Cable Voltage Drop Calculator
Calculate voltage drop in electrical cables based on length, current, and wire gauge.
Get recommendations for meeting NEC and IEC standards.
Voltage drop occurs when electrical current flows through a conductor — resistance in the wire causes some voltage to be lost as heat along the way. For short runs, voltage drop is negligible. For long runs or high-current loads, it can cause equipment to malfunction, run inefficiently, or fail entirely.
Voltage drop formula (single-phase AC or DC):
Voltage Drop (V) = 2 × L × I × R / 1000
Variable definitions:
- L = One-way cable length (feet or meters)
- I = Current (amperes)
- R = Conductor resistance (ohms per 1,000 ft or per km, from wire tables)
- Factor of 2 = accounts for the complete circuit (current travels out AND returns)
Voltage drop percentage:
VD% = (Voltage Drop / Supply Voltage) × 100
NEC (National Electrical Code) guidelines:
- Branch circuit voltage drop: ≤ 3% recommended
- Total (feeder + branch): ≤ 5% combined
- Sensitive loads (audio/video, medical, computer rooms): ≤ 1–2%
Copper wire resistance reference table:
| AWG (Imperial) | mm² (Metric) | Ohms/1000 ft | Ohms/km | Max Current |
|---|---|---|---|---|
| 14 AWG | 2.5 mm² | 3.14 | 10.3 | 15 A |
| 12 AWG | 4 mm² | 1.98 | 6.50 | 20 A |
| 10 AWG | 6 mm² | 1.24 | 4.07 | 30 A |
| 8 AWG | 10 mm² | 0.778 | 2.55 | 50 A |
| 6 AWG | 16 mm² | 0.491 | 1.61 | 65 A |
| 4 AWG | 25 mm² | 0.308 | 1.01 | 85 A |
| 2 AWG | 35 mm² | 0.194 | 0.637 | 115 A |
Worked example: A 240V, 20A circuit running 150 feet to a workshop subpanel, using 10 AWG wire:
- VD = 2 × 150 × 20 × 1.24 / 1000 = 7.44 volts
- VD% = 7.44 / 240 × 100 = 3.1% — slightly over the 3% recommendation
Solution: Upgrade to 8 AWG wire:
- VD = 2 × 150 × 20 × 0.778 / 1000 = 4.67 volts → 1.9% ✓
Voltage drop in data and AV cables: For low-voltage signals (audio, video, Ethernet, speaker wire), the concern shifts from voltage drop to signal attenuation and impedance matching. Cat6 Ethernet: maximum run of 100 meters (328 ft) per segment. Speaker wire: heavier gauge (lower AWG) recommended for runs over 50 ft to preserve audio quality.