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Calculators / Power and wiring

What size wire do you need?

Put in the load, the voltage and how far it is from the panel. You'll get the smallest wire that carries the breaker and keeps the voltage drop under 3%.

Updated · How we check our numbers

These are example numbers. Put in yours.

The load

A

Off the nameplate or the install manual. A 48 A EV charger is 48. Don't know amps? Watts ÷ volts.

Runs 3 hours or more at a time?
Voltage

The run

ft

One way, along the path the wire takes, not a straight line.

Wire metal
Wire type

Use this wire

6 AWG copper

On a 60 A breaker. It carries 65 A, and drops 1.2% over 60 ft.

Load
48 A× 1.25 continuous = 60 A
Breaker
60 A
Run
60 ft one way

Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.

breaker = 48 A × 1.25 = 60 A → 60 A standard

smallest copper wire that carries 60 A at 75 °C = 6 AWG (65 A)

drop = 2 × 60 ft × 0.491 Ω/1,000 ft × 48 A = 2.8 V = 1.2% of 240 V

Copper, THHN/THWN wire in conduit

75 °C column, 240 V
  • 14 AWGcarries 15 A, drops 18.1 V (7.5%)
    Under 60 A
  • 12 AWGcarries 20 A, drops 11.4 V (4.8%)
    Under 60 A
  • 10 AWGcarries 30 A, drops 7.1 V (3%)
    Under 60 A
  • 8 AWGcarries 50 A, drops 4.5 V (1.9%)
    Under 60 A
  • 6 AWGcarries 65 A, drops 2.8 V (1.2%)
    Use this
  • 4 AWGcarries 85 A, drops 1.8 V (0.7%)
    Also works

Ampacity from NEC Table 310.16, small wires capped by 240.4(D). Resistance from NEC Chapter 9 Table 8, at 75 °C.

Not counted

This assumes 86 °F around the wire and no more than 3 current-carrying wires together. A hot attic, or cables bundled through the same holes, can need a bigger size.

Permit

A new 240 V circuit takes a permit in most towns, and your local code has the final say. Use this to plan and price the job, then have it inspected.

How we work it out

Two things size a wire, and the bigger answer wins.

Ampacity. The wire has to carry the breaker that protects it. A load that runs 3 hours or more, like an EV charger or a heater, is sized at 125%, then rounded up to a standard breaker. We take the ampacity from NEC Table 310.16. NM-B and UF-B cable must use the 60 °C column; THHN in conduit uses 75 °C, the most a breaker terminal is rated for. 14, 12 and 10 AWG copper are capped at 15, 20 and 30 A.

breaker = load amps × 1.25 if continuous, up to the next standard size

Voltage drop. Current goes out and back, so the wire is twice the run. The NEC suggests keeping a branch circuit under 3%. We use each size's resistance at 75 °C from NEC Chapter 9 Table 8.

drop = 2 × one-way feet × ohms per 1,000 ft × amps ÷ 1,000

Not counted: hot attics over 86 °F, more than 3 current-carrying wires bundled together, and feeders. Those can push you a size bigger. Your local code and inspector have the final say.

Where the numbers come from

Guides

Sizing the load first

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