Three Phase Voltage Drop Calculator BS 7671 Compliant

Voltage drop in a three-phase circuit is the reduction in electrical potential along the cable conductors as current flows from the supply to the load. Every cable has resistance (and at larger sizes, reactance), and when current passes through this impedance, some of the supply voltage is consumed by the cable itself rather than delivered to the load.

In a three-phase system, three line conductors each carry current with a 120-degree phase displacement. This phase relationship means the voltage drop behaviour differs from single-phase circuits. The line-to-line voltage in the UK is 400 V (compared to 230 V phase-to-neutral), and the voltage drop limits are applied against this higher voltage. A three-phase power circuit is permitted up to 20 V of voltage drop (5% of 400 V), whereas a single-phase power circuit is limited to 11.5 V (5% of 230 V).

Three-phase voltage drop is a critical design consideration for cable sizing on commercial and industrial installations. Sub-main cables feeding distribution boards, motor circuits, three-phase EV chargers, and large power supplies all require accurate voltage drop calculations to ensure compliance with BS 7671:2018+A4:2026.

The square root of 3 (approximately 1.732) appears throughout three-phase electrical calculations. It arises from the geometric relationship between three sinusoidal waveforms displaced by 120 degrees. In a star-connected system, the line voltage is root 3 times the phase voltage: 230 V x 1.732 = 400 V.

A common question is whether you need to multiply by root 3 when calculating three-phase voltage drop using the BS 7671 tables. The answer is no — because the three-phase mV/A/m values in the Appendix 4 tables already incorporate the root 3 factor. The tabulated three-phase values give the line-to-line voltage drop directly when you apply the standard formula.

The relationship between the single-phase and three-phase mV/A/m values in the tables is: the three-phase value is approximately equal to the single-phase value divided by root 3 (for purely resistive cables) or calculated from the combined resistive and reactive components (for larger cables where reactance is significant).

The standard formula for calculating three-phase voltage drop using BS 7671 tabulated values is identical in structure to the single-phase formula:

The critical difference from single-phase calculations is that you must use the three-phase column from the mV/A/m tables, and the result is compared against the three-phase voltage limits (percentage of 400 V, not 230 V).

For larger cables (typically 25 mm² and above), the BS 7671 tables split the mV/A/m value into separate resistive (r) and reactive (x) components. In this case, the combined voltage drop is calculated as: VD = (mV/A/m_r x cos phi + mV/A/m_x x sin phi) x I_b x L / 1000, where cos phi is the power factor of the load. This is important for three-phase motor circuits where the power factor can be significantly less than unity.

Three-Phase Volt Drop Calculator (BS 7671)

Free three-phase voltage drop calculator to BS 7671: enter cable size, current and length to check volt drop against the 3% and 5% limits.

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A balanced three-phase load draws equal current on all three phases at the same power factor. Examples include three-phase motors, three-phase heaters with equal elements, and 22 kW EV chargers. For balanced loads, a single voltage drop calculation using the three-phase mV/A/m value and the per-phase current gives the complete answer.

An unbalanced three-phase load draws different currents on each phase. This is the typical situation in a three-phase distribution board feeding single-phase final circuits — it is almost impossible to achieve perfect balance across all three phases. The degree of imbalance depends on how the circuits are distributed across the phases.

For sub-main cables feeding three-phase distribution boards, design for the worst-case phase current. If the board is new and the circuit loading is known, check voltage drop using the highest individual phase current with the single-phase mV/A/m values. If the loading is assumed to be approximately balanced, the three-phase calculation is acceptable.

BS 7671:2018+A4:2026 Regulation 125.8 and Table 4Ab define the maximum permitted voltage drop for all installations. The limits for three-phase circuits on a public LV supply are:

For installations supplied from a private LV supply (such as a standby generator or private transformer), Regulation 125.8 permits higher limits: 8% (32 V) for power and 6% (24 V) for lighting. These higher limits recognise that the electrician typically has more control over the supply characteristics in a private installation.

Remember that the voltage drop limit applies from the origin of the installation (the meter or main incoming supply terminals) to the most remote point of the final circuit. If there are sub-main cables in the path, the voltage drop across each section of cable must be added together. The total must remain within the limit.

Instant pass/fail against BS 7671 limits

Elec-Mate automatically selects the correct limit — 20 V for three-phase power…

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Below are commonly referenced three-phase mV/A/m values from BS 7671 Appendix 4. These are approximate and for reference — always verify against the current edition of BS 7671 for your specific cable type and installation method.

Values extracted from BS 7671:2018+A4:2026, Tables 4D2B and 4E4B. Three-phase column (3-core or 4-core cable, three-phase a.c.). Copper conductors. Always verify against the current edition for your specific installation method.