Wire Size Calculator
Enter your load current, one-way run distance and source voltage to get a recommended AWG wire size — checked against both NEC ampacity and a voltage drop target, whichever calls for the larger conductor. Works for AC or DC, copper or aluminum, 12V solar/battery runs through 480V three-phase.
What this calculator does
This tool checks two independent limits for your circuit — the NEC ampacity minimum for your current (with a 125% adjustment if you flag the load as continuous), and the smallest wire size that keeps voltage drop under your target at your distance and voltage — then recommends whichever size is larger, since both requirements must be satisfied. Already have a wire size in mind? Check its voltage drop directly.
How the math works
- Ampacity minimum: the smallest AWG whose NEC Table 310.16 (75°C) ampacity is at or above your current (× 1.25 if continuous).
- Voltage-drop minimum: the smallest AWG keeping 2 × I × D × R ÷ 1,000 (or √3 × I × D × R ÷ 1,000 for three-phase) under your target percentage of source voltage.
- Recommended size: whichever of the two is the larger conductor.
Worked example
A 20 A, 120 V load 150 ft away on copper: the ampacity minimum is 14 AWG (20 A), but at that distance 14 AWG's voltage drop comes out to over 15% — far above the 3% target — so the voltage-drop check pushes the recommendation up to 6 AWG, the smallest size that keeps drop under 3% (2.46% at this distance and current).
Frequently asked questions
What wire size do I need for a given amperage?
It depends on both current and distance. Short runs are usually set by the NEC ampacity table alone (20 A needs at least 12 AWG copper); longer runs can need a larger size once voltage drop is factored in.
How is a DC wire size calculated differently than AC?
The underlying math is the same, but DC systems (solar, 12V/24V battery, automotive) often run at much lower voltage, so the same wattage produces a much larger percentage voltage drop — DC runs frequently need a larger wire than an equivalent 120V AC circuit.
Does this calculator size ground or neutral wires?
No — it sizes the current-carrying (hot) conductor. Equipment grounding conductor size comes from NEC Table 250.122 based on overcurrent protection rating, not load current or distance, so it's a separate lookup.
What does "continuous load" mean for wire sizing?
A load running at maximum current for 3+ hours. NEC 210.19(A)/215.2(A) require sizing ampacity at 125% of that current, which can push the recommended wire size up a gauge or more.
Limitations
This tool estimates from standard published ampacity and resistance tables. It doesn't account for ambient temperature, conduit fill, bundled-conductor derating, or ground/neutral sizing. It's for planning purposes only — confirm final sizing with a licensed electrician or your local code before installing.
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Cite this calculator
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