Voltage drop is one of the five checks in the cable sizing process. BS 7671 Regulation 525.1 sets the maximum permitted voltage drop at 3% for lighting circuits and 5% for other circuits. This guide covers the limits, the formula, worked examples, and when voltage drop matters most.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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Voltage drop is the reduction in voltage that occurs as electrical current flows through a cable. All cables have resistance, and that resistance causes a voltage loss between the supply end (the distribution board) and the load end (the socket, light, or appliance). The longer the cable, the higher the current, and the smaller the conductor — the greater the voltage drop.
Excessive voltage drop can cause problems. Lighting circuits may produce dim or flickering lights. Motor loads may fail to start or run inefficiently. Electronic equipment may malfunction or shut down. In extreme cases, equipment can be damaged. BS 7671 sets maximum voltage drop limits to ensure that the voltage at the load is high enough for equipment to operate correctly and safely.
Voltage drop is part of the cable sizing process. After selecting a cable based on its current-carrying capacity and correction factors, you must verify that the voltage drop across the cable length does not exceed the BS 7671 limits. If it does, you need to increase the cable size until the voltage drop is within limits.
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BS 7671 Regulation 525.1 sets the maximum permitted voltage drop from the origin of the installation to the load. The limits differ depending on the circuit type:
| Circuit Type | Percentage | 230V Single-Phase | 400V Three-Phase |
|---|---|---|---|
| Lighting | 3% | 6.9V | 12V |
| All other circuits | 5% | 11.5V | 20V |
These limits apply from the origin of the installation (the main switch or distribution board) to the most distant point on the circuit. For installations supplied from a private LV supply (such as a generator or transformer), the limits may be relaxed — BS 7671 permits up to 6% for lighting and 8% for other circuits in these cases (Regulation 525.1.1).
Important: The voltage drop values of 6.9V and 11.5V assume a nominal supply voltage of 230V. The actual UK supply voltage can vary between 216.2V and 253V (230V -6% / +10%). The voltage drop limit applies at the nominal voltage, not the worst-case low voltage.
The voltage drop formula for single-phase circuits is straightforward:
VD = (mV/A/m x Ib x L) / 1000
VD = Voltage drop in volts
mV/A/m = Millivolts per amp per metre (from Appendix 4 tables)
Ib = Design current in amps (the actual load current)
L = One-way cable length in metres (route length, not straight-line distance)
The mV/A/m value is specific to each cable type, cable size, and installation method. It is found in the voltage drop columns of the Appendix 4 current-carrying capacity tables. Each table provides both single-phase (two-column) and three-phase (three-column) mV/A/m values.
The result is in volts. Compare it against the appropriate limit — 6.9V for lighting or 11.5V for other circuits (single-phase 230V). If the voltage drop exceeds the limit, you need to increase the cable size or reduce the cable length.
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Circuit: Ring final circuit, 2.5mm2 twin and earth, Reference Method C (clipped direct)
Design current (Ib): 25A (typical domestic ring circuit load)
Cable length: 50m total ring length (longest route to the most distant socket = 25m)
mV/A/m: 18 mV/A/m (from Table 4D5A, 2.5mm2, column for Reference Method C)
VD = (18 x 25 x 25) / 1000 = 11.25V
11.25V is within the 5% limit (11.5V) — but only just. Marginal pass.
Note: For a ring circuit, the voltage drop is calculated using the design current and half the total ring length (the longest route to the most distant point). If the ring becomes unbalanced (which happens in practice), the actual voltage drop may be higher. Consider 4mm2 cable for long ring circuits.
Circuit: Lighting radial, 1.5mm2 twin and earth, Reference Method A (enclosed in insulated wall)
Design current (Ib): 6A (typical domestic lighting circuit)
Cable length: 20m to the furthest luminaire
mV/A/m: 29 mV/A/m (from Table 4D5A, 1.5mm2)
VD = (29 x 6 x 20) / 1000 = 3.48V
3.48V is within the 3% lighting limit (6.9V). Compliant.
Circuit: 32A radial for 7.4kW EV charger, 6mm2 twin and earth, Reference Method C
Design current (Ib): 32A
Cable length: 30m (consumer unit to detached garage)
mV/A/m: 7.3 mV/A/m (from Table 4D5A, 6mm2)
VD = (7.3 x 32 x 30) / 1000 = 7.01V
7.01V is within the 5% power limit (11.5V). Compliant.
