Wire Size Calculator (AWG)

Find the smallest copper or aluminum wire for your load, run length and voltage-drop limit. Results update as you type.

Quick examples

Example values are shown. Change any field to calculate your own circuit.

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Enter your circuit

All fields are required unless marked optional.

Load 40 A

Use the amps on the nameplate or breaker. Choose Watts above to enter power instead.

Run 50 ft one-way

Enter the distance from the source to the load, not the round trip.

Circuit AC single-phase · 240 V · Copper · 3% max drop

Nominal supply voltage: 120 or 240 V for AC homes, 12 V for most vehicles.

3% is a common design target; use 1–2% for sensitive electronics.

Hotter wire has more resistance. 75 °C is the conservative NEC reference.

Advanced settings

Based on NEC Table 310.16 ampacities and Chapter 9 Table 8 resistance. No temperature or bundling derating. Check with a licensed electrician. How this is calculated

On this page

How to size a wire in four steps

  1. Find the load current. Use the amps on the nameplate or fuse. If you only know watts, type the watts and the calculator divides by the voltage for you.
  2. Choose the system and voltage. DC (12 V, 24 V, 48 V), AC single-phase (120 V, 240 V) or AC three-phase (208 V, 480 V).
  3. Enter the one-way length from the source to the load. The calculator accounts for the return path itself, so do not double the distance.
  4. Pick a voltage-drop limit. 3% is a common design target. Sensitive electronics, long LED runs and battery charging often need 1–2%.

What the calculator checks

It finds the smallest wire that passes two limits and shows which one decided the answer.

1. Ampacity (can the wire carry the current safely?)

The load is rounded up to the next standard breaker size (NEC 240.6(A)). Continuous loads, meaning loads that run for three hours or more such as EV chargers and heaters, are multiplied by 125% first. The wire must have an ampacity at or above the breaker rating from NEC Table 310.16, using the termination temperature column you select. The small-conductor limits of NEC 240.4(D) are applied, so copper 14, 12 and 10 AWG are never paired with a breaker larger than 15, 20 and 30 A (aluminum 12 and 10 AWG: 15 and 25 A).

2. Voltage drop (will the load get enough voltage?)

For DC and single-phase AC: Vdrop = 2 × I × R × L ÷ 1000. For three-phase AC the 2 becomes √3 (about 1.732). Here I is the current in amps, L is the one-way length in feet, and R is the conductor resistance in ohms per 1,000 ft. Resistance is calculated for copper or aluminum, assuming stranded conductors, and corrected to the wire temperature you choose. The values agree with NEC Chapter 9 Table 8 within about 2–3%.

The wire size returned is the larger of the two answers. Long runs are usually decided by voltage drop; short, heavy circuits by ampacity.

Wire size for common breaker sizes

Smallest wire allowed by ampacity alone for each standard breaker, using NEC Table 310.16 and the small-conductor limits of NEC 240.4(D). Long runs may need a larger wire to keep voltage drop in check, so use the calculator above for your actual length. A dash means the size is not covered by this tool's 60 °C table.

Smallest copper or aluminum wire for each breaker rating, by ampacity alone
Breaker Copper, 75 °CCopper, 60 °CAluminum, 75 °C
15 A 14 AWG14 AWG12 AWG
20 A 12 AWG12 AWG10 AWG
25 A 10 AWG10 AWG10 AWG
30 A 10 AWG10 AWG8 AWG
35 A 8 AWG8 AWG8 AWG
40 A 8 AWG8 AWG8 AWG
45 A 8 AWG6 AWG6 AWG
50 A 8 AWG6 AWG6 AWG
60 A 6 AWG4 AWG4 AWG
70 A 4 AWG4 AWG3 AWG
80 A 4 AWG3 AWG2 AWG
90 A 3 AWG2 AWG2 AWG
100 A 3 AWG — 1 AWG
110 A 2 AWG — 1/0 AWG
125 A 1 AWG — 2/0 AWG
150 A 1/0 AWG — 3/0 AWG
175 A 2/0 AWG — 4/0 AWG
200 A 3/0 AWG — 250 kcmil

Assumptions and limits

  • Resistance only. Reactance is ignored. That is fine for DC and for small AC conductors. For large AC conductors on long runs, use the impedance values in NEC Chapter 9 Table 9.
  • No derating. The ampacity values do not include corrections for ambient temperature above 30 °C, more than three current-carrying conductors in a raceway, or conduit fill. Apply NEC 310.15 corrections separately when they apply.
  • Termination temperature. The default is the 75 °C column. Many equipment terminals on circuits of 100 A or less are rated 60 °C; choose the 60 °C column if yours are. Aluminum uses the 75 °C column only in this tool.
  • DC systems. RVs, boats and vehicles are governed by other standards. NEC building-wire ampacity is used here as a conservative guide only.
  • Not a code review. Local amendments, inspection and a licensed electrician decide what is acceptable for your installation.

Common mistakes

  • Entering the round-trip length. Enter the one-way distance. The return conductor is already included in the formula.
  • Forgetting the 125% rule for continuous loads. A 40 A EV charger needs a 50 A circuit and wire rated for it.
  • Using the 90 °C column directly. Terminals limit the usable ampacity to the 60 °C or 75 °C column. The 90 °C values are used only as a starting point for derating.
  • Treating 3% as a legal limit. The NEC mentions 3% for branch circuits and 5% overall only as a recommendation in informational notes. It is a design target, not a requirement.
  • Mixing copper and aluminum values. Aluminum needs roughly two sizes larger wire for the same current and must use connectors rated for it.

Sources and method

  • Ampacities: NEC (NFPA 70) Table 310.16, as reproduced in the Cerrowire ampacity chart (2017 NEC basis). Check your adopted code edition.
  • Conductor resistance: copper and aluminum resistivity with a 2% stranding allowance and standard temperature coefficients, checked against NEC Chapter 9 Table 8.
  • Small-conductor and standard breaker rules: NEC 240.4(D) and 240.6(A).

Found an error? Please tell us. See also the disclaimer.

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