Wire Size Calculator

Wire size is settled by two things at once, the ampacity table and how far the voltage falls over a long run. Enter the load current, the voltage, the one-way distance and the circuit type, and this returns the smallest copper conductor that satisfies both, with the table ampacity, every correction factor it applied and the drop in volts and percent. Ampacities come from NEC Table 310.16 for copper at 60, 75 and 90 C, corrected for ambient temperature, for the number of current-carrying conductors in the raceway and for a continuous load when you enter those. Aluminium conductors are not covered, and the adopted local code and a licensed electrician govern what is actually installed.

Calculator Type
Distance In
Sets the distance you type and every distance in the answer. The drop formula itself is still worked in feet, because K is ohm-circular mil per foot, and the formula row shows both.
Ambient Temperature In
NEC Table 310.15(B)(1) carries both scales and this page indexes it the same way: the 96 to 104 F row is the 36 to 40 C row.

The smallest copper conductor that satisfies both the corrected ampacity and your voltage drop limit.

The current the circuit will actually carry, not the breaker size.
Panel to load along the cable route, measured one way. The formula doubles it for the return conductor.
Under 110.14(C) the terminations usually decide this: 60 C for equipment rated 100 A or less unless it is listed for 75 C, and the 90 C column is normally only a starting point for derating.
The air around the raceway. 30 C, which is 86 F, is the basis of the table, so leaving this blank applies no correction. An attic in summer is much hotter.
Count only the conductors that carry current. A grounding conductor does not count, and a neutral usually does not on a balanced system.
The NEC has no mandatory limit. 3% on a branch circuit and 5% overall are a recommendation in an informational note to 210.19(A).
Widget
Distance In
Sets the distance you type and every distance in the answer. The drop formula itself is still worked in feet, because K is ohm-circular mil per foot, and the formula row shows both.
Ambient Temperature In
NEC Table 310.15(B)(1) carries both scales and this page indexes it the same way: the 96 to 104 F row is the 36 to 40 C row.

Whether a given copper conductor carries the load and how far the voltage falls over the run.

The current the circuit will actually carry, not the breaker size.
Panel to load along the cable route, measured one way. The formula doubles it for the return conductor.
Under 110.14(C) the terminations usually decide this: 60 C for equipment rated 100 A or less unless it is listed for 75 C, and the 90 C column is normally only a starting point for derating.
The air around the raceway. 30 C, which is 86 F, is the basis of the table, so leaving this blank applies no correction. An attic in summer is much hotter.
Count only the conductors that carry current. A grounding conductor does not count, and a neutral usually does not on a balanced system.
Widget
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