Wire gauge has to clear two separate bars: carry the amps without overheating (ampacity) and deliver the voltage over the distance (voltage drop). This calculator checks both and hands you the smallest gauge that passes.
Circuit details
A
ft
Recommended wire size
—
Voltage drop at that size—
Ampacity of that size—
Sizing driven by—
Planning estimate only. It assumes two current-carrying conductors at typical temperatures and does not cover conduit fill, bundling, ambient derating, or terminations. The NEC, local amendments, and a licensed electrician always govern.
The two tests every wire must pass
Ampacity is heat: how much current the conductor can carry continuously without cooking its insulation. Voltage drop is distance: resistance eats voltage along the run, and the NEC recommends keeping the loss to about 3% for branch circuits. Short runs are almost always sized by ampacity; long runs — a detached garage, a well pump, a 12V trailer feed — get sized up by voltage drop.
Required circular mils = (2 × K × amps × one-way feet) ÷ allowable volts dropped K ≈ 12.9 for copper, 21.2 for aluminum
Worked example
A 30 A, 240 V mini-split feed 120 ft away: #10 copper handles the amps, and at that distance drops about 2.9 V (1.2%) — comfortably inside 3%, so #10 AWG copper passes both tests. Take the same 30 A load on a 120 V circuit and the percentage doubles; distance punishes low-voltage systems, which is why 12V work uses shockingly fat wire.
Where the estimate ends
Bundled cables, hot attics, aluminum terminations, motor loads, and continuous loads (the 125% rule) all change the answer. Use this to plan the material run, then confirm against NEC tables or with your electrician before you pull anything.
Frequently asked questions
What size wire do I need for a 30 amp circuit?
#10 AWG copper is the standard match for 30 A on a typical short run. If the run is long — roughly beyond 100–150 ft depending on voltage — voltage drop can force you up to #8 or larger.
Why does a long run need thicker wire even if the amps are the same?
Resistance builds with every foot of conductor, and voltage is lost across that resistance. The wire never overheats, but the equipment at the far end sees low voltage, which makes motors run hot and electronics misbehave.
Is aluminum wire okay to use?
Aluminum is common and code-legal for larger feeders (like service entrances and subpanels) and is cheaper per foot, but it needs roughly two sizes larger than copper for the same job and requires aluminum-rated connectors and anti-oxidant at terminations.