Voltage drop calculator

What is voltage drop?

Every metre of cable has resistance, so some of your supply voltage is lost between the board and the load. A little is unavoidable. Too much and lights dim and flicker, motors run hot and die young, LED drivers reset, and the installation falls outside BS 7671. That's why every circuit design includes a voltage drop calculation, and why long runs so often end up in bigger cable than the current alone would need.

The voltage drop formula

The whole check is one line:

Voltage drop (V) = mV/A/m × design current Ib (A) × length (m) ÷ 1000

Three inputs: the cable's tabulated millivolt drop per amp per metre, the current the circuit actually carries, and the one-way route length. Multiply, divide by 1000, done. The mV/A/m value comes from the tables in Appendix 4 of BS 7671 and depends on the cable size. These are the single-phase values this calculator uses for copper 70°C thermoplastic cable:

BS 7671 voltage drop limits (3% and 5%)

For an installation fed straight from the public supply, Appendix 4 recommends a maximum of 3% for lighting and 5% for everything else. On a 230 V single-phase supply that's 6.9 V and 11.5 V. On a 400 V three-phase supply it's 12 V and 20 V. Private supplies (a generator or your own transformer) get a little more headroom, 6% and 8%.

How to calculate voltage drop, step by step

1. Get the design current Ib. The current the circuit actually carries, not the breaker rating. Load in kW? Convert it with the kW to amps calculator.
2. Measure the route, one way. Board to the furthest point, following the actual cable path, not the crow's flight.
3. Look up the mV/A/m value for your cable size in the table above (or Appendix 4 of BS 7671).
4. Multiply and divide. mV/A/m × Ib × length, then divide by 1000 for volts.
5. Compare against your limit. 6.9 V lighting, 11.5 V power on a 230 V supply. Or skip all five steps and type it into the calculator above.

Worked examples

Radial circuit: a 20 A radial in 2.5 mm² T&E, 18 m from the board to the last socket: 18 × 20 × 18 ÷ 1000 = 6.48 V. The power limit is 11.5 V, so it passes with room to spare. Stretch that same circuit to 35 m and you're at 12.6 V, a fail, and the reason garden office runs so often end up in 4 mm² or 6 mm².

Ring final circuit: rings get a shortcut. The current splits both ways round the ring and the worst point is the midpoint, so you divide the normal answer by 4: VD = mV/A/m × Ib × total ring length ÷ 4000. A 2.5 mm² ring with 68 m of cable carrying 26 A: 18 × 26 × 68 ÷ 4000 = 7.96 V. Pass.

Three-phase sub-main: a 60 A sub-main in 16 mm² over 40 m, using the three-phase value: 2.4 × 60 × 40 ÷ 1000 = 5.76 V against a 20 V limit. Loads of headroom, which is exactly why long outbuilding feeds love a three-phase supply.

Does temperature change the answer?

Slightly, in your favour. The tabulated mV/A/m values assume the conductor is sat at its full 70°C operating temperature. A lightly loaded cable runs cooler, has less resistance, and drops a touch less than the calculation says. There is a formal correction factor for design work, but on site the tabulated value is the safe, conservative call, if it passes on paper, it passes in copper.

• Measure the run one way, board to the furthest point. The tables already account for the return path.
• Use the design current Ib, not the breaker rating. A 32 A ring loaded to 26 A drops less than the breaker suggests.
• Voltage drop is usually what forces long runs up a size, not current-carrying capacity. Check both, the cable size calculator does them together.