Wire Size Calculator (AWG and mm^2)

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DC systems and AC single-phase use a factor of 2 for the round-trip length; three-phase uses the square root of 3.
Copper is the most common for residential and commercial wiring. Aluminum is used for large service conductors and overhead runs.
The continuous current the wire must carry, in amperes.
A
System voltage: 12 V, 24 V, 120 V, 240 V, 208 V, 480 V etc.
V
Distance from the power source to the load. The calculator doubles this for the round-trip (DC/single-phase) or applies the three-phase factor automatically.
ft
NEC recommends no more than 3% for branch circuits and 5% for feeders + branch combined. Low-voltage DC systems may need 2% or less.
%
The temperature rating of the wire insulation determines which NEC ampacity column to use. Most residential wiring uses 60 C or 75 C.
Recommended wire sizeWithin NEC 3% limit
AWG 12

Smallest standard AWG that meets both the voltage-drop limit and the NEC ampacity limit

Cross-sectional area2.92mm^2
Actual voltage drop3.18V
Voltage drop2.65%
Wire resistance0.1588ohm
Wire ampacity25A
Wire diameter2.053mm
2.65 %
NEC branch limit (3%)<3NEC feeder limit (5%)3-5Excessive drop5+

Use AWG 12 for this circuit.

  • The voltage drop of 2.65% is within the NEC-recommended 3% for branch circuits.
  • AWG 12 has an ampacity of 25 A at the selected insulation rating, giving 5 A of headroom above your load.
  • Always size wire to the larger of the voltage-drop result and the NEC ampacity limit - this calculator does that automatically for copper and aluminum.

Next stepVerify that your circuit breaker rating does not exceed the ampacity of the wire you select.

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