Voltage Drop Calculator
Every metre of cable eats a little voltage. Enter your wire size, current, and run length, and see exactly how many volts arrive at the far end — and whether the drop stays inside the 3% guideline.
Voltage drop
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Estimates for planning only. Always verify against your local electrical code and consult a licensed electrician.
How this voltage drop calculator works
Drop (V) = k × ρ × length × amps ÷ cross-section, with k = 2 for DC/single-phase (the current travels out and back) or √3 for balanced three-phase, and ρ the conductor resistivity (copper 0.0175, aluminum 0.0282 Ω·mm²/m). The percentage is that drop over the system voltage, and the power chip (I² × R of the run) shows the watts your cable turns into heat.
The verdict line uses the common guideline: under 3% is good, 3–5% is marginal, over 5% needs thicker wire, a shorter run, or a higher system voltage.
Voltage drop FAQ
How do I calculate voltage drop?
Drop = 2 × resistivity × one-way length × current ÷ cross-section (√3 instead of 2 for three-phase). 20 A through 15 m of 2.5 mm² copper drops 4.2 V — 3.5% on a 120 V circuit.
What percentage voltage drop is acceptable?
3% for branch circuits, 5% total with the feeder. Long 12/24 V solar and battery runs should target 2% since each lost volt is a bigger share of the system voltage.
What happens if voltage drop is too high?
Lights dim, motors run hot and struggle to start, chargers slow, and the lost power heats your walls. Chronic undervoltage shortens the life of pumps and compressors.
How do I reduce voltage drop?
Thicker wire, shorter runs, or higher voltage — going from 12 V to 48 V cuts current and percentage drop by 4× for the same power. Over long distances, raising voltage beats thickening cable.