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:
The AWG and mm² sizes on each row are the usual trade substitutes for one another, but they are not the same conductor, so each has its own resistance. The calculator uses whichever column matches the unit system you picked.
| AWG (Imperial) | AWG Ω/1000 ft | AWG Ω/km | mm² (Metric) | mm² Ω/km | Max Current |
|---|---|---|---|---|---|
| 14 AWG (2.08 mm²) | 3.14 | 10.3 | 2.5 mm² | 7.41 | 15 A |
| 12 AWG (3.31 mm²) | 1.98 | 6.50 | 4 mm² | 4.61 | 20 A |
| 10 AWG (5.26 mm²) | 1.24 | 4.07 | 6 mm² | 3.08 | 30 A |
| 8 AWG (8.37 mm²) | 0.778 | 2.55 | 10 mm² | 1.83 | 50 A |
| 6 AWG (13.3 mm²) | 0.491 | 1.61 | 16 mm² | 1.15 | 65 A |
| 4 AWG (21.2 mm²) | 0.308 | 1.01 | 25 mm² | 0.727 | 85 A |
| 2 AWG (33.6 mm²) | 0.194 | 0.637 | 35 mm² | 0.524 | 115 A |
The gap is bigger than it looks. A 2.5 mm² conductor has about 20% more copper in it than 14 AWG, so it drops about 28% less voltage over the same run. Treating them as identical, which plenty of online calculators do, overstates the drop on every metric job. The AWG figures come from NEC Chapter 9 Table 8 (uncoated stranded copper) and the metric ones from IEC 60228, both at 20 °C.
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% ✓
The same job in metric: A 230V, 16A circuit running 40 metres on 4 mm² cable.
- VD = 2 × 40 × 16 × 4.61 / 1000 = 5.90 volts
- VD% = 5.90 / 230 × 100 = 2.6% ✓ inside the 3% guideline
Run that with 12 AWG resistance instead, on the assumption that 4 mm² and 12 AWG are interchangeable, and you get 8.32 V and 3.6%, which sends you shopping for thicker cable you do not need.
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.
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