Voltage Drop & Cable Size Calculator
Calculate voltage drop for copper or aluminum cable — single phase, three phase or DC — and get the recommended cross-section.
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Voltage Drop Math
Single-phase and DC circuits drop ΔU = 2·ρ·L·I ÷ S (current travels out and back); three-phase drops ΔU = √3·ρ·L·I ÷ S, with resistivity ρ ≈ 0.0175 Ω·mm²/m for copper and 0.028 for aluminum. Practice keeps the drop under 3% for most circuits (5% absolute ceiling for feeders) — beyond that, motors overheat and LEDs flicker. The tool shows the drop in volts and percent, flags the status, and suggests the smallest standard cross-section that stays under 3%.
Voltage drop is only half the story: ampacity (current-carrying capacity by installation method) and breaker coordination also bind the choice — confirm against your electrical code or an electrician.
Frequently Asked Questions
What is an acceptable voltage drop?
Common practice: up to 3% for branch circuits and 5% total from source to load. Sensitive electronics and long motor runs deserve tighter margins.
Why does three-phase drop less than single-phase for the same load?
The factor is √3 ≈ 1.73 instead of 2, and the same power flows at higher voltage with lower current — both cut the drop, which is why long runs are pulled three-phase.
Copper or aluminum?
Aluminum is ~60% more resistive, so it needs about 1.6× the cross-section for the same drop — but it is lighter and cheaper, which is why utilities love it for feeders. Terminations need aluminum-rated connectors.
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Voltage Drop & Cable Size Calculator