Wire Size Calculator

Ampacity: NEC Table 310.16 (2023)

Small-conductor limits: NEC 240.4(D)

Smallest copper size by load: 100 ft, 240 V single-phase, 75 °C, 3% drop

How to choose a wire size

1. Find the load current. Use the amps on the nameplate, or divide watts by volts (single-phase) or by √3 × volts (three-phase). 7,200 W ÷ 240 V = 30 A.
2. Add 25% for a continuous load. A load that runs 3 hours or more is counted at 125%: NEC 210.19(A)(1) for branch circuits and 215.2(A)(1) for feeders. 30 A becomes 37.5 A. A non-continuous load counts at 100%.
3. Pick the smallest size with enough ampacity. Read Table 310.16 in the column for the lowest-rated part of the circuit (the 60 °C column for most equipment of 100 A or less, unless it is marked 75 °C). The example needs 8 AWG copper.
4. Check the small-conductor limits. For 14, 12 and 10 AWG copper and 12 and 10 AWG aluminum, NEC 240.4(D) caps the breaker at 15, 20 and 30 A for copper and 15 and 25 A for aluminum, whatever the table says. A 25 A load therefore cannot use 12 AWG copper even though its 75 °C column reads 25 A.
5. Check the voltage drop. Drop = 2 × length × amps × resistance ÷ 1000 (√3 in place of 2 for three-phase), compared with your limit, 3% of the voltage by default. The example needs 6 AWG copper.
6. Use the larger of the answers. The size must pass all three tests, so take the biggest result: 6 AWG copper for the example. Then have an electrician check conduit fill, bundling and ambient derating, the terminals and local code.

Frequently asked questions

What size wire do I need for a 20 amp, 30 amp or 50 amp circuit?

On a short run (10 ft, not continuous, 60 °C terminals) the calculator gives 12 AWG copper for 20 A, 10 AWG for 30 A and 6 AWG for 50 A, following Table 310.16 and the 240.4(D) limits. A longer run or a continuous load can push the size up one or two steps: the same 30 A load over 200 ft as a continuous load is 6 AWG in the example above.

Why does a continuous load need 125%?

A load expected to run for 3 hours or more is a continuous load. NEC 210.19(A)(1) (branch circuits) and 215.2(A)(1) (feeders) require the conductors to carry 125% of it, so the breaker is not held at its limit for hours. In the example 30 A counts as 37.5 A. Without the 125% option the example needs 10 AWG copper for ampacity, instead of 8 AWG copper. The voltage drop is still worked on the actual 30 A.

Which temperature column of Table 310.16 should I use?

The one that matches the lowest-rated part of the circuit: the wire insulation, the breaker terminal or the device terminal. NEC 110.14(C)(1) says equipment rated 100 A or less (or marked for 14 to 1 AWG conductors) is evaluated at 60 °C unless it is marked for 75 °C, and equipment over 100 A at 75 °C. A 90 °C wire such as THHN can start from the 90 °C column for derating (heat, bundling), but the final ampacity still cannot exceed the terminal column.

What is NEC 240.4(D)?

It limits the overcurrent protection of small conductors: 14 AWG copper to 15 A, 12 AWG copper to 20 A, 10 AWG copper to 30 A, 12 AWG aluminum to 15 A and 10 AWG aluminum to 25 A (aluminum includes copper-clad aluminum). It is why 12 AWG copper is not used on a 25 A breaker even though Table 310.16 lists 25 A at 75 °C: for a 25 A non-continuous load the calculator picks 10 AWG copper and shows "NEC 240.4(D) limit" as the governing constraint.

How much voltage drop is allowed?

The NEC sets no hard limit for ordinary circuits. Informational notes to 210.19(A) and 215.2(A) recommend no more than 3% on a branch circuit or feeder and 5% overall. Informational notes are advice, not requirements, but equipment instructions and some local codes do set limits. The default here is 3% and you can change it: at 5% the example needs 8 AWG copper instead of 6 AWG copper.

How do I size wire from watts?

Change "Load in" to watts. Single-phase amps = watts ÷ volts; three-phase amps = watts ÷ (√3 × volts); the calculator assumes a power factor of 1. 7,200 W on 240 V is 30 A and gives 6 AWG copper in the example. 10,000 W on 208 V three-phase is 27.76 A and, over 100 ft with a 3% limit, 10 AWG copper.

Is aluminum wire sized differently from copper?

Yes. For the same size it carries less current (Table 310.16 lists lower values) and has more resistance, so it normally goes up one or two sizes: the example run is 4 AWG aluminum instead of 6 AWG copper. Aluminum is listed from 12 AWG, and it needs terminals and connectors marked for it.

What are the limits of this calculator?

It uses Table 310.16 (30 °C ambient, no more than three current-carrying conductors) and does not apply ambient temperature correction, adjustment for more than three conductors in a conduit, conduit fill, motor or air conditioner rules, grounding conductor sizing, or the rounding of the breaker up to the next standard size. It does not cover wire in free air, underground or aluminum service-entrance cable, and it stops at 500 kcmil: larger loads need parallel conductors and an engineer or electrician.

Can I use this to wire my house or EV charger myself?

It can show what size to expect and help you talk to the electrician, but it is not a substitute for a licensed electrician, a permit or an inspection. Local code may add to or change the NEC, and an EV charger or a range has its own rules. Have a licensed electrician check any real installation.