Volt drop
Voltage drop calculator
Work out the volt drop on a run from the cable size, design current and length, and check it against the BS 7671 limits of 3% for lighting and 5% for other circuits.
Single phase 230 V, 70°C thermoplastic copper, mV/A/m from BS 7671 Table 4D2B. Guide only.
Why volt drop decides long runs
Two cables can both carry a circuit's current, and only one of them be acceptable, because a longer run loses more voltage along the conductor before it reaches the load. BS 7671 keeps that loss within 3% of the supply voltage for lighting (6.9 V at 230 V) and 5% for everything else (11.5 V). Lighting gets the tighter figure because lamps dim and flicker as the voltage drops.
The numbers behind the result
Each cable size has a millivolts-per-amp-per-metre value (mV/A/m) from the BS 7671 tables. A 2.5 mm² twin and earth is 18, a 1.5 mm² is 29. The drop is that figure times the design current times the route length, divided by 1000. The table values assume the conductor is at its 70°C operating temperature, so a lightly loaded cool circuit performs slightly better than the figure here, never worse.
When a run fails
If a run is over the limit you have three options: go up a size to lower the mV/A/m, shorten or re-route the cable, or on bigger jobs move the board closer to the load. Going up a size also helps the current capacity and the loop impedance, so it is often the simplest fix on a borderline circuit.
FAQs
What is the maximum permitted voltage drop?
BS 7671 recommends 3% of supply voltage for lighting (6.9 V at 230 V) and 5% for other uses (11.5 V), measured from the origin of the installation to the load.
How do you calculate voltage drop?
Volt drop in volts is the cable's mV/A/m figure times the design current times the cable length, divided by 1000. Express it as a percentage of 230 V to compare with the limit.
Why does a longer cable need a bigger size?
Volt drop builds up with length, so a long run can breach the limit even when the cable easily carries the current. A larger conductor has a lower mV/A/m figure, which brings the drop back within limits.