Voltage drop calculator to BS 7671

The voltage drop formula

Voltage drop in volts = mV/A/m × Ib × L ÷ 1,000. Ib is the design current in amps, L is the length of the run in metres, and mV/A/m is the cable's tabulated drop: millivolts lost for every amp, over every metre. Dividing by 1,000 turns millivolts into volts. This is the method NICEIC publishes for BS 7671 Appendix 4.

Measure L along the route the cable actually takes, not in a straight line across the plan. Up the wall, along the joists, round the chimney breast and down again all count. Use the length once: the mV/A/m figure for a single-phase cable already counts both the line and the neutral.

For cables up to 16mm², NICEIC notes that inductance can be ignored and the tabulated figure used as it is. That covers every size this calculator offers.

Worked example: a 32A radial 35 metres away

A 32A radial to the far end of a long single-storey extension, in 4mm² twin and earth clipped direct, with the full 32A taken as the design current. The route from the consumer unit is 35 metres. These are the figures the calculator opens with.

• Current check first: 4mm² clipped direct is rated 37A, so it carries 32A.
• Voltage drop: 11 mV/A/m × 32A × 35m ÷ 1,000 = 12.32V.
• As a percentage: 12.32V ÷ 230V = 5.36%. The limit for a power circuit is 5%, which is 11.5V. Fails.
• Longest run 4mm² allows at 32A: 11.5V × 1,000 ÷ (11 × 32) = 32.6m. The run is more than 2m too long.
• Try 6mm²: 7.3 × 32 × 35 ÷ 1,000 = 8.18V, or 3.55%. Passes, and 6mm² would reach 49.2m.
• Answer: the cable carries the current but loses too much voltage, so the run goes up to 6mm².

The 3% and 5% limits

For an installation supplied straight from the public network, BS 7671 deems the voltage drop acceptable if it stays within 3% for lighting and 5% for other uses. NICEIC gives the source as section 6.4 of Appendix 4, under regulation 525.202. On a 230V supply that is 6.9V for lighting and 11.5V for everything else.

The drop is measured from the origin of the installation, usually the supply terminals, to the light, the socket or the fixed equipment. NICEIC also notes that a motor may be allowed a bigger drop while it starts, and so may other equipment with a high inrush current.

Lighting gets the tighter limit, so lighting runs hit it at lower currents than you would expect. A 1.0mm² lighting circuit carrying 6A over 30 metres loses 44 × 6 × 30 ÷ 1,000 = 7.92V. That is 3.44%, a fail against 6.9V. In 1.5mm², it is 29 × 6 × 30 ÷ 1,000 = 5.22V, or 2.27%, a pass.

If the circuit is fed from a distribution board in a garage or outbuilding, add the drops together: the 3% or 5% has to cover the sub-main to that board and the final circuit after it. NICEIC's own example has a distribution cable dropping 1.65% and a final circuit dropping 2.19%, 3.84% in total and inside 5%. This calculator works out one twin and earth circuit at a time, so run it for each such cable and add the percentages.

The longest run for each cable size

Turn the formula round and you get the longest run a size allows: limit in volts × 1,000 ÷ (mV/A/m × Ib). The calculator shows it for the size you pick and rounds it down to the nearest 10cm, so you never read a length that fails.

These are the longest runs at 32A against the 5% limit of 11.5V, using the mV/A/m column of Table 4D5, as reproduced by Caledonian Cables and Eland Cables. Halve the current and the length doubles. Lengths only: each size must still carry 32A for the way it is installed.

• 2.5mm² (18 mV/A/m): 19.9m. It cannot carry 32A as a radial in any case.
• 4mm² (11 mV/A/m): 32.6m.
• 6mm² (7.3 mV/A/m): 49.2m.
• 10mm² (4.4 mV/A/m): 81.6m.
• 16mm² (2.8 mV/A/m): 128.3m.

Ways to bring a run back inside the limit

Voltage drop depends on three things in the formula, so there are three levers. Go up a cable size, cut the length, or reduce the current the circuit has to carry.

Going up a size is the usual answer. Each step cuts the mV/A/m figure by a third or more: 18 for 2.5mm², 11 for 4mm², 7.3 for 6mm². Check the bigger cable still fits the accessory terminals and the breaker before you order a drum.

A shorter route helps more than people expect. Every metre you save off the run comes straight off the drop, so a route through the floor void can beat one round the skirting.

Splitting the load works too. Two radials each carrying half the current lose half the voltage over the same distance. That is a design decision for whoever certifies the work, not something to improvise on site.

What the answer leaves out

It is a guide only. The mV/A/m figures assume the conductor is at its full 70°C operating temperature, so the result errs on the safe side and can point to a bigger cable than a detailed calculation would. NICEIC notes that Appendix 4 section 6 has a fuller method that corrects for conductor temperature and power factor. This calculator does not use it.

No allowance is made for cables bunched together, a hot room or the type of breaker or fuse, and earth fault loop impedance and disconnection times are not checked either. It covers flat twin and earth (6242Y) from 1.0 to 16mm² only. To pick the size from the load in the first place, use the cable size calculator.

Only a competent person working to BS 7671 should design, test and sign off the circuit. A new circuit in England is notifiable under Approved Document P, which means an electrician in a competent person scheme, or building control, has to certify it.

Common questions