Voltage Drop Calculator Calculator
Calculate the voltage lost in a cable run from its gauge, length and current — and check it against the usual 3 % limit.
How to use it
- Enter the gauge number — use 0 for 1/0, −1 for 2/0 and −2 for 3/0.
- Read the diameter, cross-section and resistance from the result tiles.
- For a cable run, use the voltage-drop calculator and keep the result under 3 %.
This calculator works out voltage drop calculator from awg number, one-way length and current. It shows the substituted formula underneath every result, so you can check the arithmetic or copy the working straight into a report.
The formula
Vdrop = 2 × ρ × L ÷ A × I · three phase: √3 instead of 2
Why cable sizing matters
A cable that is too thin does two things, and both are bad. It drops voltage, so the equipment at the far end sees less than it should — lights dim, motors run hot and controls behave oddly. And it dissipates that lost power as heat inside the insulation.
The usual guideline is to keep voltage drop under 3% on a branch circuit and 5% from the supply to the furthest load. When the calculated figure creeps above that, the answer is a thicker conductor rather than a longer extension lead.
Everything runs in your browser
There is no server involved in the calculation. The numbers you type never leave your device, nothing is logged, and the page keeps working with no connection once it has loaded. That is not a marketing line — it is simply how the tool is built, because a calculation like this has no reason to touch a network.
Questions people ask
Where does the AWG formula come from?
American Wire Gauge is defined geometrically: gauge 36 is 0.005 in and gauge 0000 is 0.46 in, with 39 equal ratio steps between them. That gives d(mm) = 0.127 × 92^((36 − n) ÷ 39).
How much voltage drop is acceptable?
The usual guideline is 3 % on a branch circuit and 5 % from the supply to the furthest load. Above that, lights dim visibly and motors run hot, so step up to a thicker conductor.
Does temperature change the answer?
Yes. Copper resistance rises about 0.4 % per °C, so a hot cable in a conduit drops more voltage than the 20 °C figures used here. Allow some margin for a hot installation.
Last updated 17 August 2026