Why long runs lose voltage
Every wire has a little resistance, so some voltage is used up pushing current along it before it reaches the load. On a short run it hardly matters. On a long run to a shed, a pump, a garage or a string of lights, the drop can be enough to dim lamps, slow motors and waste power as heat.
This voltage drop calculator works out the drop for a copper or aluminium conductor, by AWG gauge or by metric size in mm², for DC, single phase or three phase supplies. It shows the voltage left at the load, the power lost in the wires, and the conductor’s resistance.
How to use it
- Choose AWG or metric sizing, and pick the conductor size and metal.
- Choose single phase or DC, or three phase, and enter the supply voltage and the current the load draws.
- Enter the one-way length of the run, from the supply to the load. The calculator counts the return path itself.
- Read the drop in volts and as a percentage, and the voltage left at the load.
The voltage drop formula
single phase or DC: drop = 2 × length × current × resistance per length
three phase: drop = √3 × length × current × resistance per length
resistance per length = resistivity × (1 + α × (T − 20 °C)) ÷ cross section area
AWG diameter (mm) = 0.127 × 92^((36 − gauge) ÷ 39)The resistance comes from the metal itself rather than from a printed table: annealed copper has a resistivity of 1.7241 × 10⁻⁸ ohm metres at 20 °C (the international copper standard), aluminium about 61% of copper’s conductivity, and both rise with temperature. For 12 AWG copper at 75 °C that gives 1.93 ohms per 1,000 feet.
Worked example: 16 A over 100 feet of 12 AWG copper
- Resistance: 1.9315 ohms per 1,000 ft at 75 °C
- Drop: 2 × 100 ft × 16 A × 0.0019315 ohms per ft = 6.18 V
- As a share of 120 V: 6.18 V (5.15%)
- Voltage at the load: 113.82 V
The load receives 113.82 V, and about 99 W is lost as heat in the wires. Many designers aim for a smaller drop on a branch circuit; moving up to 10 AWG would cut this one to about 3.24%.
Conductor resistance by size
| Size | Area | Copper | Aluminium |
|---|---|---|---|
| 14 AWG | 2.081 mm² | 3.0712 | 5.0576 |
| 12 AWG | 3.309 mm² | 1.9315 | 3.1807 |
| 10 AWG | 5.261 mm² | 1.2147 | 2.0004 |
| 8 AWG | 8.366 mm² | 0.764 | 1.2581 |
| 6 AWG | 13.302 mm² | 0.4805 | 0.7912 |
| 4 AWG | 21.151 mm² | 0.3022 | 0.4976 |
| 2 AWG | 33.631 mm² | 0.19 | 0.3129 |
| 1/0 AWG | 53.475 mm² | 0.1195 | 0.1968 |
| 4/0 AWG | 107.219 mm² | 0.0596 | 0.0982 |
Every three gauge numbers smaller roughly doubles the area and halves the resistance. Stranded and coated wires differ slightly from these solid conductor figures.
Keeping the drop down
- Use a larger conductor: the most direct fix.
- Shorten the run, or move the supply closer to the load.
- Use a higher supply voltage where the equipment allows it; the same power then needs less current.
- For current from power, see the watts to amps converter; for the basic relationships, the Ohm’s law calculator.
Frequently asked questions
How do I calculate voltage drop?
Multiply twice the one-way length by the current and by the wire’s resistance per unit length. For three phase, use √3 in place of 2. For 16 A over 100 feet of 12 AWG copper at 1.93 ohms per 1,000 feet, the drop is about 6.18 volts, or 5.15% of 120 V.
What is an acceptable voltage drop?
Electrical codes and design guides set the limits, so check the rules that apply where you are. Many designers aim to keep the drop on a circuit to a few percent so lights stay bright and motors run properly. The calculator shows the percentage so you can compare.
Why is the length doubled?
Current flows out along one conductor and back along the other, so in a single phase or DC circuit it travels the run twice. Enter the one-way distance from the supply to the load, and the calculator doubles it. For three phase, the factor is √3, about 1.732.
Does aluminium wire have more voltage drop than copper?
Yes, for the same size. Aluminium conducts about 61% as well as copper, so its resistance is about 1.64 times higher and the drop is larger. That is why aluminium conductors are usually chosen a size or two bigger than copper for the same job.
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