Calculate voltage drop for any cable type and installation method. Instantly check compliance with the 3% lighting and 5% power limits from BS 7671 Table 4Ab.
Calculate voltage drop using BS 7671 Appendix 4 tabulated values
Ib - design current
One-way route length
Voltage drop is the reduction in electrical potential (voltage) along a conductor as current flows through it. Every cable has resistance, and when current passes through that resistance, some of the supply voltage is lost as heat in the cable itself. The voltage available at the load end of the cable is therefore lower than the voltage at the supply end.
In practical terms, excessive voltage drop means that the equipment connected at the far end of a cable run does not receive its full rated voltage. For lighting circuits, this causes lamps to produce less light and can lead to visible flickering, particularly with LED drivers. For motors and other power equipment, low voltage causes higher current draw, reduced efficiency, overheating, and premature failure. On very long cable runs — such as those found in commercial buildings, farms, and industrial sites — voltage drop is often the factor that determines the minimum cable size rather than the current-carrying capacity alone.
The amount of voltage drop depends on three main factors: the resistance of the cable per unit length (determined by its cross-sectional area and conductor material), the current flowing through it, and the length of the cable run. Copper conductors have lower resistance than aluminium for the same cross-sectional area, and larger cables have lower resistance than smaller ones. This is why increasing the cable size is one of the primary methods of reducing voltage drop. Use the cable sizing calculator alongside this tool to ensure the selected cable meets both current-carrying capacity and voltage drop requirements. The trunking fill calculator helps plan containment for the sized cables.
BS 7671:2018+A4:2026, the 18th Edition of the IET Wiring Regulations, sets out the maximum permitted voltage drop for electrical installations in the United Kingdom. These limits are defined in Regulation 525.1 and quantified in Table 4Ab.
For installations supplied directly from a public low-voltage distribution system (i.e. most domestic and commercial properties connected to the DNO network), the limits in Table 4Ab are:
3%
of nominal voltage = 6.9 V on a 230 V supply
5%
of nominal voltage = 11.5 V on a 230 V supply
For installations supplied from a private LV supply (such as a generator or transformer), the permitted voltage drop is higher: 6% for lighting and 8% for other uses. This is because the supply point is typically closer to the installation and the electrician has more control over the supply characteristics.
It is important to note that these percentage limits apply from the origin of the installation (the supply terminals) to the most distant point of the circuit. If an installation has sub-distribution boards, the voltage drop accumulates along the entire path from the main incoming supply through any sub-mains to the final circuit. The total voltage drop across all sections of cable must remain within the limit.
The standard voltage drop formula used with BS 7671 tables is straightforward:
VD = (mV/A/m × Ib × L) ÷ 1000
VD = voltage drop in volts
mV/A/m = millivolts per ampere per metre (from BS 7671 tables)
Ib = design current of the circuit in amperes
L = length of the cable run in metres
The mV/A/m value is the key figure. It represents the voltage drop per ampere of current per metre of cable length, expressed in millivolts. This value is specific to each cable type, conductor material, cross-sectional area, and whether the circuit is single-phase or three-phase. These values are tabulated in BS 7671 Appendix 4, in the "B" series of tables (4D1B, 4D2B, 4E1B, etc.).
For example, a 2.5 mm² copper twin and earth cable (flat thermoplastic, Table 4D5B) has a tabulated voltage drop of 18 mV/A/m for single-phase circuits. If this cable carries 20 A over a length of 25 metres, the voltage drop is:
VD = 18 × 20 × 25 ÷ 1000 = 9.0 V
This is 3.91% of 230 V — which passes the 5% power limit (11.5 V) but would fail the 3% lighting limit (6.9 V). If this were a lighting circuit, you would need to increase the cable to 4 mm².
The division by 1000 is necessary because the tabulated values are in millivolts, and we need the result in volts. Always remember to express the final answer as a percentage of the nominal supply voltage to compare against the BS 7671 limit: (VD ÷ 230) × 100 for single-phase, or (VD ÷ 400) × 100 for three-phase.
