FixMeasure
Basics
Voltage drop: why long cables need thicker wire
Lights that dim when a motor starts at the end of a long cable, or a tool that struggles in a garden workshop, are often caused by voltage drop. This guide explains what it is and how it is limited.
1. Safety first
This guide is for understanding, not for carrying out electrical work. New circuits and outdoor or buried cables are usually reserved for licensed electricians, and the rules differ by country. See Electrical wire sizes and types explained for wire sizes and types.
2. What voltage drop is
Every conductor has resistance. When current flows, some voltage is lost along the cable as heat, so the equipment at the end receives less than the supply voltage. The longer and thinner the cable and the higher the current, the larger the drop.
3. How to calculate it
For a single-phase circuit:
Voltage drop ≈ 2 × length × current × resistance per meter
The factor 2 accounts for the outgoing and return conductors. Copper has a resistivity of about 0.0175 Ω·mm²/m, so a 2.5 mm² copper conductor has about 0.007 Ω per meter. A 30 m run carrying 16 A loses about 2 × 30 × 16 × 0.007 ≈ 6.7 V, or about 2.9% of 230 V. The Voltage drop calculator does this for any size.
4. Typical limits
A common design target is a drop of no more than about 3% for final circuits and about 5% in total from the supply. National codes set the exact values; some are stricter for lighting. On 120 V systems the same power needs twice the current, so voltage drop matters even more.
5. How to reduce it
Choose the next larger wire size, shorten the route, or reduce the load. For a long run to an outbuilding, a larger feeder cable with a small distribution board at the far end is common. Remember that the cable must also be protected by a correctly rated breaker.
Common mistakes
- Choosing wire size by current only and ignoring length.
- Forgetting the factor of two for the return conductor.
- Using extension leads as a permanent supply to an outbuilding.
- Ignoring that motors draw higher current at start-up.
- Doing fixed wiring without the required qualifications.
Frequently asked questions
What is an acceptable voltage drop?
Commonly up to about 3% for final circuits and 5% in total, depending on national rules.
Does voltage drop waste energy?
Yes. The lost voltage becomes heat in the cable.
Why does a long extension lead get warm?
Its resistance turns some of the current into heat, the same effect that causes voltage drop.
Is voltage drop worse on 120 V?
For the same power, yes, because the current is higher.
How do I fix excessive voltage drop?
Use a thicker cable, shorten the run or reduce the load; ask an electrician.