If the cable run were 50m instead: VD = (7.3 x 32 x 50) / 1000 = 11.68V — this would exceed the 5% limit and require upsizing to 10mm2 cable.
The mV/A/m values used in voltage drop calculations come from the voltage drop columns in the Appendix 4 current-carrying capacity tables of BS 7671. Each table covers a specific cable type, and within each table, the mV/A/m values vary by cable size and number of cores.
Each table provides two sets of mV/A/m values: one for single-phase (two conductors carrying current) and one for three-phase (three conductors carrying current). Ensure you use the correct column for your circuit type. The mV/A/m values account for both the resistance and the reactance of the cable — for smaller cables, resistance dominates; for larger cables, reactance becomes significant.
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Start free trialThe mV/A/m values in Appendix 4 are given at the conductor's maximum operating temperature (70C for PVC, 90C for XLPE). In practice, the conductor operating temperature depends on the load current and the ambient temperature. If the cable is not fully loaded, the conductor temperature will be lower than the maximum, and the actual voltage drop will be less than the calculated value.
BS 7671 Appendix 4 provides a correction formula for when a more precise voltage drop is needed. The corrected mV/A/m value accounts for the actual conductor temperature:
mV/A/m (corrected) = mV/A/m (tabulated) x [230 + tp - (Ca2 - Ib2/It2) x (tp - 30)] / (230 + tp)
tp = Maximum conductor operating temperature (70C for PVC)
Ca = Ambient temperature correction factor applied
Ib = Design current
It = Tabulated current-carrying capacity of the cable
In most domestic situations, the simpler calculation using the tabulated mV/A/m value directly is sufficient and gives a conservative (worst-case) result. The temperature correction is most useful in borderline cases where the calculated voltage drop is just above the limit — the correction may bring it within limits and avoid unnecessarily upsizing the cable.
Voltage drop is not always the determining factor in cable sizing — for short cable runs with moderate loads, the current-carrying capacity usually dictates the cable size, and voltage drop is well within limits. However, there are common situations where voltage drop becomes the critical factor:
Any cable run over 20 metres should be checked for voltage drop. Circuits to detached garages, garden offices, outbuildings, and EV chargers in driveways often involve cable runs of 30-50 metres or more. At these distances, voltage drop can easily exceed the 5% limit and may require a cable size one or two steps larger than the current-carrying capacity alone would suggest.
Lighting circuits have the tighter 3% limit (6.9V), making voltage drop more likely to be the critical factor. This is particularly relevant for long lighting circuits in commercial buildings, warehouses, and large domestic properties where cable runs to distant luminaires can be substantial.
Electric showers (32-45A), cookers (30-45A), and EV chargers (32A) draw high currents that produce significant voltage drop even on moderate cable lengths. A 10.8kW shower on a 15-metre run of 10mm2 cable produces a voltage drop of about 3V — well within limits. But the same shower on a 40-metre run produces about 8V, pushing close to the limit.
Motors draw significantly higher current during starting than during running. If the voltage drop is already near the limit at running current, the starting current may cause sufficient voltage drop to prevent the motor starting at all. For motor circuits, it is good practice to keep the running voltage drop well below the 5% limit to allow for starting current.
For three-phase circuits, the same formula applies, but you use the three-phase mV/A/m values from Appendix 4 (these are different from the single-phase values) and compare against the three-phase limits:
VD = (mV/A/m [3-phase] x Ib x L) / 1000
3% lighting limit: 400V x 0.03 = 12V
5% other limit: 400V x 0.05 = 20V
Three-phase mV/A/m values are lower than single-phase values for the same cable because the voltage drop calculation accounts for three conductors sharing the load. The three-phase values are typically about 87% (1/root 3) of the single-phase values for the resistive component.
Circuit: Three-phase distribution to a sub-board, 16mm2 4-core SWA, Reference Method D (direct buried)
Design current (Ib): 60A per phase
Cable length: 35m
mV/A/m (3-phase): 2.4 mV/A/m (from Table 4D4A, 16mm2 SWA, three-phase column)
VD = (2.4 x 60 x 35) / 1000 = 5.04V
5.04V is within the 5% limit (20V for 400V three-phase). Compliant.
Elec-Mate's voltage drop calculator handles both single-phase and three-phase circuits. Select the supply type, enter the cable details and load…
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