The voltage drop per ampere per metre values are found in the "B" series of current-carrying capacity tables in Appendix 4 of BS 7671. Each table covers a specific cable construction. The most commonly used tables for UK domestic and commercial installations are:
Within each table, find the row for your cable cross-sectional area (1.0 mm², 1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², 16 mm², 25 mm², etc.). Then read across to the column for your circuit type: "2-core cable, single-phase a.c. or d.c." or "3-core or 4-core cable, three-phase a.c.". Some tables split the value into separate resistive (r) and reactive (x) components for larger cables where reactance becomes significant.
The Elec-Mate voltage drop calculator has all of these tables built in. Simply select your cable type and cross-sectional area, and the correct mV/A/m value is automatically applied to the calculation. This eliminates manual table look-up errors and saves considerable time when designing circuits or verifying installations on site.
Identify the design current (Ib) of the circuit in amps, the length of the cable run (L) in metres from the distribution board to the furthest point, and whether the circuit is single-phase or three-phase. Also note the type of circuit: lighting (3% limit) or power (5% limit).
Determine the cable type you are using (e.g. twin and earth 6242Y, SWA, singles in conduit) and the installation method (clipped direct, in trunking, buried, etc.). This determines which table in BS 7671 Appendix 4 you will reference.
Open the appropriate voltage drop table in BS 7671 (Tables 4D1B through 4J4B). Find the row matching your cable cross-sectional area (e.g. 2.5 mm², 4 mm², 6 mm²) and read off the mV/A/m value for your circuit type (single-phase or three-phase).
Calculate: Voltage Drop = mV/A/m × Ib × L ÷ 1000. For example, a 2.5 mm² twin and earth cable carrying 20 A over 30 metres with a tabulated value of 18 mV/A/m gives: 18 × 20 × 30 ÷ 1000 = 10.8 V.
Compare your calculated voltage drop against the maximum allowed: 6.9 V for lighting (3% of 230 V) or 11.5 V for power (5% of 230 V). If the voltage drop exceeds the limit, you must either increase the cable size, reduce the cable run length, or split the circuit.
If the voltage drop is marginal, consider applying the conductor temperature correction factor from Appendix 4 of BS 7671. When the cable is not fully loaded, the actual voltage drop will be lower than the tabulated value. This correction can sometimes allow a smaller cable size to be used.
A ring final circuit supplies socket outlets in a domestic kitchen. The circuit is wired in 2.5 mm² flat twin and earth cable (Table 4D5B). The total cable length of the ring is 50 metres. The design current is 30 A (though the ring is protected by a 32 A MCB). What is the voltage drop?
For a ring circuit, the effective length used in the volt drop calculation is one quarter of the total ring length (because current flows in both directions around the ring). Effective length = 50 ÷ 4 = 12.5 m.
VD = 18 × 30 × 12.5 ÷ 1000 = 6.75 V (2.93%)
Result: PASS — 6.75 V is within the 11.5 V (5%) limit for power circuits.
A lighting circuit in a large workshop runs 45 metres from the consumer unit to the furthest luminaire. The circuit is wired in 1.5 mm² flat twin and earth cable (Table 4D5B, mV/A/m = 29). The design current is 8 A.
VD = 29 × 8 × 45 ÷ 1000 = 10.44 V (4.54%)
Result: FAIL — 10.44 V exceeds the 6.9 V (3%) limit for lighting circuits. The cable must be increased to 2.5 mm² (mV/A/m = 18), giving: 18 × 8 × 45 ÷ 1000 = 6.48 V (2.82%) — PASS.
A three-phase SWA sub-main cable runs 60 metres from the main switchboard to a sub-distribution board. The cable is 25 mm² 4-core copper XLPE SWA (Table 4E4B, three-phase mV/A/m = 1.50). The design current is 80 A per phase.
