TradesHow this works
Picking a wire size is really two questions at once. First, can the conductor carry the load without overheating. That is ampacity, and it comes straight from the NEC table for the wire size, the material, and the temperature rating of your terminals. Second, will the wire still deliver usable voltage at the far end of the run. That is voltage drop, and it gets worse the farther the load sits from the panel.
This tool sizes for both. It finds the smallest conductor that carries the load, then checks the voltage drop over your run and steps up if the drop is too high. It tells you which of the two is actually driving the size, because that is the part most calculators leave out. On a short run you are usually sized by ampacity. On a long run the voltage drop takes over, and no amount of "it can carry the current" changes that.
A worked example
Say you have a 40 amp non-continuous load at 240 volts, single-phase, copper, on a 75 degree termination. By ampacity alone that is 8 AWG copper, which is good for 50 amps. Run it 25 feet and the voltage drop is small, so 8 AWG is your answer and ampacity is what set it. Now push the same load out to 250 feet. The wire still carries 40 amps fine, but the voltage drop climbs past 3 percent, so the tool steps you up to a larger conductor. Same load, same breaker, bigger wire, and the reason is distance.
What it does not do
It does not apply ambient temperature or conduit-fill derating, it does not size a service or a feeder for a permit, and it does not certify anything. Treat it as a fast way to reason about a run and a starting point you confirm against the code that applies where you work.
Common questions
Does this replace a real electrician or an inspection?
No. It is a planning tool. It helps you think through a conductor size and see what is driving it, but it does not certify code compliance and it does not replace a licensed electrician, a permit, or an inspection.
When does voltage drop matter more than ampacity?
On longer runs. Ampacity is about heat in the wire and sets a floor on size. Voltage drop grows with distance, so on a long run the wire that carries the load fine can still lose too much voltage. Past a certain length the voltage drop, not the ampacity, is what forces you to a bigger conductor.
What voltage drop target should I use?
Three percent on a branch circuit is the common rule of thumb, and five percent total including the feeder. Those numbers come from an informational note in the NEC, so they are a recommendation, not a hard code requirement. You can change the target in the advanced options.
Copper or aluminum?
Copper is smaller for the same load and easier to terminate, so it is common on branch circuits. Aluminum is cheaper for larger feeders and services. Pick "Compare both" to see the two side by side for your run.
Sources: NEC Table 310.16 (ampacity) (2023) · NEC Chapter 9 Table 8 (conductor properties) (2023) · NEC 240.4(D), 240.6(A), 210.19 (overcurrent + voltage-drop note) (2023). Engine version 1.0.0. Risk tier 2.