Wire Size Calculator: Ampacity & Voltage Drop
A conductor must pass two tests: it must safely carry the current (ampacity) and it must not drop too much voltage over the run. This calculator checks both and returns the smallest size that passes. Ampacity figures are the NEC Table 310.16 75 °C column; resistance follows NEC Chapter 9 Table 8. Always confirm the final choice against the edition of the code and the installation conditions that apply to your job.
Method: smallest size whose 75 °C ampacity ≥ load current AND whose voltage drop ≤ your limit (VD = 2·I·L·R/1000 single-phase, √3·I·L·R/1000 three-phase)
Worked example: a 32 A single-phase load, 40 m from a 230 V source, copper, 3% limit: 10 AWG carries 35 A (passes ampacity) but drops about 3.6% over the run, so it fails; 8 AWG drops about 2.3% and passes — the answer is 8 AWG copper. On short runs ampacity decides; on long runs voltage drop usually decides.
Real installations add derating for ambient temperature, grouping and conduit fill, and the breaker must match the conductor and the load — see the voltage drop calculator to check any size you already have in mind.
Frequently asked questions
Why did the calculator pick a bigger wire than ampacity alone needs?
Because voltage drop grows with length. A size that safely carries the current on a short run can still lose too many volts on a long run — the calculator enforces whichever test is stricter.
What do the ampacity figures assume?
The NEC Table 310.16 75 °C column for conductors in raceway at 30 °C ambient, with no more than three current-carrying conductors. Hotter ambients, bundled cables and other conditions derate the ampacity further.
Can I use this for a house or a factory?
As a first check, yes — but the final selection must follow the code edition, the installation method and the breaker coordination for the actual job. When in doubt, go one size up and have the design checked.