How voltage drop is calculated
Every wire has resistance. When current flows, part of the supply voltage is lost along the wire, and the load sees less. The drop depends on the current, the length, the wire size and the conductor material:
Single-phase and DC: VD = 2 · L · I · R · pf
Three-phase: VD = 1.732 · L · I · R · pf
R = ρ ÷ A
- L
- one-way length of the circuit
- I
- load current in A
- R
- resistance of one conductor per unit length
- ρ
- resistivity: copper 0.0172, aluminum 0.0282 Ω·mm²/m at 20 °C, about 22% higher at 75 °C
- A
- conductor cross-section in mm² (12 AWG = 3.31 mm²)
- pf
- power factor of the load
The factor 2 counts the wire to the load and the wire back. In a balanced three-phase circuit no current returns on the neutral, so the factor is √3.
Worked example
A 20 A load at 120 V sits 100 ft from the panel on 12 AWG copper. At 75 °C copper has ρ = 0.0210 Ω·mm²/m. One conductor has 0.0210 ÷ 3.31 = 0.00634 Ω per meter, and 100 ft is 30.5 m. The drop is 2 × 30.5 × 20 × 0.00634 = 7.7 V, or 6.4%. That is well over the 3% recommendation; 10 AWG brings it to 4.0% and 8 AWG to 2.5%.
Maximum length for 3% drop, copper at 75 °C
| Wire size | 15 A at 120 V | 20 A at 120 V | 30 A at 240 V |
|---|---|---|---|
| 14 AWG | 39 ft | – | – |
| 12 AWG | 62 ft | 47 ft | – |
| 10 AWG | 99 ft | 74 ft | 99 ft |
| 8 AWG | 157 ft | 118 ft | 157 ft |
| 6 AWG | 250 ft | 187 ft | 250 ft |
Single-phase, power factor 1. Sizes that are not rated for the current are left empty.
Voltage drop limits by country
| Standard | Limit |
|---|---|
| US, NEC 210.19 and 215.2 (recommendation) | 3% branch circuit, 5% feeder plus branch |
| UK, BS 7671 Appendix 4 | 3% lighting, 5% other uses |
| Australia and New Zealand, AS/NZS 3000 | 5% from point of supply |
| Canada, CEC Rule 8-102 | 3% branch or feeder, 5% in total |
What the calculator does not check
Voltage drop is only one condition for choosing a wire. It must also carry the current safely for its insulation and installation method, and the breaker must protect it. Use the cable size calculator to check both together. Fixed installations must follow local code and be done by a licensed electrician.