Free online tool
Voltage Drop Calculator
Already have a cable size in mind? Check how much voltage it will drop over your run, and whether that stays within a safe margin.
Voltage drop
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Fill in the form to see your result.
Engineering estimate for planning purposes only. Confirm against your local electrical code before installation.
How voltage drop is calculated
Voltage drop happens because every real conductor has resistance: current flowing through it "spends" some of the supply voltage along the way. The longer and thinner the cable, and the higher the current, the more voltage is lost before it reaches the load. This calculator uses the standard formula Vdrop = (2 or √3) × ρ × L × I ÷ A, where ρ is the conductor's resistivity, L is the one-way run length, I is the load current, and A is the cross-sectional area. The factor is 2 for DC and single-phase AC (current travels out and back), or √3 for a balanced three-phase circuit.
If your result comes back over your limit, the fix is almost always to increase the cable size. Go back to the Cable Size Calculator to find the next standard size up.
Frequently asked questions
What voltage drop percentage is acceptable?
A common guideline is up to 3% on a branch circuit and 5% total from the source to the load. Some equipment is designed to a tighter limit, often 1-2%: long solar wiring, motor starting, and precision electronics all tend to be less forgiving.
Does voltage drop matter if the cable already passes the ampacity check?
Yes. Ampacity and voltage drop are two independent checks. A cable can be well within its safe current rating and still lose an unacceptable amount of voltage over a long run.
Does aluminium have more voltage drop than copper?
Yes. Aluminium has roughly 1.6 times the resistance of copper for the same cross-section, so it needs a larger size than copper for the same voltage drop over the same run.