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Wire Size Calculator

Find the minimum AWG wire size for a circuit from the current, voltage, length and maximum voltage drop. Copper or aluminum, NEC ampacity. Free.

Minimum wire size
8 AWG
Voltage drop at this size
6.25 V (2.6%)
Ampacity at 75 °C
50 A
Smallest size for the current alone
8 AWG

Based on NEC 310.16 at 75 °C with no more than 3 current-carrying conductors. Circuits of 100 A or less often must use the 60 °C column (a size larger) unless the equipment is listed for 75 °C. Check local code before buying.

Picking the right wire gauge

A wire must be thick enough for two reasons: to carry the current without overheating (its ampacity), and to keep voltage drop low on long runs. This calculator checks both, using NEC 310.16 ampacities at 75 °C and a voltage-drop limit you set. A 40 A, 240 V single-phase load 100 ft away needs 8 AWG copper: it carries 50 A and drops 2.6%. Aluminum needs a bigger size for the same load. Small copper sizes are limited by their breaker: 14 AWG to 15 A, 12 AWG to 20 A and 10 AWG to 30 A.

How to use it

  1. 1

    Choose the conductor

    Copper or aluminum.

  2. 2

    Enter the circuit

    Voltage, phase, current and one-way length.

  3. 3

    Set the voltage drop limit

    Usually 3%, then read the minimum wire size.

Frequently asked questions

What size wire for 30 amps?

10 AWG copper on a short run. Long runs may need 8 AWG to keep voltage drop under 3%.

What size wire for 50 amps?

8 AWG copper is rated 50 A at 75 °C, but on circuits of 100 A or less most terminals are rated 60 °C, where you need 6 AWG. Use 6 AWG unless the equipment is listed for 75 °C.

Why does distance matter?

Resistance grows with length, so long runs lose more voltage and need thicker wire.

Is this a substitute for an electrician?

No. Temperature, conduit fill and local code can change the size. Have a qualified electrician check.

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