How this voltage drop calculator works
Wire is a resistor, and long runs spend voltage on it. The classic estimating formula prices that loss from four numbers:
% drop = VD ÷ system volts · NEC note: ≤ 3% branch, ≤ 5% total
Worked example: voltage drop on a 100 ft run
15 A over 100 ft of 12 AWG copper (6,530 CM): VD = 2 × 12.9 × 15 × 100 ÷ 6,530 = 5.93 V — 4.9% of 120 V, leaving 114.07 V at the load. Over the 3% note: the first listed gauge that passes is 8 AWG.
What wire size for the run? — 3% chart
| One-way run | Gauge |
|---|---|
| 25 ft | 14 AWG |
| 50 ft | 12 AWG |
| 75 ft | 10 AWG |
| 100 ft | 8 AWG |
| 150 ft | 8 AWG |
| 200 ft | 6 AWG |
| Gauge | Circular mils |
|---|---|
| 14 AWG | 4,107 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
Voltage drop FAQ
How do I calculate voltage drop?
The standard 2-wire estimating formula: VD = 2 × K × I × L ÷ CM, with K = 12.9 for copper, I the amps, L the one-way feet, and CM the wire’s circular mils. The default 15 A over 100 ft of 12 AWG drops 5.93 V — 4.9% of a 120 V circuit.
What is an acceptable voltage drop?
The NEC’s informational note recommends no more than 3% on a branch circuit and 5% total with the feeder — a recommendation for performance, not a code mandate in most cases. Motors, compressors and electronics are why you honor it anyway.
What size wire do I need for a 100 ft run?
For 15 A at 120 V, 8 AWG copper is the first listed gauge inside 3% at 100 ft — two sizes fatter than the 14 AWG that ampacity alone would allow. Distance, not just load, sizes long runs; the chart below shows the crossover lengths.
Does voltage drop matter less at 240 V?
Yes — the same volts lost are half the percentage at 240 V, which is why long feeders to shops and barns usually run 240. Low-voltage systems invert that: at 12 V even short runs drop brutal percentages, as landscape-lighting installers learn fast.
Is aluminum wire worse for voltage drop?
Aluminum’s K is 21.2 against copper’s 12.9, so it needs about 64% more circular mils for the same drop — typically two AWG sizes up. It still wins on cost for big feeders, which is why service conductors are commonly aluminum.
Does this tell me the wire is code-legal?
No — it checks voltage drop only. The conductor must first be sized to the breaker per the NEC ampacity tables, with derating, termination ratings and local amendments on top. Treat the result as planning math and have a licensed electrician confirm.
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