Wire size calculator
Get the minimum copper wire size for a single-phase or three-phase circuit, checking both the ampacity and the maximum voltage drop.
How it is calculated
The wire size is chosen with two criteria, and the larger result wins:
- Ampacity (thermal criterion). The current must not exceed what the conductor carries without exceeding its rated temperature. For AWG the calculator uses NEC Table 310.16 at 75 °C; for mm², the IEC 60364-5-52 tables.
- Voltage drop. The voltage lost along the wire must stay below the chosen percentage.
I = P / (V · PF) single-phase
I = P / (√3 · V · PF) three-phase
A = 2 · L · P / (γ · e · V) single-phase
A = L · P / (γ · e · V) three-phase
P is the power in W, L the one-way length in m, e the allowed drop in V, V the voltage and γ the conductivity of copper at operating temperature (about 47 m/(Ω·mm²) at 75 °C).
The NEC informational notes recommend a maximum of 3 % on a branch circuit and 5 % for feeder plus branch. Cable reactance is ignored, which is reasonable below about 1/0 AWG (50 mm²). Always have the final design checked by a licensed electrician.
Frequently asked questions
What size wire do I need for 30 amps?
Under the NEC, 10 AWG copper is the minimum for a 30 A circuit. On long runs, check the voltage drop: over roughly 25 m (80 ft) at 240 V you may need 8 AWG.
What size wire for a 50 amp circuit?
6 AWG copper (65 A at 75 °C) is the usual choice for 50 A, for example for an EV charger or range, subject to voltage drop on long runs.
Does the calculator work for aluminium wire?
No: it uses copper tables. Aluminium needs roughly two AWG sizes larger for the same current.