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Wire Size Calculator

Enter the system type and voltage, the current, the one-way distance and the wire material. The calculator finds the smallest AWG size that can carry the current and keeps the voltage drop within your limit, and lists every size it checked.

Your numbers

Result

How we calculate it

Two checks decide the wire. Ampacity is how much current the wire can carry without overheating; it comes from NEC Table 310.16 in the column that matches the temperature rating of your terminals and breaker (use 60 °C if you do not know, 75 °C is common). A load that runs three hours or more is sized at 125% of its current.

Voltage drop is the voltage lost along the wire. For DC and single phase it is 2 × current × resistance × distance, because the current goes out and comes back; for three phase it is 1.732 × current × resistance × distance. Resistance in ohms per 1000 feet comes from NEC Chapter 9 Table 8. A common design limit is 3% for a branch circuit.

The smallest wire that passes both checks is shown, with the size you would pick for the current alone and for the voltage drop alone. Long runs and low voltages (12 V and 24 V) are usually decided by voltage drop, not ampacity.

Assumptions and limits

  • This is a planning estimate, not electrical advice. Wiring must follow the electrical code adopted where you live, and a licensed electrician or permit inspector decides what is safe for your job.
  • Values are from the 2023 NEC tables (the 2026 edition adds rows for 16 AWG copper and 14 AWG copper-clad aluminum and is reported to leave these ampacities unchanged) for up to three current-carrying conductors at 30 °C. Hot locations, more conductors in a conduit, wire bundles and local amendments need corrections that are not included.
  • Sizes run from 14 AWG to 4/0 AWG. The smallest wire sizes also have a breaker limit (NEC 240.4(D)): 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG.

Frequently asked questions

What size wire do I need for 50 amps?

With 75 °C terminals, 8 AWG copper (50 A ampacity) carries a 50 A load that is not continuous. If the load is continuous it is sized at 125%, 62.5 A, which needs 6 AWG (65 A). A long run needs a bigger wire again for voltage drop, so enter your distance in the calculator.

What is an acceptable voltage drop?

The NEC recommends about 3% for a branch circuit and 5% total from the service to the load. For low voltage DC such as 12 V, even a 3% drop is a large part of the available power, so many designs use less.

Why is wire size bigger at 12 V and 24 V?

The same power needs much more current at a low voltage, and the voltage drop in volts is a larger share of the total. A 100 ft run at 12 V carrying 20 A usually needs far thicker wire than the same power at 120 V.

Copper or aluminum?

Aluminum is lighter and cheaper but needs a larger size for the same current and must be used with connectors rated for it. Copper is the usual choice for branch circuits.

Does this replace an electrician?

No. Use it to plan and compare. A licensed electrician or your local inspector confirms the wire, breaker and installation for your job.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: System = Single phase AC; Voltage = 240 V; Load current (amps) = 40; One-way distance (feet) = 100; Wire material = Copper; Voltage drop limit (%) = 3; Terminal temperature rating = 75 °C; Continuous load (3 hours or more) = No.

  • Smallest wire size 8 AWG copper
  • Ampacity at 75 C 50 A
  • Voltage drop 6.22 V
  • Voltage drop percent 2.59%
  • Voltage at the load 233.78 V
  • Power lost in the wire 249.0 W
  • Wire for the current alone 8 AWG
  • Wire for the voltage drop alone 8 AWG

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.