. For a 100 amp load:
At 240V: 100 ft with 1 AWG AL or 3 AWG CU. 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 100 amp wire size requirement comes from NEC Table 310.16, Article 225. 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.
Detached Garage Sub-Panel
Running a 100A sub-panel to a detached garage typically requires 1 AWG aluminum SER cable, which is far more economical than copper for this application. NEC 225.30 requires a disconnecting means at the detached structure, and a separate grounding electrode system per NEC 250.32.
Conduit vs. Romex Installation
For residential work, SER cable or individual conductors in conduit is the standard choice for concealed wiring in walls and attics. For exposed runs in garages, basements, or outdoors, use THHN 3 AWG copper or 1 AWG aluminum in EMT or PVC conduit. In commercial and industrial settings, individual THHN 3 AWG copper or 1 AWG 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 3 AWG may be less than 100A, requiring you to upsize. Use our Derating Calculator to check.
Wire Cost Estimate
3 AWG copper wire costs approximately $2.50-4.00 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.