What is voltage drop in electrical wiring?

Voltage drop is the reduction in voltage along a cable caused by the resistance of the conductor. A long or thin cable carrying a high current will lose more voltage than a short or thick one. If the drop is too large, appliances at the end of the run receive less than the designed supply voltage, which can cause lamps to dim and motors to run hot.

What are the BS 7671 voltage drop limits?

BS 7671 Regulation 525 sets the limit at 3% of the nominal supply voltage for lighting circuits (6.9 V on a 230 V supply) and 5% for power and other circuits (11.5 V). These limits apply from the origin of the installation — typically the main switch in the consumer unit — to the furthest point on each circuit.

How do I find the mV/A/m value for my cable?

The mV/A/m figure is the voltage drop per ampere per metre of cable run. You can find it in Appendix 4 of BS 7671 (the 18th Edition Wiring Regulations) or in the On Site Guide tables. For a 2.5 mm² twin and earth flat cable clipped direct, the figure is around 18 mV/A/m. For 1.5 mm² it is around 29 mV/A/m.

Does voltage drop matter on short cable runs?

On runs under about 15 metres at normal domestic loads, voltage drop rarely causes a problem with standard cable sizes. It becomes significant on long runs to garden buildings, workshop supplies or upstairs lighting circuits fed from a distant consumer unit.

Understand Voltage Drop in UK Domestic Wiring

Helpful video reference. GSH Electrical's video "Voltage Drop: What Our Socket Test Revealed!" shows a real-world measurement on a UK domestic socket circuit. Gary walks through exactly what the readings mean against BS 7671 limits, which gives a good sense of the numbers before you attempt a calculation on paper.

Before you start. Voltage drop is a design and verification calculation, not a live test. You do not need to touch any conductors to work it out. All you need is the circuit schedule or a tape measure for the cable length, the load in watts or amps, and the BS 7671 Appendix 4 tables. If you are verifying an existing circuit that you suspect has a problem, use a calibrated multifunction tester — never improvise.

1. Find the design current (Ib)

The design current is the maximum current the circuit will carry at full load. For a lighting circuit, add up the lamp wattages and divide by 230: a circuit serving six 10 W LED fittings has an Ib of 60 / 230 = 0.26 A. For a socket circuit, use the actual connected load if you know it, or apply the diversity factors in BS 7671 Chapter 31 for a realistic worst-case figure.

Do not confuse design current with MCB rating. A 16 A MCB protects a circuit whose design current may be only 4 A. The MCB rating is the upper safety limit; Ib is what actually flows under normal use.

2. Measure or confirm the cable route length

Voltage drop calculation needs the one-way cable length — the distance from the consumer unit to the furthest point on the circuit, following the actual cable route through walls, floors and ceiling voids, not the straight-line distance. Add 10% to a measured route length to allow for tails and terminations inside accessory back boxes.

For a new circuit, design to the longest possible route. For an existing circuit, the circuit schedule or original drawings should give you this figure; failing that, a tape measure along the accessible run is the only option.

3. Look up the mV/A/m value

Open BS 7671 Appendix 4 (or the On Site Guide tables at the back) and find the row for your cable size and installation method. The mV/A/m column gives the voltage drop per ampere of current per metre of cable. Common values for twin and earth flat cable clipped direct or in conduit:

1.0 mm² — approximately 44 mV/A/m
1.5 mm² — approximately 29 mV/A/m
2.5 mm² — approximately 18 mV/A/m
4.0 mm² — approximately 11 mV/A/m
6.0 mm² — approximately 7.3 mV/A/m
10 mm² — approximately 4.4 mV/A/m

These figures assume resistive load at operating temperature. For inductive loads, use the impedance (mV/A/m) column rather than the resistance column — the difference is small at domestic frequencies but the standard requires it.

4. Calculate the voltage drop

The formula is straightforward:

Vd (volts) = (mV/A/m × Ib × L) ÷ 1000

Where Ib is the design current in amps and L is the one-way cable length in metres.

Worked example: a 2.5 mm² cable (18 mV/A/m) running 20 metres to a socket outlet carrying 13 A:

Vd = (18 × 13 × 20) ÷ 1000 = 4680 ÷ 1000 = 4.68 V

The limit for a power circuit is 5% of 230 V = 11.5 V. 4.68 V is well within that, so the cable size is acceptable.

5. Compare against BS 7671 Regulation 525 limits

The calculated drop must stay within:

Lighting circuits: 3% of 230 V = 6.9 V maximum
Power and other circuits: 5% of 230 V = 11.5 V maximum

These limits apply from the origin of the installation to the furthest point. If the consumer unit is already some distance from the incoming supply, that additional drop should be considered, although in practice most UK domestic supplies are metered at or very close to the consumer unit.

6. Upsize the cable if the drop is too high

If the calculated drop exceeds the limit, move to the next cable size up and repeat. Going from 2.5 mm² to 4 mm² reduces the mV/A/m from 18 to 11 — a 39% reduction in drop for the same run. That is usually enough to bring a borderline circuit into compliance without going any further.

In some installations — a very long armoured cable to a garden outbuilding being the most common domestic example — you may need to go up two sizes. Run the numbers at each step rather than guessing.

Stop and call an electrician if: your calculation shows the existing cable is outside the BS 7671 limits, lights at the end of a circuit are noticeably dimmer than those closer to the consumer unit, sockets on a long radial feel warm in use, or any circuit schedule on the original electrical installation certificate does not match what you find when you measure the actual run.

When to call us

Voltage drop matters most during the design stage of a new circuit or when fault-finding a circuit that is not performing as expected. Richard carries out circuit design calculations as part of every new installation and EICR follow-up. If you have a long cable run to a garden room, outbuilding or loft conversion and you are not sure the existing cable size is adequate, a site visit and calculation takes less than half an hour.

Need a circuit designed or checked in Sandwich?

Richard carries out voltage drop calculations as standard on every new circuit and checks them during EICR inspections. Based in Sandwich CT13.

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Understand voltage drop in UK domestic wiring