Panel & Electrical Tool

Wire & Cable Sizing Calculator

Size a conductor for both ampacity and voltage drop in one step. Enter your load and run, set the insulation rating, ambient and conductor count, and get the smallest copper or aluminum size that satisfies both limits.

NEC recommends 3% on a branch circuit and 5% total to the load (informational, not enforced).

Recommended conductor
10 AWG
governed by ampacity
Derated ampacity
30.8 A
Voltage drop
1.61%
Load
24 A
Ampacity needs
10 AWG
after temp x0.88 and bundle x1
Voltage drop needs
12 AWG
to stay under 3%
Per-size breakdown (copper, 75 C column)
SizeBase ADerated AVD %Verdict
14 AWG2017.64.08%amp low · vd high
12 AWG25222.57%amp low · vd ok
10 AWGPICK3530.81.61%amp ok · vd ok
8 AWG50441.01%amp ok · vd ok
6 AWG6557.20.64%amp ok · vd ok
4 AWG8574.80.4%amp ok · vd ok
3 AWG100880.32%amp ok · vd ok
2 AWG115101.20.25%amp ok · vd ok
1 AWG130114.40.2%amp ok · vd ok
1/0 AWG1501320.16%amp ok · vd ok
2/0 AWG1751540.13%amp ok · vd ok
3/0 AWG2001760.1%amp ok · vd ok
4/0 AWG230202.40.08%amp ok · vd ok
250 kcmil255224.40.07%amp ok · vd ok
300 kcmil285250.80.06%amp ok · vd ok
350 kcmil310272.80.05%amp ok · vd ok
400 kcmil335294.80.04%amp ok · vd ok
500 kcmil380334.40.03%amp ok · vd ok
600 kcmil420369.60.03%amp ok · vd ok
700 kcmil460404.80.02%amp ok · vd ok
750 kcmil4754180.02%amp ok · vd ok

Educational estimate based on NEC Table 310.16 with 310.15(B)(1) temperature and 310.15(C)(1) bundling adjustments. It does not handle continuous-load 125% factors, conductor termination temperature limits, parallel sets, or local amendments. Verify against the current code edition and a licensed engineer before installing.

Questions

Frequently Asked

How do I calculate wire size from amps?

Start with the load current, then pick the smallest conductor whose NEC ampacity meets or exceeds it after temperature and bundle derating. Then check voltage drop over your run length and step up a size if the drop is too high. The larger of the two results is your wire size. A short run is usually limited by ampacity; a long run is usually limited by voltage drop.

What is the acceptable voltage drop for a circuit?

The NEC recommends a maximum of 3% on a branch circuit and 5% total from the service to the load. These figures are informational, not enforced, but most panel and motor circuits are designed to stay within them so the load gets enough voltage to run correctly. You can set your own limit in the calculator.

Why does ambient temperature change the wire size?

Conductor ampacity is rated at a 30C ambient. Hotter surroundings, like a packed enclosure or a rooftop run, reduce how much current the insulation can safely carry, so NEC 310.15(B)(1) applies a correction factor that lowers the usable ampacity. The hotter the ambient, the larger the conductor you need for the same load.

Does bundling wires in a conduit reduce ampacity?

Yes. Once you have more than three current-carrying conductors in the same raceway or cable, NEC 310.15(C)(1) requires an adjustment factor because the conductors heat each other. Four to six conductors drop to 80%, seven to nine drop to 70%, and ten to twenty drop to 50%. The calculator applies this automatically from the conductor count you enter.

Is copper or aluminum better for control panel wiring?

Copper carries more current per size and terminates easily, which is why control panels almost always use it. Aluminum is cheaper and lighter for large feeders, but it needs larger conductors for the same load and torque-rated, listed terminations. The calculator supports both so you can compare sizes directly.

Is anything sent to a server?

No. All sizing math runs in your browser. No load data, lengths or project details leave your machine.