How It Works
Every conductor has resistance per foot. Multiply by the total path length (out and back, hence factor 2) and current to get volts lost: VD = 2 × R × L × I. Percent drop is VD ÷ system voltage.
Calculation Explanation
Example: #12 copper, 20A, 100 ft, 120V → 2 × 1.588/1000 × 100 × 20 = 6.35V → 5.3% drop. That's why a 120V 20A circuit shouldn't run 100 ft on #12 — bump to #10 (3.3%).
If the drop is too high, use the Wire Size Calculator to pick a bigger gauge, or check the breaker size for the circuit.
Voltage Drop Quick Reference (Copper, 3% target)
| AWG | 15A @ 120V | 20A @ 120V | 30A @ 240V |
|---|---|---|---|
| #14 | ~50 ft | — | — |
| #12 | ~75 ft | ~55 ft | — |
| #10 | ~120 ft | ~90 ft | ~170 ft |
| #8 | ~190 ft | ~140 ft | ~270 ft |
| #6 | — | ~225 ft | ~430 ft |