What size wire do you need?

What size wire do I need?

Pick the smallest wire whose rated current covers the load, then check the voltage drop over the length of the run. For household copper wire, 14 AWG is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A. A 20 A circuit 50 feet from the panel at 120 V can use 12 AWG for the current, but it drops 3.3% — so 10 AWG, at 2.1%, is the better choice.

What is the 60 °C and 75 °C column?

NEC Table 310.16 lists how much current a wire can carry by the temperature rating of its insulation and terminals. Nonmetallic cable (Romex) and many small breakers are limited to the 60 °C column; larger feeders with 75 °C-rated terminals can use the 75 °C column. When in doubt, use 60 °C.

Why does the length of the run matter?

Current has to go out and back, and every foot of wire has some resistance, so long runs lose voltage. The NEC recommends keeping the drop under 3% for a branch circuit and 5% overall; a bigger wire has less resistance and less drop.

What is a continuous load?

A load that runs three hours or more at a time — such as a heater, an EV charger or lighting in a store. The NEC sizes the wire for 125% of a continuous load, so a 32 A EV charger needs wire rated for 40 A.

Copper or aluminum?

Aluminum carries less current for the same size — 12 AWG aluminum is limited to 15 A where copper carries 20 A — so it needs a larger size, but it is cheaper for large feeders. Use connectors and breakers rated for aluminum.

Why is the wire for a long run bigger than the current alone requires?

Because the voltage drop limit can be stricter than the current limit. A 30 A, 240 V copper load 30 feet away is served by 10 AWG (the current alone needs 10 AWG), but the same load 150 feet away needs 8 AWG to stay within a 3% drop, one size larger than the 10 AWG that suffices for the current. The calculator reports both, so you can see which one is driving the choice.

How much bigger does aluminum need to be?

For the same 30 A, 240 V, 150 foot run, aluminum needs 4 AWG against 8 AWG for copper. Aluminum carries less current for a given size and has higher resistance, so it moves up one or two sizes, which is usual for feeders.

How the size is chosen

The calculator takes the smallest standard wire that passes two tests. First, the current: the load (multiplied by 1.25 if it is continuous) must not exceed the wire's allowable current from NEC Table 310.16, in the 60 °C or 75 °C column, and for small copper wire the limits of NEC 240.4(D), which are 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. Second, the voltage drop of the run, computed with the formula 2 × K × I × L ÷ CM for a single phase, must not exceed the percentage you allow, 3% by default. The result is the larger of the two answers.

Worked example: an EV charger and a long feeder

A 32 A EV charger is a continuous load, so the wire must carry 32 × 1.25 = 40 A. On a 240 V, 60 foot copper run in the 60 °C column, the smallest wire that works is 8 AWG (drop 3.00 V, 1.25%); the 75 °C column, usable only if every terminal in the circuit is rated for it, gives 8 AWG here too. A 30 A, 240 V load 150 feet from the panel on copper: 8 AWG (7.03 V, 2.93%), against 4 AWG in aluminum (4.57 V, 1.90%).

The wire is only one part of the circuit

The wire has to match the breaker: the breaker protects the wire from overheating, so the wire's allowable current should be at least the breaker size. Conditions that this calculator does not model, such as many conductors bundled in one conduit, high ambient temperature, long runs through insulation, and different rules for motors, derating and grounding conductors, can require a larger wire. The terminals on the breaker and the device have their own temperature ratings, and the lowest one governs. The code in your area and the inspector decide, and a licensed electrician should do or check the work.

Common mistakes

Entering the round-trip length instead of the one-way distance doubles the drop twice. Sizing by current alone for a long run, and ignoring the voltage drop, can leave a motor or charger undervoltage. Using the 75 °C column when the cable or a device terminal is rated only 60 °C overstates the allowable current. Forgetting the 125% factor on a continuous load is common with chargers and heaters. And a bigger wire is not a substitute for the right breaker: a thicker wire on a larger breaker defeats the protection for the equipment connected to it.

Sources: National Electrical Code (NFPA 70): Table 310.16 (allowable ampacities), 240.4(D) (small conductors), 210.19(A) and 210.20(A) (continuous loads), and the informational notes on voltage drop (3% branch, 5% total); the voltage drop formula with K = 12.9 for copper and 21.2 for aluminum. Related: voltage drop calculator, watts to amps calculator and wire resistance calculator.