Wire Size & Ampacity Calculator

About the Wire Size & Ampacity Calculator

Picking a wire gauge involves two separate questions: is the conductor big enough to carry the current safely, and is it big enough to deliver acceptable voltage at the far end of a long run? This calculator answers the second, sizing a conductor so voltage drop stays within your chosen percentage over the distance you specify, which is the constraint that usually governs feeders, subpanels, and outbuildings.

Electricians, solar installers, and anyone running power to a shop, well pump, or detached garage face this all the time: the ampacity table says #10 is fine for the amps, but 200 feet away the voltage has sagged too far. The tool works the voltage-drop formula in reverse, telling you the minimum circular mils and the smallest standard gauge that will hold the drop under your limit.

You supply the system voltage, load current, one-way distance, conductor material, and the maximum drop you are willing to accept, and it returns both the required circular mils and a recommended AWG size from the common range of #14 up through 1/0. It is a fast way to right-size a run before you price copper.

How the Wire Size & Ampacity Calculator works

The tool first converts your allowable drop percentage into volts: allowed drop = system voltage x max drop percent / 100. That is the voltage budget you are willing to lose across the conductor.

It then rearranges the standard drop formula to solve for cross-section instead of drop: required CM = (2 x K x I x L) / allowed-drop-volts. K is 12.9 for copper or 21.2 for aluminum, I is the load current, L is the one-way distance, and the 2 covers the out-and-back conductor path. The result is the minimum circular mils needed.

Finally it scans the standard AWG table, #14 (4,107 CM), #12 (6,530), #10 (10,380), #8 (16,510), #6 (26,240), #4 (41,740), #2 (66,360), 1/0 (105,600), and recommends the first size whose circular mils meet or exceed the requirement. If the run needs more than 1/0, it tells you to size up beyond the table.

Worked example

Say you need to feed a 40-amp load at 240 volts to a shop 120 feet away in copper, and you want to hold voltage drop to 3%. First the voltage budget: 240 x 3 / 100 = 7.2 volts allowed.

Required circular mils = (2 x 12.9 x 40 x 120) / 7.2 = 123,840 / 7.2 = 17,200 CM. That is the minimum cross-section that keeps the drop at or under 7.2 volts.

Scanning the table, #8 copper offers 16,510 CM, just short of the 17,200 needed, so it is rejected. The next size up, #6 copper at 26,240 CM, clears the requirement easily, so the calculator recommends 6 AWG. Note that #8 might satisfy ampacity for 40 amps, but voltage drop over 120 feet is what forces the jump to #6.

Tips & common mistakes

• This sizes for voltage drop only. Always cross-check the result against the NEC ampacity tables (310.16) and your local code so the conductor is also rated to carry the current and survive a fault.
• Tighten the max-drop target for sensitive or continuous loads. Well pumps, EV chargers, and long feeders benefit from a 2% budget, while a short lighting branch can usually live with 3%.
• Switching material to aluminum will push the recommendation up a size or two because its higher K needs more circular mils for the same drop; weigh the cost savings against the larger conduit and terminations required.
• When the recommended size lands just above a gauge boundary, as in the #8-to-#6 example, double-check your distance and current inputs, since a small measurement error can bump you an unnecessary size and add real cost on a long pull.
• Remember terminations and breakers have their own temperature ratings. A conductor sized for drop still has to land on lugs rated for its ampacity at the correct 60/75/90 degree C column.

Wire Size & Ampacity Calculator: frequently asked questions