Work out the voltage drop
VD = mV/A/m × Ib × L ÷ 1000. Limits for an installation fed from a public low-voltage supply (BS 7671 Appendix 4, Table 4Ab).
Check against BS 7671 and the manufacturer's data; this is a guide, not a design tool.
Voltage drop table for twin & earth
mV/A/m values from BS 7671 Table 4D5 (70 °C thermoplastic flat cable, conductors at operating temperature).
| Cable | mV/A/m | Ltg 6 A | 16 A | 20 A | 32 A |
|---|---|---|---|---|---|
| 1.0 mm² | 44 | 26 m | 16 m | 13 m | 8 m |
| 1.5 mm² | 29 | 40 m | 25 m | 20 m | 12 m |
| 2.5 mm² | 18 | 64 m | 40 m | 32 m | 20 m |
| 4 mm² | 11 | 105 m | 65 m | 52 m | 33 m |
| 6 mm² | 7.3 | 158 m | 98 m | 79 m | 49 m |
| 10 mm² | 4.4 | 261 m | 163 m | 131 m | 82 m |
| 16 mm² | 2.8 | 411 m | 257 m | 205 m | 128 m |
Lighting column: 3% (6.9 V) at 6 A. Other columns: 5% (11.5 V) at the current shown, single-phase 230 V. Lengths are for voltage drop only — check current-carrying capacity and Zs too.
How voltage drop is checked
BS 7671 Regulation 525 requires the voltage at the equipment to be high enough for it to work safely. Appendix 4 gives deemed-to-comply limits measured from the origin of the installation:
- Public low-voltage supply: 3% for lighting (6.9 V at 230 V, 12 V at 400 V) and 5% for other uses (11.5 V at 230 V, 20 V at 400 V).
- Private supply (own transformer or generator): 6% for lighting and 8% for other uses.
The formula
Voltage drop (V) = mV/A/m × Ib × L ÷ 1000, where mV/A/m is the tabulated value for the cable, Ib is the design current in amps and L is the length of run in metres. Example: 2.5 mm² twin & earth (18 mV/A/m), 20 A, 30 m → 18 × 20 × 30 ÷ 1000 = 10.8 V, which is 4.7% — inside the 5% limit for a socket circuit.
Remember the whole route
The limit covers the drop from the origin to the point of use. If the circuit is fed from a sub-main, add the sub-main's voltage drop to the final circuit's. For a ring final circuit, the usual approximation is to use a quarter of the end-to-end ring length with the full load, or follow the method in the On-Site Guide.
Three-phase circuits
For a balanced three-phase circuit use the three-phase mV/A/m column for the cable (for example in Table 4D2B or 4E2B) and compare with the 400 V limits. Twin & earth is single-phase only, so the three-phase option asks you to enter the value.
Questions electricians ask
What is the maximum voltage drop allowed in BS 7671?
For an installation supplied from a public low-voltage network, Appendix 4 of BS 7671 gives 3% for lighting circuits and 5% for other uses, measured from the origin. At 230 V that is 6.9 V and 11.5 V. Private supplies are allowed 6% and 8%.
How do you calculate voltage drop?
Multiply the cable's tabulated mV/A/m by the design current and the length in metres, then divide by 1000. For example 2.5 mm² twin and earth at 18 mV/A/m carrying 20 A over 30 m drops 10.8 V.
What is the mV/A/m of 2.5 mm² twin and earth?
BS 7671 Table 4D5 gives 18 mV/A/m for 2.5 mm² flat twin and earth, 29 mV/A/m for 1.5 mm², 11 for 4 mm², 7.3 for 6 mm² and 4.4 for 10 mm².
Does voltage drop include the sub-main?
Yes. The 3% and 5% limits apply from the origin of the installation to the equipment, so the drop in a sub-main or distribution circuit must be added to the final circuit.
More free tools for electricians
Max Zs table — MCB, RCBO and fuse limits + 80% check› Cable size calculator — Twin & earth, Table 4D5, Ca · Cg · Ci›Test sheets on the phone
EIC, EICR and Minor Works with live Zs, insulation and RCD checks, printed on the official IET forms.
See Graftum for electriciansCheck against BS 7671 and the manufacturer's data; this is a guide, not a design tool.