Voltage Drop Calculator
Voltage drop for a cable run in volts and percent, the voltage left at the load and every wire size compared, for AWG or mm² cable, AC or DC.
Voltage drop is the voltage lost in the wires between the supply and whatever they power. A 20 A circuit at 120 V that runs 100 ft in 12 AWG copper loses 7.9 V, or 6.6%, leaving 112.1 V at the far end. 10 AWG cuts that to 4.1% and 8 AWG to 2.6%. In metric, 2.5 mm² cable carrying 20 A for 30 m at 230 V drops 10.6 V, or 4.6%. This calculator works out the drop in volts and as a percentage, the voltage left at the load and the power the cable turns into heat. It handles single-phase AC, three-phase AC and DC, AWG wire in copper or aluminium, and metric copper cable from 1 to 120 mm². A table lists every wire size side by side, so you can see which ones keep the drop under 3% or 5%.
- Voltage drop
- 7.9 V
- Drop
- 6.58%
- Voltage at the load
- 112.1 V
- Power lost in the cable
- 158 W
12 AWG, 100 ft one way
over 5%
from 120 V
as heat, at this current
| Size | Drop | Percent | At the load |
|---|---|---|---|
| 18 AWG | 31.85 V | 26.54% (over 5%) | 88.15 V |
| 16 AWG | 20 V | 16.67% (over 5%) | 100 V |
| 14 AWG | 12.55 V | 10.46% (over 5%) | 107.4 V |
| 12 AWG | 7.9 V | 6.58% (over 5%) | 112.1 V |
| 10 AWG | 4.97 V | 4.14% (3–5%) | 115 V |
| 8 AWG | 3.13 V | 2.6% (under 3%) | 116.9 V |
| 6 AWG | 1.97 V | 1.64% (under 3%) | 118 V |
| 4 AWG | 1.24 V | 1.03% (under 3%) | 118.8 V |
| 3 AWG | 0.98 V | 0.82% (under 3%) | 119 V |
| 2 AWG | 0.78 V | 0.65% (under 3%) | 119.2 V |
| 1 AWG | 0.62 V | 0.51% (under 3%) | 119.4 V |
| 1/0 AWG | 0.49 V | 0.41% (under 3%) | 119.5 V |
| 2/0 AWG | 0.39 V | 0.32% (under 3%) | 119.6 V |
| 3/0 AWG | 0.31 V | 0.26% (under 3%) | 119.7 V |
| 4/0 AWG | 0.24 V | 0.2% (under 3%) | 119.8 V |
How it was worked out
- Single-phase (out and back): VD = 2 × K × I × L ÷ CM
- 2 × 12.9 (copper) × 20 A × 100 ft ÷ 6,530 cmil (12 AWG) = 7.9 V
- 7.9 V ÷ 120 V = 6.58%, leaving 112.1 V at the load
Resistance is taken at the conductor's working temperature (75 °C for the US K-factor, 70 °C for metric cables, as BS 7671 does). Reactance is left out, which matters little below about 1/0 AWG or 35 mm²; above that, AC drops read a little low.
Common use cases
- Check that a long run to a garage, shed or outbuilding won't lose too much voltage
- Size the cable for a 12 V or 24 V DC system in a van, boat or solar setup
- Compare wire sizes before buying cable for a pump, heater or EV charger circuit
- Check a run against the 3% and 5% figures in the NEC, BS 7671 or AS/NZS 3000
How to use
- Choose AWG and feet or mm² and metres, and your country for the guidance on limits.
- Pick the supply: single-phase AC, three-phase AC or DC. The voltage fills in for your country, and you can change it.
- Enter the current in amps and the length of the run one way, from the supply to the load.
- Choose the wire size, and copper or aluminium for AWG.
- Read the drop in volts and percent, and compare every size in the table.
FAQ
What is an acceptable voltage drop?
In the US, informational notes in the NEC suggest no more than 3% on a branch circuit and 5% in total from the service to the load. They're recommendations, not requirements. In the UK, BS 7671 Appendix 4 suggests 3% for lighting and 5% for other uses on a public supply. In Australia and New Zealand, AS/NZS 3000 limits the drop from the point of supply to 5%.
How is voltage drop calculated?
For AWG wire: drop = 2 × K × I × L ÷ CM for single-phase AC and DC, or 1.732 × K × I × L ÷ CM for three-phase. K is 12.9 for copper and 21.2 for aluminium, I is the current, L is the one-way length in feet and CM is the wire's area in circular mils (6,530 for 12 AWG). For metric cable, multiply the cable's mV/A/m figure by the amps and the metres, then divide by 1,000. 2.5 mm² cable is about 18 mV/A/m.
Why does voltage drop matter more on 12 V systems?
The drop in volts depends on the current and the cable, not the supply voltage, so the same loss is a much bigger share of 12 V. 10 A over 20 ft in 12 AWG drops 0.79 V, or 6.6% of 12 V: the same share as 20 A over 100 ft at 120 V. 10 AWG brings it to 4.1% and 8 AWG to 2.6%. In metric, 10 A over 6 m drops 5.5% in 4 mm² cable and 3.7% in 6 mm².
Why is the drop lower on three-phase?
On a balanced three-phase circuit the current doesn't all come back on one wire, so the formula uses √3 (1.732) instead of 2. The percentage is also taken of a higher voltage. 6 mm² cable carrying 32 A for 50 m drops 11.8 V (5.1%) on a 230 V single-phase circuit, and 10.2 V (2.6%) on a 400 V three-phase one.
Is the length one way or there and back?
Enter the one-way length, from the supply to the load. The formulas already count the current going out and coming back.
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Results are estimates worked out from the figures you enter and published tables, for planning only. They are not electrical, engineering or professional advice. Mains electricity can kill and start fires. Check cable and circuit sizes against the wiring rules where you live (the NEC and local amendments in the US, BS 7671 in the UK, AS/NZS 3000 and 3008 in Australia and New Zealand), and use a licensed electrician where the law requires one.