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Voltage drop calculator
Enter the supply, load current, cable length and conductor size. See the voltage drop, the percentage, the voltage at the load and the smallest cable size that keeps you within your limit.
- Free to use
- No sign-up
- Runs in your browser
- Nothing you type is sent anywhere
Your circuit
Results
An estimate using conductor resistance at about 75°C. It leaves out reactance, so it reads a little low on very large cables and long runs. Always check against your local electrical code and the equipment manufacturer.
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How voltage drop is calculated
Every conductor has resistance, so some voltage is lost along the cable when current flows. The longer the run, the higher the current and the smaller the conductor, the more voltage you lose before it reaches the load.
The formula
For single phase and DC circuits the current flows out and back, so the drop is 2 × current × the resistance of one leg. For a balanced three phase circuit it is √3 × current × the resistance of one conductor. Resistance is the conductor's resistivity × length ÷ cross-sectional area.
Copper or aluminium
Aluminium has about 1.6 times the resistance of copper for the same size, so an aluminium run usually needs to be one or two sizes larger to give the same voltage drop.
What limit should you use?
Many electricians aim for no more than 3% on a branch circuit and 5% for the feeder and branch circuit together. In the US these figures come from informational notes in the NEC. In the UK, Australia and New Zealand the wiring rules set their own limits, often 3% for lighting and 5% for other uses in the UK and 5% in total in Australia and New Zealand. Your local code always takes priority.
This calculator gives an estimate based on DC resistance at about 75°C. For very large cables, long feeders or motor starting, use your code's tables or a full design calculation.
Read more: Voltage drop explained: how to size cable for long runs.
Frequently asked questions
Is the length one way or there and back?
Enter the one-way length from the panel to the load. The calculator accounts for the return path for you.
Why does the result say to size up?
The voltage drop is over the limit you chose. Use the smallest size shown in the results, then check it still meets ampacity and your local code.
Does it work for EV chargers and solar?
Yes. Enter the charger or inverter current. Continuous loads like EV chargers are often sized for 125% of the load current, so check your code.
Is this an official code calculation?
No. It is a quick estimate for planning and quoting. Always confirm the final design against your local electrical code and the manufacturer's instructions.
Does anything I type get sent anywhere?
No. The calculator runs entirely in your browser.
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