100 Amp Wire Size Calculator

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Single-phase covers most residential panels. Three-phase is typical for commercial and industrial services.
The voltage at the source panel. Common values: 120 V, 208 V, 240 V, 480 V.
V
Aluminum is lighter and less expensive but requires a larger gauge than copper for the same ampacity. Aluminum is not rated for gauges smaller than 12 AWG.
Use 75 C for most residential and commercial work. NEC 110.14(C) limits terminations to 75 C unless all equipment is rated for 90 C.
Air temperature surrounding the conduit or cable. Attics can reach 50-60 C in summer. Higher ambient means lower effective ampacity (NEC 310.15(B)(2)(a)).
C
The length from the source panel to the load panel or equipment - one way. The voltage-drop formula multiplies by 2 (or sqrt(3) for three-phase) to account for the return path.
ft
NEC recommends 3% drop for branch circuits and 5% total (feeder + branch). Going below 3% improves efficiency; 5% is the absolute ceiling most inspectors allow.
%
Minimum wire gaugeVoltage drop OK
1 AWG

Smallest AWG size that meets the 80% NEC ampacity rule at your temperature and material

Derated ampacity130A
Voltage drop3.08V
Voltage drop (%)1.28%
Voltage drop verdictWithin limit (1.28% < 3% limit)
1.28 %
Excellent<2Acceptable2-3Marginal3-5Excessive5+

Use 1 AWG copper for this 100 A circuit.

  • The NEC 80% rule requires 125 A of rated ampacity to safely carry a continuous 100 A load. 1 AWG copper meets this threshold at your insulation rating and ambient temperature.
  • At 1.28% voltage drop, this gauge is within your 3% limit - no upsize needed for run length.
  • Always verify your final wire selection with a licensed electrician and local authority having jurisdiction, as local codes may be more restrictive than NEC minimums.

Next stepCheck conduit fill: three conductors plus a ground in 1-1/4" EMT is typical for 1/0 AWG copper. Confirm your breaker, disconnect, and panel lugs are all rated for the conductor size and material.

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