Voltage Drop Limits: What the 3% and 5% Figures Mean

The figures usually quoted are 3% for lighting and 5% for other uses. In the UK they come from BS 7671, Appendix 4, for an installation supplied from the public low-voltage network. They limit the voltage drop in the cable as a percentage of the supply voltage. They are not the same everywhere, and the limit that applies to a real job is the one in the project specification and the local wiring rules.

What each source says

• UK, BS 7671 (Appendix 4). 3% for lighting and 5% for other uses on an installation supplied from the public network. Higher figures of 6% and 8% are given for an installation with a private supply.
• IEC 60364-5-52. It gives informative guidance on voltage drop. The limit depends on the supply and on the country, so no single IEC figure is quoted here.
• US, NEC. The informational notes to sections 210.19 and 215.2 suggest 3% on a branch circuit and 5% for the feeder and branch circuit together. They are recommendations, not requirements.

Because the figures differ by supply, country and circuit type, the calculator offers 3% and 5% and lets you set your own limit.

How the percentage is worked out

The voltage drop in volts is divided by the supply voltage and multiplied by 100. For a single-phase 230 V circuit that is Vd ÷ 230 × 100. For three-phase, the drop between two lines is compared with the line-to-line voltage, such as 400 V. The guide on three-phase voltage drop shows the working.

Worked example: a 230 V circuit against 3% and 5%

A single-phase 230 V circuit carries 20 A over 30 m of 2.5 mm² copper at 70 °C, a power factor of 1. The cable's resistance at 70 °C is 7.41 × 1.1965 = 8.87 Ω/km, so the drop is 2 × 20 × 0.030 × 8.87 = 10.64 V.

So the same cable passes a 5% limit and fails a 3% one, and the smallest size in the table that meets 3% is 4 mm². The size that meets the voltage drop is only one requirement: the cable must also carry the current safely, and the voltage drop calculator says so beside its answer.

Why a limit exists

Equipment is designed to work from a supply within a range of voltage, so the voltage lost in the cable has to leave enough at the load. A motor also needs enough voltage at its terminals while it starts, which this calculator does not check.

Common mistakes

• Mixing up circuit parts. A 3% figure may apply to one circuit, and a 5% figure to the whole run from the origin. The two drops add up, so a limit on the total is used up by every part of the run.
• Using the wrong reference voltage. Divide a three-phase line-to-line drop by the line-to-line voltage.
• Treating a recommendation as a rule. The NEC notes are informational. What binds a project is its specification and the wiring rules where it is built.

Checking a cable

Enter the circuit in the voltage drop calculator, pick 3%, 5% or your own limit, and it shows whether the drop is within it and the smallest size that is. The results are estimates: check the limit that applies to your job with the person responsible for the design.