Voltage drop reduces available power at the load. NEC recommends max 3% for branch circuits and 5% total from service to final outlet.
1φ: Vdrop = 2 × I × R × L ÷ 1000. 3φ: use √3 instead of 2.
Max 3% on branch circuits, 5% total. Exceeding causes dim lights and motor issues.
Wire sizing, long cable runs, solar panel wiring, industrial feeders.
Current, wire gauge (resistance), cable length, and phase type all affect drop.
Calculate voltage lost across conductors.
I = current (amps), R = wire resistance (Ω per 1000ft), L = one-way length (ft). For 3-phase: use √3 instead of 2.
4 steps to determine voltage drop.
Determine the load current in amps.
One-way distance from panel to load in feet.
Look up Ω/1000ft for your wire gauge (e.g., 12 AWG = 1.588).
Vdrop = 2 × I × R × L ÷ 1000. Check if % drop is within limits.
Resistance in Ω per 1000 ft for common wire gauges.
| AWG | Ω/1000ft | Max Amps (60°C) | Typical Use |
|---|---|---|---|
| 14 | 2.525 | 15 | Lighting circuits |
| 12 | 1.588 | 20 | General outlets |
| 10 | 0.999 | 30 | Dryers, AC units |
| 8 | 0.628 | 40 | Ranges, feeders |
| 6 | 0.395 | 55 | Large appliances |
| 4 | 0.249 | 70 | Feeders, sub-panels |
| 2 | 0.156 | 95 | Service entrance |
Real-world cable sizing scenarios.
20A on 12 AWG, 75ft run, 120V:
2 × 20 × 1.588 × 75 ÷ 1000 = 4.76V (3.97% — OK)
100A on 4 AWG, 200ft, 480V 3φ:
√3 × 100 × 0.249 × 200 ÷ 1000 = 8.63V (1.80% — excellent)
30A on 10 AWG, 150ft, 240V:
2 × 30 × 0.999 × 150 ÷ 1000 = 8.99V (3.75% — borderline)
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Common questions about voltage drop.
NEC recommends max 3% for branch circuits and 5% total. Some jurisdictions are stricter.
Use larger wire gauge, shorten cable runs, or increase voltage (e.g., 240V instead of 120V).
Wire resistance. Longer runs, smaller wire, and higher current all increase voltage drop.
Yes. Voltage drop creates heat in the wire (I²R losses), wasting energy and potentially causing fires.
The basic formula is similar, but AC adds impedance effects. For most residential/commercial work, the resistive formula is adequate.