Voltage Drop Calculator

Every real conductor has some resistance, even though a wire is often treated as an "ideal" zero-resistance connection when sketching a circuit. That small resistance, spread over the length of a cable, consumes a bit of the supply voltage before it ever reaches the load — the longer or thinner the cable, the more of the original voltage is lost to it rather than delivered to whatever is plugged in at the far end.

This matters in practice because a motor, light or appliance is designed to run at a specific voltage. If too much of the supply is lost along the way, the device at the end of the run receives less than it needs — motors run hot and lose torque, older lighting dims, and sensitive electronics can behave unpredictably or fail to start at all.

Cable resistance depends on three things: the material’s resistivity, how long the run is, and how thick the conductor is. Resistivity is a property of the metal itself — copper is a noticeably better conductor than aluminium, so a copper cable of a given size drops less voltage than an aluminium cable of the same size carrying the same current over the same distance. Aluminium is lighter and cheaper, which is why it remains common on long overhead and underground distribution runs despite needing a larger cross-section to match copper’s performance.

Length matters because resistance accumulates continuously along the conductor — twice the distance is, all else equal, twice the resistance and twice the voltage lost. Cross-sectional area works the other way: a thicker conductor gives current more room to flow, so resistance — and voltage drop — falls as area increases, roughly in inverse proportion.

Current does not just travel to a load — it also has to return to complete the circuit. In a single-phase or DC circuit, current goes out along one conductor and back along another, so the resistance it encounters is effectively double the one-way cable length. Three-phase circuits are arranged so the three phase currents partially cancel each other at the return path, which is why the standard voltage-drop formula for three-phase uses a factor of √3 — about 1.73 — rather than a flat 2.

This is also one reason three-phase power is often preferred for larger loads over long distances: for the same power delivered, three-phase wiring generally experiences less voltage drop and needs less conductor material than an equivalent single-phase run.

Electrical codes in most countries do not typically mandate a hard voltage-drop limit as a safety rule the way they do for overcurrent protection, but both major wiring standards — the US National Electrical Code and the International Electrotechnical Commission’s guidance — converge on the same widely used recommendation: no more than roughly 3% drop on a final branch circuit, and no more than about 5% total from the service origin to the load. These figures are a design guideline for acceptable equipment performance, not an absolute physical limit.

Staying inside them is less about safety in the fire-risk sense and more about making sure connected equipment actually gets the voltage it was designed for. A cable sized only for current-carrying capacity can still satisfy a code’s ampacity rules while producing a voltage drop well outside these guidelines on a long run — exactly the kind of gap a voltage-drop check like this one is meant to catch.