Voltage Drop Calculator UK

BS 7671 Regulation 525 limits voltage drop to 3% for lighting circuits and 5% for other circuits including power, heating, and motors. These are measured from the origin of the installation to any load point. On a 230V single-phase supply, this means 6.9V maximum for lighting and 11.5V maximum for power.

For single phase circuits: VD = (mV/A/m × current × length) ÷ 1000. For three phase: VD = (mV/A/m × current × length × 0.866) ÷ 1000. The mV/A/m values from Table 4D1B already account for both conductors at 70°C operating temperature, so do NOT multiply by 2. This is the official BS 7671 method.

Excessive voltage drop causes equipment malfunction, motor overheating, LED dimming, reduced equipment lifespan, and BS 7671 non-compliance. Proper calculation during the design phase prevents costly installation failures and EICR certification failures.

You have three options. First, increase the cable size to reduce the mV/A/m value — going from 2.5mm² (18 mV/A/m) to 4mm² (11 mV/A/m) cuts voltage drop by 39%. Second, reposition the distribution board closer to the load to shorten the run. Third, split a large load across multiple smaller circuits.

The mV/A/m values in BS 7671 Table 4D1B already account for both conductors (live and neutral) at the cable's actual 70°C operating temperature. The older "multiply resistance by 2" method uses conductor resistance at 20°C, which is a different approach and underestimates real-world voltage drop by approximately 20%. Don't mix methods — use mV/A/m without the factor of 2.

The limit applies to the TOTAL voltage drop from the origin of the installation (typically the consumer unit or distribution board) to the load point. This includes distribution circuits AND final circuits combined. You cannot exceed 3% for lighting or 5% for power as a total.

Three-phase circuits have inherently lower voltage drop because the load is balanced across three conductors. The formula uses a 0.866 factor (√3/2) to convert single-phase tabulated mV/A/m values, and the percentage is calculated against 400V line-to-line rather than 230V single phase.

The default calculator uses BS 7671 Table 4D1B which covers thermoplastic (PVC) insulated copper conductors. SWA cables use Table 4D4B with slightly different mV/A/m values. For most domestic SWA submains the difference is small, but for precise compliance work consult Table 4D4B directly. We have a dedicated SWA Armoured Cable calculator that uses the correct values.

BS 7671 recognises that transient voltage drops during motor starting (where inrush current is typically 5–7× running current) are acceptable if they don't cause equipment damage or hazardous operation. However, design should still minimise starting inrush voltage drop. Continuous operation must meet the stated 3% or 5% limits.

Design for anticipated future maximum demand rather than current load. This avoids costly cable upgrades later. For example, if a distribution board might eventually need 40A, design for 40A now rather than upgrading from 20A in five years' time. Voltage drop scales linearly with current, so over-sizing the cable today saves significant disruption later.

No. The Table 4D1B values are already at 70°C operating temperature. Unlike the conductor resistance method which uses 20°C and needs correction, mV/A/m values represent real operating conditions under design current load.

Yes. All calculations follow BS 7671:2018+A2:2022 Regulation 525 requirements and use mV/A/m values from Table 4D1B for thermoplastic (PVC) insulated copper conductors at 70°C operating temperature. The methodology matches what a qualified electrician would use during design.

No. BS 7671 voltage drop is measured from the origin of the installation (typically the consumer unit or distribution board) to the load. The DNO supply cable from the substation to the cutout has its own permitted drop separately under ESQCR (Electricity Safety, Quality and Continuity Regulations).

Yes. EV chargers and heat pumps are power circuits subject to the 5% voltage drop limit. Both typically have long runs from the consumer unit, making voltage drop a critical sizing factor. Use the actual continuous design current — 32A for a 7.4kW EV charger, or the heat pump nameplate rating. We have dedicated EV Charger and Heat Pump calculators with pre-filled scenarios.

No. Voltage drop is one of three checks for cable sizing under BS 7671. The other two are current-carrying capacity (the cable must safely carry the design current without overheating) and disconnection time under fault conditions. A fully compliant cable size satisfies all three checks. This calculator handles voltage drop only — verify CCC and disconnection time separately.