This calculator helps you plan a circuit. Electrical work must follow the National Electrical Code and local codes; when in doubt, hire a licensed electrician.
Two reasons to choose a bigger wire
A wire must be thick enough for two separate reasons:
- Ampacity: it must carry the current without overheating. For copper building wire, the usual minimum is 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A (NEC small-conductor rule), and larger sizes from the NEC ampacity table.
- Voltage drop: over a long run, the wire’s resistance wastes voltage. Motors run hot, lights dim and electronics misbehave. The NEC recommends no more than 3% drop on a branch circuit and 5% in total from the service to the outlet.
The calculator works out the cross-section needed for the voltage drop, compares it with the usual gauge for the current, and recommends the larger of the two.
The formulas
- Single-phase or DC: cross-section S = 2 × ρ × L × I ÷ ΔV
- Three-phase: S = √3 × ρ × L × I ÷ ΔV
- L is the one-way length, I the current and ΔV the allowed drop in volts. ρ is the resistivity at operating temperature: 0.0214 Ω·mm²/m for copper and 0.0352 for aluminum, the values behind the NEC’s K constants of 12.9 and 21.2.
- The AWG number then comes from the diameter: d (mm) = 0.127 × 92^((36 − AWG) ÷ 39). The calculator rounds to the next even gauge used for building wire.
Worked example: 16 A at 120 V over 100 ft of copper, 3% maximum
- Allowed drop: 3% of 120 V = 3.6 V.
- Cross-section: 2 × 0.0214 × 30.48 m × 16 ÷ 3.6 = 5.81 mm².
- 10 AWG (5.26 mm²) is too small, so the calculator picks 8 AWG (8.37 mm²).
- With 8 AWG, the actual drop is 2.50 V, or 2.08%, and the cable wastes about 40 W.
- For the current alone, 12 AWG would have been enough: the length is what pushes the gauge up.
Gauge for other lengths (16 A, 120 V, 3%)
| One-way length | Gauge | Voltage drop |
|---|---|---|
| 25 ft | 12 AWG | 1.32% |
| 50 ft | 12 AWG | 2.63% |
| 100 ft | 8 AWG | 2.08% |
| 150 ft | 6 AWG | 1.97% |
| 200 ft | 6 AWG | 2.62% |
| 300 ft | 4 AWG | 2.47% |
Tips
- Use 240 V when you can: for the same power, the current halves and the percentage drop is divided by four.
- Aluminum needs about two sizes larger than copper, and connectors rated for it.
- Low-voltage DC (12 V solar, RV, landscape lighting) is where voltage drop bites hardest: a 3% drop on 12 V is only 0.36 V.
- The calculator does not apply derating for bundled cables, high ambient temperature or conduit fill, which can require a larger wire.