Wire Size Calculator — Ampacity + Voltage Drop

How to use

1. Enter the load in amps and the circuit voltage (120, 240, 277, 480…).
2. Enter the one-way run length in feet and a max drop — 3% is standard.
3. Pick single-phase or three-phase.
4. Read the ampacity pick, its voltage-drop result, and the smallest gauge passing both.

About this tool

Undersized wire runs hot and drops voltage; oversized wire just costs money — this wire size calculator finds the smallest copper conductor that passes both tests at once. Ampacity comes from the NEC 75°C column with the small-conductor caps electricians apply: 14 AWG tops out at 15 amps, 12 AWG at 20, and 10 AWG at 30. Voltage drop uses the standard VD = 2 × K × I × L ÷ CM formula with K = 12.9 for copper, evaluated against your one-way run length, circuit voltage and a configurable drop limit — 3 percent is the usual branch target. The result names the gauge that satisfies ampacity, shows whether it also passes the drop check, and if it does not, keeps stepping up until one does, printing the numbers behind both tests. Long runs, low voltage and big loads are where voltage drop quietly wins. Educational estimate — always verify against the current NEC before pulling wire.

FAQ

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