Voltage Drop Calculator
With supply voltage 240 V, load current 20 A, one-way run length 150 ft, voltage drop comes to 7.46 V — voltage dropped. It is reached in 8 steps, the last of which is 2 * 12.9 * 20 * 150 / 10380, and each one is printed on the page with its numbers filled in. The formula is the one published by NFPA 70, not an approximation fitted to it.
Voltage lost along a copper or aluminium run, as volts and as a percentage, against the 3% the NEC recommends for a branch circuit.
Formula and sources checked · How we check
Supply voltage 240 V, Load current 20 A, One-way run length 150 ft, Conductor Copper
7.46 V
Voltage dropped for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Circular mils
10,380- Resistivity constant K
12.9- Voltage dropped in the run
2 * 12.9 * 20 * 150 / 103807.457 V- Drop as a percentage
7.4566474 / 240 * 1003.107 %- Voltage reaching the load
240 - 7.4566474232.543 V- Power burned in the wire
7.4566474 * 20149.133 W- Longest run that stays under 3%
10380 * 0.03 * 240 / (2 * 12.9 * 20)144.837 ft- Percentage points under the 3% guideline
3 - 3.1069364-0.107 %
Ask about this in the chatCompare: copper against aluminiumVoltage Drop Chart: 10 AWG Copper at 20 A
Worked example
20 A at 240 V down 150 ft of 10 AWG copper loses 7.46 V. That is 3.11% of the supply, just past the 3% the NEC’s informational note recommends: the load sees 232.5 V and 149 W is burned heating the cable. Holding this circuit under 3% means either stopping the run at 145 ft or moving up to 8 AWG.
How to work it out yourself
- 1.Measure the run one way. The formula already counts the return conductor on a single-phase circuit, which is where the 2 comes from; three-phase uses 1.732 because the return path is shared.
- 2.Size for heat first, then check the drop. Ampacity — how much current the insulation can carry without cooking — is a code requirement; voltage drop is a performance guideline, and it is the one that decides whether a motor at the end of a long run starts properly.
- 3.If the drop is over 3%, the fix is a fatter conductor, not a longer one: each step up in AWG size adds roughly 26% more circular mils, and the drop falls in proportion.
- 4.Aluminium carries about 60% as well as copper for the same size, so an aluminium run needs two sizes up to match a copper one on drop.
- 5.A drop bigger than the supply voltage is the arithmetic telling you the conductor cannot carry that load over that distance at all — the negative figure is not a result, it is the circuit failing. Size up until the drop is a few percent.
Drop along the run, 10 AWG copper at 20 A on 240 V
| One-way run length (ft) | Voltage dropped | Drop as a percentage | Voltage reaching the load |
|---|---|---|---|
| 25 | 1.24 V | 0.52 % | 238.76 V |
| 50 | 2.49 V | 1.04 % | 237.51 V |
| 75 | 3.73 V | 1.55 % | 236.27 V |
| 100 | 4.97 V | 2.07 % | 235.03 V |
| 150 | 7.46 V | 3.11 % | 232.54 V |
| 200 | 9.94 V | 4.14 % | 230.06 V |
| 250 | 12.43 V | 5.18 % | 227.57 V |
| 300 | 14.91 V | 6.21 % | 225.09 V |
The drop is linear in length: double the run, double the loss.
The formula
- Circular mils
- Resistivity constant K
- Voltage dropped in the run
2 * 12.9 * 20 * 150 / 10380 - Drop as a percentage
7.4566474 / 240 * 100 - Voltage reaching the load
240 - 7.4566474 - Power burned in the wire
7.4566474 * 20 - Longest run that stays under 3%
10380 * 0.03 * 240 / (2 * 12.9 * 20) - Percentage points under the 3% guideline
3 - 3.1069364
Source: NFPA 70 — National Electrical Code (free online access), Copper Development Association — ampacity adjustments for wire and cable
Questions people actually ask
- Does the NEC require voltage drop under 3%?
- No. The 3% figure for a branch circuit, and 5% for feeder and branch combined, appear in informational notes — 210.19(A) and 215.2(A) — which are recommendations rather than enforceable rules. Some local amendments and some load types do make it mandatory, and equipment manufacturers often specify their own limit.
- What is the voltage drop formula?
- VD = 2 × K × I × L ÷ CM for single-phase, and 1.732 × K × I × L ÷ CM for three-phase, where K is 12.9 for copper or 21.2 for aluminium, I is amps, L is the one-way run in feet and CM is the conductor’s circular mils. It is a resistance calculation with the round trip built into the 2.
- Why does a long run make lights dim and motors struggle?
- Because the wire is a resistor. Every volt lost along it is a volt the load never sees, and the power that vanishes — 149 watts on the run above — heats the cable instead. Motors are the worst affected: torque falls with the square of voltage, so a 5% drop costs about 10% of starting torque.
- Should I use the 75°C or 90°C column?
- Neither changes this calculation. K = 12.9 is copper at about 75°C, which is the normal working assumption; a conductor running hotter has slightly higher resistance and drops slightly more, and a cold run drops slightly less. Ampacity, not drop, is what the temperature columns are for.
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