Voltage Drop
Work out the voltage drop over a cable run — volts lost, drop as a percentage, the voltage left at the load, pass/fail against a 3% or 5% limit, the smallest conductor that would meet it, and the power wasted as heat. Metric (mm², IEC 60228) and imperial (AWG/kcmil, NEC) sizes, fully offline.
Open Voltage Drop →What is the voltage drop calculator?
A free cable voltage-drop calculator that works entirely in your browser. Give it the system voltage, the load current, the one-way length of the run and the conductor size, and it returns the volts lost in the cable, that loss as a percentage, the voltage actually left at the load, and a clear pass or fail against your chosen limit. It also names the smallest standard conductor that would meet the limit and shows the power wasted as heat in the run. Copper and aluminium, DC, single-phase and three-phase are all supported, in metric sizes using IEC 60228 conductor resistance or imperial AWG and kcmil sizes using the NEC method. Conductor resistance is corrected to the working temperature you enter, because a hot cable drops noticeably more voltage than a cold one. Your entries stay on your device.
How to use Voltage Drop
- Choose your units — Switch between Metric (mm², metres, °C) and Imperial (AWG/kcmil, feet, °F) at the top right.
- Set the supply and material — Pick DC, single-phase or three-phase, then copper or aluminium. Three-phase voltage is line-to-line.
- Enter the run — Type the system voltage, the load current and the one-way length — not the there-and-back length. The calculator adds the return path itself.
- Pick a size and a limit — Choose the conductor size and set the drop limit, typically 3% for a final circuit or 5% overall. The verdict and the smallest acceptable size update as you type.
Frequently asked questions
Should I enter the one-way length or the total conductor length?
One-way — the distance from the supply to the load. For DC and single-phase the calculator doubles it for the return conductor, and for three-phase it applies the √3 factor instead.
What voltage drop is acceptable?
3% for a branch or final circuit and 5% overall are the usual figures — recommendations in the NEC informational notes, and the lighting and general limits in BS 7671 and IEC 60364. Always confirm against the edition in force where you are working.
Why does conductor temperature matter?
Copper resistance rises roughly 0.4% per degree Celsius. A conductor running at its 70 °C or 75 °C rating has about 20% more resistance than at 20 °C, so ignoring temperature understates the drop.
Why do metric and imperial give slightly different answers?
They use different published data — IEC 60228 stranded conductor resistance for mm² sizes, and circular mils with the NEC constant for AWG. The two agree closely but are not identical, which reflects the standards themselves.
Tips
- Long runs usually fail on voltage drop before they fail on current-carrying capacity, so check the drop before you settle on a cable size.
- The power lost as heat is money burnt in the cable — on a heavily loaded long run, the next size up often pays for itself.
- If the run is close to the limit, raise the conductor temperature to the cable's rating and check it still passes on a hot day.