Wire Size Calculator

Find the smallest copper or aluminium conductor that both carries the load current and keeps voltage drop within your limit. Pick a use-case preset or enter the circuit yourself. Sizes in AWG or mm2 for single-phase, three-phase and DC.

How It Works

Two limits, take the larger

A conductor must satisfy two independent requirements, and the correct size is the larger of the two:

  • Ampacity: the conductor must be rated to carry the load current at the chosen termination temperature (60, 75 or 90°C).
  • Voltage drop: over the run length, the drop must stay within your limit (commonly 3% on a branch circuit, 5% total).

On short, high-current runs ampacity usually wins. On long runs the voltage-drop limit forces a larger conductor than ampacity alone would need.

Worked example

A 48 A EV charger at 240 V over 50 ft of copper: #8 AWG covers the ampacity (50 A at 75°C) and holds voltage drop about 1.5%, so #8 AWG is the pick, set by ampacity.

Ampacities are reference values for not more than three current-carrying conductors in a standard installation. They do not include ambient-temperature or conductor-bundling derating, and local wiring rules take precedence. Verify against the applicable standard.

Frequently Asked Questions

How do I choose a wire size?

Size for ampacity first (the conductor must carry the current), then check voltage drop over the run. Use whichever requirement calls for the larger conductor.

What does the termination rating mean?

Terminations are rated 60, 75 or 90°C. You generally size ampacity from the lowest-rated component in the circuit, most often the 75°C column.

Why does a long run need a bigger wire?

Voltage drop grows with length. On a long run the drop can exceed the limit even when the conductor easily carries the current, so a larger size is needed.

Copper or aluminium?

Aluminium has lower ampacity and higher resistance for a given size, so an aluminium conductor is usually one to two sizes larger than copper for the same job.

Does this include derating?

No. The ampacities are base values for a standard installation. Apply ambient-temperature and bundling derating factors separately, and follow your local wiring rules.

Wire size by breaker rating

Already know the circuit rating? These pages give the answer straight out, with the 60/75/90°C table and the maximum run length before voltage drop bites.

20 Amp Wire Size 30 Amp Wire Size 40 Amp Wire Size 50 Amp Wire Size 60 Amp Wire Size 70 Amp Wire Size 80 Amp Wire Size 100 Amp Wire Size 125 Amp Wire Size 150 Amp Wire Size 200 Amp Wire Size 400 Amp Wire Size

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