Understanding 100 Amp Wire Sizing
Sizing a wire for a 100 amp circuit means satisfying two independent requirements at once: the conductor must be rated to carry 100A continuously without overheating (ampacity), and it must be thick enough that the voltage lost to resistance over its length stays within an acceptable percentage (voltage drop). This calculator checks both requirements and recommends whichever gauge is larger — that larger size is the one that actually works.
The ampacity requirement
NEC Table 310.16 lists how much current a given wire gauge and insulation type can carry continuously. At the commonly used 75°C column (THWN, THW), 3 AWG copper or 1 AWG aluminum is rated for exactly 100A — the minimum size for a straight 100A load. A smaller gauge is not permitted regardless of run length, because it would overheat under sustained load.
The voltage-drop formula
Even when a wire is rated to carry the current, a long run can lose too much voltage to resistance before it reaches the load. The standard formula is VD = (2 × K × I × D) / CM for single-phase circuits, or VD = (1.732 × K × I × D) / CM for three-phase, where I is current in amps, D is the one-way length in feet, CM is the conductor's cross-sectional area in circular mils, and K is a resistivity constant — 12.9 ohm-cmil/ft for copper, 21.2 for aluminum. Rearranging for the required circular mils shows the smallest conductor area that keeps drop under your target percentage.
Reading the results
The calculator finds the smallest wire that satisfies ampacity alone, the smallest that satisfies voltage drop alone, and recommends whichever is larger. Short runs are almost always ampacity-limited, so 3 AWG copper is enough. Longer runs at 100A often become voltage-drop-limited and need a larger gauge, even though 3 AWG would technically carry the current safely.
Assumptions and limits
- Ampacity uses the NEC 75°C column, the most common rating for building wire; the 60°C and 90°C columns give different numbers and may apply depending on your terminals and insulation type.
- The voltage-drop constant K assumes typical building temperatures and does not correct for conduit fill, heavy derating, or unusual ambient heat.
- This tool sizes the conductor only — it does not size the circuit breaker or overcurrent device, which follows separate NEC rules. For any permit-required installation, confirm sizing with a licensed electrician.