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Calculate BS 7671 volt drop for any UK circuit — get voltage drop in volts and percentage, check compliance with the 5% limit, and find the maximum compliant cable run length.
Design current of the circuit (Ib). For socket circuits use the circuit protective device rating.
One-way cable run from supply point to load. Do not double it — the mV/A·m method accounts for both conductors.
WARNING 3–5%
Within 5% limit but above 3% recommended for lighting
Volt Drop
7.2 V
at 230V nominal
Volt Drop %
3.13%
BS 7671 limit: 5%
Next step
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This calculator uses the mV/A·m (millivolt per amp per metre) method from BS 7671 Table 4Ab — the standard approach used by UK electricians and accepted by all major certification schemes including NICEIC and NAPIT.
BS 7671 Table 4Ab gives the voltage drop in millivolts per amp of current per metre of cable run for each copper conductor size. For 70°C thermoplastic cable, values range from 44 mV/A·m for 1mm² down to 1.75 mV/A·m for 25mm². For 90°C XLPE insulation, multiply by 1.08 to account for the higher resistance at the elevated conductor operating temperature.
For single-phase circuits, the Table 4Ab mV/A·m values already account for both the outgoing live conductor and the return neutral. For three-phase circuits, the geometric relationship between phases means the effective volt drop is reduced by √3/2 (≈ 0.866) — so multiply the mV/A·m value by 0.866 before applying it to the formula.
Vd (V) = (mV/A·m × I × L) ÷ 1,000, where I is the design current in amps and L is the one-way route length in metres. The ÷ 1,000 converts millivolts to volts. Route length is one-way — do not double it, as the Table 4Ab values already model the complete circuit across both conductors.
Volt drop percentage = (Vd ÷ nominal voltage) × 100. Nominal voltage is 230V for single-phase and 400V (line-to-line) for three-phase. BS 7671 Appendix 12 sets the maximum at 5% for final circuits (11.5V at 230V; 20V at 400V) with a recommended 3% limit for lighting (6.9V at 230V). The maximum compliant run at 5% is: L_max = (0.05 × Vnom × 1,000) ÷ (mV/A·m × I).
WORKED EXAMPLE
A 20A socket circuit feeds a workshop 35m from the consumer unit. Cable is 2.5mm² 70°C thermoplastic, single-phase. The electrician needs to confirm whether 2.5mm² meets the BS 7671 5% volt drop limit or whether 4mm² is required.
mV/A·m — 2.5mm² 70°C
18 mV/A·m
Vd = 18 × 20 × 35 ÷ 1,000
12.6 V
Vd% = 12.6 ÷ 230 × 100
5.48%
BS 7671 5% limit
11.5 V
Result
FAIL — size up to 4mm²
5.48% exceeds the 5% limit · 4mm² gives 3.35% — well within BS 7671 ✓
With 4mm² cable: mV/A·m = 11, Vd = (11 × 20 × 35) ÷ 1,000 = 7.7V = 3.35% — compliant. Current-carrying capacity of 4mm² clipped direct is 32A, comfortably above the 20A design current.
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Common questions about volt drop and BS 7671 cable compliance.
Volt drop is the reduction in voltage that occurs along a cable due to its resistance as current flows through it. Ohm's Law means that the longer or thinner the cable, the greater the resistance and therefore the greater the voltage loss between the source and the load. BS 7671 (the 18th Edition Wiring Regulations) sets a maximum volt drop of 5% for final circuits and recommends no more than 3% for lighting circuits, to ensure that equipment operates within its rated voltage range and does not suffer reduced performance, overheating, or damage.
BS 7671 Appendix 12 specifies that the total voltage drop from the origin of an installation to any socket outlet or fixed current-using equipment must not exceed 5% of the nominal voltage — that is, 11.5V on a 230V single-phase circuit or 20V on a 400V three-phase circuit. For lighting circuits, a more stringent 3% (6.9V) limit applies. These limits are cumulative across the whole installation, so if distribution cables already account for some volt drop, the remaining budget for the final circuit is reduced accordingly.
The simplest fix is to increase the cable cross-sectional area (CSA). A larger conductor has lower resistance per metre, so the voltage drop for the same current and run length is reduced. Alternatively, you can reduce the circuit length by moving the distribution board or adding a local sub-board closer to the load. For three-phase systems, balancing loads across phases also reduces current per phase and therefore volt drop. Reducing the connected load is another option but often impractical.
Cable sizing has two separate constraints: current-carrying capacity (the cable must not overheat) and volt drop (the voltage loss must not exceed the BS 7671 limit). A cable that is large enough to carry the current safely may still fail the volt drop check on long runs — and a cable sized purely for volt drop compliance may be larger than needed for current rating. You must satisfy both criteria simultaneously. The cable sizing calculator on this site handles both checks together.
Cable insulation type affects the maximum operating temperature of the conductor, which in turn affects its resistance. XLPE (cross-linked polyethylene) cables are rated to 90°C, meaning the conductor resistance at operating temperature is approximately 8% higher than a 70°C thermoplastic cable of the same size. This means volt drop is marginally worse for XLPE cables at full load — approximately 1.08× the 70°C values. However, XLPE cables have a higher current-carrying capacity at the same CSA, which often allows a smaller cable to be selected when current rating is the limiting factor.
For single-phase circuits, the current flows out on the live conductor and returns on the neutral, so the mV/A·m value already assumes a two-conductor circuit. For three-phase systems, the geometric relationship between the three phases means the effective volt drop is reduced by a factor of √3/2 (approximately 0.866) compared to single-phase for the same conductor size and current. The BS 7671 Table 4Ab values for three-phase are pre-adjusted for this factor.
For a 20A ring final circuit (standard domestic socket circuit), 2.5mm² is adequate both for current capacity and volt drop on typical house runs of up to about 30–35 metres (one-way). For runs longer than this, or for 32A radial circuits, 4mm² becomes necessary to stay within the 5% volt drop limit. 4mm² is also common on cooker circuits, dedicated EV charger feeds, and shower circuits where the run from consumer unit to outlet exceeds 15–20 metres. Always check both current capacity (derated for installation method) and volt drop.
Route length is the one-way cable run from the supply point (consumer unit, distribution board, or junction box) to the furthest load on the circuit. Do not double it — the mV/A·m method already accounts for the return conductor. Measure the actual cable route, not the straight-line distance: include vertical drops to floor level, runs along ceiling voids, and any diversions around obstacles. For a ring circuit, measure the longer of the two routes from the consumer unit to the furthest point on the ring — this is the worst-case volt drop scenario.