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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How It's Calculated
Cable cross-section is never chosen from a single number — it must satisfy two independent criteria, and the tighter one wins. The first is ampacity: how much current a conductor can carry continuously without its insulation overheating, which depends on the conductor material, insulation type, ambient temperature and installation method (conduit, free air, buried, bundled with other cables). The second is voltage drop: the resistive loss along the length of the run, which only becomes significant on long circuits. This calculator focuses on the voltage-drop side of the decision.
For single-phase and DC circuits, current travels out through one conductor and back through another, so the loss doubles: ΔU = 2·ρ·L·I ÷ S, where ρ is resistivity (≈0.0175 Ω·mm²/m for copper, 0.028 for aluminum), L is one-way length in meters, I is current in amps and S is cross-section in mm². Three-phase circuits share the load across three conductors 120° apart, so the geometry works out to ΔU = √3·ρ·L·I ÷ S instead of double.
Worked example: a single-phase 230V circuit carrying 16A over a 25m one-way run through 2.5mm² copper: ΔU = 2 × 0.0175 × 25 × 16 ÷ 2.5 = 5.6V, or 2.4% of 230V — comfortably under the usual 3% ceiling. Stretch that same run to 60m and the drop climbs past 13V, nearly 6%, at which point the next standard size (4mm² or 6mm²) is needed to bring it back under 3%.
What You Should Know
An undersized cross-section is a fire and safety risk, not just an efficiency one. When a conductor is too thin for the current it carries, it heats up under normal load — insulation can degrade, soften or, in extreme or faulted conditions, ignite. That is why ampacity, not voltage drop, is usually the hard legal minimum in electrical codes (IEC 60364 in most of Europe, NEC/NFPA 70 in North America): a cable must first be thick enough to carry the current safely, and only then does voltage drop get checked for longer runs.
- Under 3% drop: generally fine for lighting and general branch circuits.
- 3-5% drop: acceptable for the total run from source to load in many codes, but motors run hotter and less efficiently, and LED drivers can flicker or shorten their lifespan.
- Over 5%: commonly flagged as non-compliant — step up to the next standard cross-section or shorten the run.
This tool gives an approximate, educational figure for planning purposes. Real installations must also account for grouping/derating factors (several cables bundled together carry less current each safely), ambient temperature corrections, breaker trip curves, and short-circuit withstand — all of which sit outside a simple voltage-drop formula. Always confirm the final cross-section against your local electrical code or with a licensed electrician before wiring anything permanent.
Frequently Asked Questions
What counts as an acceptable voltage drop?
Common practice keeps branch circuits under 3% and the total run from source to final load under 5%. Those are not hard physics limits — they are conservative margins codes and engineers converge on because beyond them, motors run hot and lose torque, lighting dims or flickers, and electronic equipment can misbehave on the low voltage. Sensitive electronics and long motor feeders often deserve tighter margins than the code minimum.
Why does three-phase drop less than single-phase for the same load?
The formula's multiplier is √3 (≈1.73) for three-phase instead of 2 for single-phase, because the return current is shared and phase-shifted across three conductors instead of doubling back through a single path. On top of that, the same power is typically delivered at a higher voltage with proportionally lower current in three-phase systems, which independently shrinks the I in the drop formula — the two effects compound, which is exactly why long industrial and commercial feeders are almost always run three-phase.
Should I use copper or aluminum?
Aluminum's resistivity is roughly 60% higher than copper's, so for the same voltage drop it needs about 1.6 times the cross-sectional area. It is significantly lighter and cheaper per amp of capacity, which is why utilities favor it for long overhead feeders and service entrances. The trade-off is at the terminations: aluminum conductors need connectors and lugs specifically rated for aluminum, because aluminum oxidizes and cold-flows differently than copper — using copper-only hardware is a known cause of loose, overheating joints.
My voltage drop calculation looks fine — why did an electrician still specify a thicker cable?
Voltage drop is only one of the two constraints. The cable also has to satisfy ampacity for its actual installation conditions — several circuits bundled in one conduit, high ambient temperature, or a long buried run all reduce how much current a given cross-section can carry safely, sometimes forcing a jump to the next size even when the drop percentage alone would have allowed something thinner. Breaker coordination and short-circuit withstand can add further constraints beyond this tool's scope.
Does "length" in this calculator mean one-way distance or the full circuit loop?
Enter the one-way distance from the source to the load — the calculator already accounts for the return path internally through the ×2 (single-phase/DC) or √3 (three-phase) factor in the formula. Doubling the length yourself would overstate the drop by roughly double.
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