Precision Utility
UK Voltage Drop
Calculator
Standard
BS 7671
Limit
Max 5% Drop
Check whether your cable run meets BS 7671 voltage drop limits in seconds. Enter the circuit current, cable length, conductor size and supply voltage — the calculator shows the exact voltage drop, percentage and a clear pass or fail verdict against the 5% maximum. Built for UK electricians and electrical designers.
Circuit Parameters
Voltage Drop
10.80 V
Voltage Drop
10.80 V
Drop %
4.70%
Voltage at Load
219.20 V
BS 7671 Verdict
PASS
How the voltage drop calculator works
Start by entering the design current of the circuit in amps. This is the expected load current — the amount of current flowing through the cable under normal conditions.
Next, enter the one-way cable length in metres. This is the distance from the distribution board to the furthest point of use — not the total length of cable in the circuit.
Select the cable cross-sectional area from the dropdown. Each size has a corresponding mV/A/m value taken from BS 7671 Table 4D5 for twin and earth thermoplastic (PVC) cables. Larger conductors have lower mV/A/m values, meaning less voltage is lost per metre.
The calculator applies the standard formula: VD = (mV/A/m x I x L) / 1000, where I is the current in amps and L is the cable length in metres. It then compares the percentage drop against the 5% limit set by BS 7671 for most lighting and power circuits.
What you need to know about voltage drop in the UK
BS 7671 (the IET Wiring Regulations) sets the standard for electrical installations in the United Kingdom. Regulation 525 states that voltage drop between the origin of the installation and any load point should not exceed the values given in Appendix 12.
Key points for 2025/26:
- The general limit is 5% of the nominal voltage for installations supplied directly from a public supply
- For a 230 V single-phase supply, that equals a maximum drop of 11.5 V
- For a 400 V three-phase supply, the maximum is 20 V
- BS 7671 splits the 5% allowance further — 3% for lighting circuits and 5% for other circuits in some cases
- The mV/A/m values used here are for twin and earth thermoplastic (PVC) flat cable at the conductor operating temperature
If your circuit fails the 5% check, the usual fix is to upsize the cable. Going from 2.5 mm² to 4.0 mm² nearly halves the voltage drop. Alternatively, you can shorten the cable run or reduce the load current by splitting the circuit.
Frequently asked questions
What is the maximum voltage drop allowed under BS 7671?
BS 7671 recommends that voltage drop should not exceed 5% of the nominal supply voltage for most installations. For a 230 V single-phase supply, that means no more than 11.5 V drop from the origin to the furthest point of the circuit.
How do I calculate voltage drop for a cable?
Multiply the cable's mV/A/m value (from BS 7671 tables) by the circuit current in amps and the cable length in metres, then divide by 1000 to get the voltage drop in volts. This calculator does it automatically — just enter the current, length and cable size.
What does mV/A/m mean in cable voltage drop tables?
mV/A/m stands for millivolts per amp per metre. It is the voltage drop per ampere of current flowing through one metre of cable. Each cable size has a different mV/A/m value — smaller cables have higher values, meaning more voltage drop per metre.
Why does cable size affect voltage drop?
Larger cables have lower resistance per metre because their conductor cross-sectional area is bigger. Lower resistance means less voltage is lost as heat along the cable. A 10 mm² cable has roughly one-tenth the resistance per metre of a 1.0 mm² cable.
What happens if voltage drop exceeds 5%?
Equipment at the far end of the circuit may not operate correctly. Motors can overheat, lights may dim, and sensitive electronics can malfunction. The circuit would also fail to comply with BS 7671 and could be flagged as a defect on an EICR.
How do I reduce voltage drop on a long cable run?
The most common solution is to increase the cable size — a larger cross-sectional area means lower resistance and less voltage drop. You can also reduce the circuit length by re-routing the cable, or split the load across two separate circuits to lower the current.