AmpSizerWire size and voltage drop calculators
Everything that decides how thick a conductor has to be — what it can carry, how far it can carry it, and what the code makes you run alongside it.
Voltage drop calculator
Amps, feet and a conductor in. Percent drop and the wire that fixes it out.
12 volt wire size calculator
Low-voltage DC, where a short run still wants surprisingly thick copper.
Extension cord size calculator
Amps and length in. The gauge that keeps the tool at full voltage out.
Ground wire size calculator
Breaker rating in, equipment grounding conductor out — with the upsizing rule.
Romex wire size calculator
Circuit amps in, the cable marking to ask for at the counter out.
Wire ampacity calculator
Table 310.16 with the ambient and bundling corrections actually applied.
Wire size calculator
Load and distance in. The conductor, the breaker, the ground and the pipe out.
Sizing a conductor is two questions that look like one. Can it carry the current without overheating, and will there be enough voltage left at the far end? The first is a table. The second is arithmetic on the length of the run. Almost every tool on the internet answers one of them and leaves you to notice the other.
These six answer both, and each one says which rule decided. Short runs are settled by ampacity; long ones by voltage drop; and the point where they change places moves with the load, the voltage and the metal you are pulling.
Frequently asked questions
Which of these should I start with?
The wire size calculator, because it runs both rules at once and hands back the breaker, the ground and the conduit with the answer. Come to the voltage drop calculator when the conductor is already chosen and you want to know whether the run is too long for it, and to the ampacity calculator when the conduit is crowded or the space is hot.
Do they all use the same tables?
Yes. Ampacity is Table 310.16, resistance is Chapter 9 Table 8, ground wires are Table 250.122, and every page reads the same data file. Two pages on this site cannot give you different numbers for the same question — that is the point of building it this way.
Why is there a separate calculator for extension cords?
Because a cord is not a raceway and the code puts it in a different table. Flexible cord ampacity is Table 400.5(A)(1), and cords also get coiled, dragged and run in the sun, so the sizing here is deliberately more conservative than a fixed conductor in conduit.
Why does 12-volt wiring need its own page?
Because at 12 volts a 3% target is 0.36 of a volt, so distance dominates completely and ampacity almost never decides anything. A 12 V run that looks trivially short on a 240 V chart can want four sizes more copper.
How we calculate this
k × amps × R × one-way feet ÷ 1,000 using the resistance figures in Chapter 9,
Table 8. The larger of the two requirements is the answer. Devices come from 240.6(A) with the
small-conductor ceiling of 240.4(D) applied, grounds from Table 250.122, and raceways from Chapter 9,
Table 4 at the 40% fill limit.Sources: