Voltage Drop Calculator
Works out the voltage lost along a cable run from its length, conductor area and current. Long runs at low voltage are where this bites — a 12 V circuit has no headroom to give away.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
single-phase & DC: Vdrop = 2 × I × ρL/A · three-phase: Vdrop = √3 × I × ρL/A
Current flows out and back, so a single-phase or DC circuit sees twice the one-way length. A balanced three-phase circuit uses √3 instead, because the return currents partly cancel.
What to watch out for
What the numbers mean in practice:
- Length counts twice. Current goes out and comes back, so a 30 m run is 60 m of conductor. This is the most common error in a hand calculation.
- Low voltage suffers most. A 3 V drop is 1.3% of 230 V but 25% of 12 V. Solar, automotive and LED lighting runs are where voltage drop actually fails.
- Aluminium has about 1.6× the resistivity of copper, so the same drop needs a larger conductor.
- The lost power becomes heat in the cable. That is the I²R term, and it is why an undersized cable is a fire risk rather than just a performance problem.
This page does not size cable. It reports the drop. Choosing a conductor also depends on ampacity, installation method, grouping, ambient temperature and derating — all set by NEC, BS 7671 or your local code, which revise on a cycle. Use this figure alongside those tables, not instead of them.
Frequently Asked Questions
What is an acceptable voltage drop?
The usual limits are 3% for a final circuit and 5% overall from the origin. BS 7671 allows 3% for lighting and 5% for other uses; NEC recommends 3% on a branch circuit and 5% total. Local code governs.
Why is the run length doubled?
Because current has to return. A single-phase or DC circuit uses two conductors, so a 30 m run means 60 m of copper. Three-phase is the exception and uses √3.
Why does voltage drop matter more on 12 V?
Because the percentage is what counts. Losing 1 V on 230 V is 0.4%; losing 1 V on 12 V is 8.3%. Low-voltage runs need much larger conductors than people expect.
Does this tell me what cable to buy?
No — it tells you the drop for a cable you specify. Sizing also depends on current-carrying capacity and derating factors from your wiring code, which this deliberately does not reproduce.