VD = 1.50 × 80 × 60 ÷ 1000 = 7.2 V (1.8% of 400 V)
Result: PASS — 7.2 V is within the 20 V (5% of 400 V) limit for a three-phase power circuit. However, remember that the voltage drop in the final circuits downstream of the sub-distribution board must also be added, and the total must remain within 5%.
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BS 7671 Table 4Ab sets the maximum voltage drop limits for installations supplied directly from a public low-voltage distribution system. For lighting circuits, the limit is 3% of the nominal supply voltage (6.9 V on a 230 V supply). For all other circuits including power and socket outlets, the limit is 5% of the nominal supply voltage (11.5 V on a 230 V supply). These limits apply from the origin of the installation to the most distant point of the circuit.
Use the formula: Voltage Drop (V) = mV/A/m × Ib × L ÷ 1000. First, look up the mV/A/m value from BS 7671 Table 4D1B to 4J4B for your specific cable type and installation method. Then multiply by the design current (Ib) in amps and the cable length (L) in metres. Divide by 1000 to convert millivolts to volts. For three-phase circuits, use the three-phase mV/A/m columns from the tables.
The mV/A/m (millivolts per ampere per metre) values are found in the current-carrying capacity tables in Appendix 4 of BS 7671. For thermoplastic (PVC) cables, refer to Tables 4D1A/4D1B (single-core) and 4D2A/4D2B (multicore). For thermosetting (XLPE/SWA) cables, refer to Tables 4E1A/4E1B through 4E4A/4E4B. The "B" tables contain the voltage drop values, while the "A" tables contain the current ratings. Always use the correct table for your cable type, conductor material (copper or aluminium), and installation method.
Yes. The mV/A/m values in the BS 7671 tables are given at the conductor operating temperature, not ambient temperature. When a cable is lightly loaded, it runs cooler and its resistance is lower, meaning the actual voltage drop will be less than the tabulated figure. You can apply a correction using the formula in BS 7671 Appendix 4, which accounts for the difference between the tabulated conductor operating temperature and the actual conductor temperature. This correction can be beneficial on long cable runs where voltage drop is marginal.
For single-phase circuits, you use the single-phase mV/A/m column (labelled "2-core cable, single-phase a.c. or d.c." in the tables). For three-phase circuits, you use the three-phase mV/A/m column (labelled "3-core or 4-core cable, three-phase a.c."). The three-phase values are lower because of the way voltage is distributed across the phases. The formula is the same in both cases: Voltage Drop = mV/A/m × Ib × L ÷ 1000. For single-phase, the result is compared against the percentage of 230 V. For three-phase, it is compared against the percentage of 400 V.
Yes. A circuit can have perfectly acceptable earth fault loop impedance, insulation resistance, and continuity values and still fail the voltage drop check. This is common on long circuit runs — for example, an outbuilding circuit running 40 to 50 metres from the distribution board, or a socket circuit in a large commercial unit serving the far end of a building. Excessive voltage drop causes equipment to run below its design voltage, which reduces the performance of motors and reduces the light output of certain lamp types. It can also cause electronic equipment to operate erratically or shut down. When you encounter high voltage drop on a circuit, the options are to increase the cable cross-sectional area, reduce the circuit length by adding a sub-distribution board closer to the load, or in some cases accept the situation with written agreement if the loads are tolerant of lower voltages. Elec-Mate's voltage drop calculator highlights circuits that exceed the BS 7671 limits and suggests alternative cable sizes.
Below are the most commonly referenced mV/A/m values from BS 7671 Table 4D5B for flat twin and earth cable (copper conductors, single-phase). These are the cables used in the vast majority of domestic and small commercial installations.
These values are extracted from BS 7671:2018+A4:2026, Table 4D5B (Reference Method C — clipped direct). Always verify against the current edition of the regulations for your specific installation method.
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