Wire Size Calculator
Find the minimum copper wire gauge (AWG) for a circuit from voltage, current, run length, and your max voltage drop. Sizes wire to keep drop under 3%.
How this calculator works
The calculator reverses the voltage-drop formula: given how much drop you’ll accept, it finds the required conductor size in circular mils, then picks the smallest standard copper gauge that meets it.
Formula: Required circular mils = (2 × 12.9 × length ft × amps) ÷ (voltage × max drop %). Then round up to the next standard AWG.
Worked example
A 120V circuit, 20A load, 100-foot one-way run, 3% maximum drop:
- Required circular mils: (2 × 12.9 × 100 × 20) ÷ (120 × 0.03) ≈ 14,333 CM
- Smallest copper gauge that meets it: 8 AWG (16,510 CM)
- Actual drop at 8 AWG: ≈ 2.60% (under the 3% limit)
Reference (120V, 20A, 3% limit)
| Run length | Minimum copper wire |
|---|---|
| 25 ft | 12 AWG |
| 50 ft | 10 AWG |
| 100 ft | 8 AWG |
| 150 ft | 6 AWG |
How to use
Enter the circuit voltage, load current, one-way run length, and your acceptable drop. Then check the recommended gauge also satisfies the breaker’s required ampacity, and confirm with the Voltage Drop Calculator.
Conductor sizing is code-governed and safety-critical. Confirm with a licensed electrician and your local code before wiring.
Frequently asked questions
How is wire size chosen?
Two things set the minimum: the circuit's ampacity (the current the wire can safely carry) and voltage drop over distance. This calculator sizes for voltage drop — it finds the smallest copper gauge that keeps drop under your chosen limit. Always also confirm the gauge meets the ampacity required for the breaker.
What max voltage drop should I use?
3% is the NEC recommendation for a branch circuit and a safe default. Use 2% for sensitive electronics or very long runs; some feeders are allowed up to 5% total, but tighter is always better for performance.
Does a lower AWG number mean thicker wire?
Yes — the AWG scale runs backwards. 14 AWG is thin, 12 is thicker, and the numbers keep dropping to 1, then 1/0 (entered as 0 here) and larger. Thicker wire has more circular mils and less voltage drop.
Is this ampacity or voltage-drop sizing?
This is voltage-drop sizing. For short runs, ampacity usually governs (for example, 20A needs at least 12 AWG regardless of distance). For long runs, voltage drop pushes you to a thicker wire than ampacity alone would require — use the larger of the two.
What about aluminum wire?
This calculator sizes copper. Aluminum carries less current per gauge and drops more voltage, so an aluminum run typically needs to be one or two sizes larger. Have an electrician confirm aluminum sizing.
Estimates only. Verify quantities with your supplier before purchasing.