VD = mV/A/m (three-phase) × Ib × L ÷ 1000, using the BS 7671 Appendix 4 tables. The √3 is already inside the tabulated value. Handles balanced and unbalanced loads, SWA and multicore cables, with instant pass/fail against the 20 V and 12 V limits on a 400 V supply.
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Calculate voltage drop using BS 7671 Appendix 4 tabulated values
Ib - design current
One-way route length
Origin → this board (submains). Leave blank if this circuit starts at the origin.
Empty: Design Current, Cable Length, Cable Size.
Set the supply to 400 V (three phase) and the result is checked against the Table 4Ab limits — 12 V for lighting, 20 V for other uses. The tool applies the two-core mV/A/m value, which over-states the drop by roughly 15%, so a pass here is a genuine pass. For a final design, take the three-phase figure from the three-phase mV/A/m table below.
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VD = (mV/A/m3ph × Ib × L) / 1000
VD = line-to-line voltage drop in volts
mV/A/m3ph = three-phase millivolts per ampere per metre, from the BS 7671 Appendix 4 tables
Ib = design current per phase in amperes
L = run length in metres
Two things separate this from the single-phase calculation. You must read the three-phase column of the table rather than the single-phase column, and the answer is compared against a percentage of 400 V rather than 230 V.
Appendix 4, Section 6 is explicit about what the tabulated number already contains: for three-phase circuits the tabulated mV/A/m values relate to the line voltage, and balanced conditions have been assumed. The root 3 is inside the figure. Multiply the length by the design current, divide by 1000, and you have the line-to-line drop in volts.
Working from conductor resistance and reactance rather than the tables, the line-to-line drop on a balanced three-phase circuit is:
VD = √3 × I × L × (R cosφ + X sinφ)
√3 ≈ 1.732, converting the per-phase drop to a line-to-line drop
R, X = resistance and reactance of one conductor in ohms per metre
cosφ = load power factor (sinφ follows from it)
For a resistive load (cosφ = 1) it reduces to VD = √3 × I × R × L. The BS 7671 three-phase mV/A/m values are this same calculation done for you at the conductor operating temperature, which is why you never multiply a tabulated three-phase value by √3 again.
For conductors of 16 mm² or less, Appendix 4, Section 6 says the inductance can be ignored and only a single mV/A/m value is tabulated. Above 16 mm² the table gives an impedance value (mV/A/m)z together with a resistive component (mV/A/m)r and a reactive component (mV/A/m)x. Using the impedance value on its own is safe but pessimistic.
Where the load power factor is worth taking into account, Section 6.2 gives the design value as cos φ × (mV/A/m)r + sin φ × (mV/A/m)x. This is the same relationship as the first-principles form √3 × I × L × (R cos φ + X sin φ), with the √3 and the per-metre resistance folded into the tabulated values. It matters most on three-phase motor circuits, where the power factor can be well below unity.
The tabulated voltage drop values apply, for a.c. operation, to frequencies in the range 49 to 61 Hz. Appendix 4 warns that the voltage drop for cables operating at higher frequencies may be substantially greater. On the output side of a variable-frequency drive, the tabulated values are not the right tool.
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Enter cable size, current and length to check volt drop against the 3% and 5% limits.
BS 7671:2018+A4:2026 Regulation 525.202 states that the voltage drop between the origin of the installation (usually the supply terminals) and a socket-outlet or the terminals of fixed current-using equipment shall not exceed the values in Appendix 4, Section 6.4. Those values are Table 4Ab, and they are expressed as a percentage of the nominal voltage of the installation.
| Supply | Lighting | Other uses | On a 400 V three-phase supply |
|---|---|---|---|
| (a) Low voltage installations supplied directly from a public low voltage distribution system | 3% | 5% | 12 V lighting, 20 V other uses |
| (b) Low voltage installation supplied from a private LV supply | 6% | 8% | 24 V lighting, 32 V other uses |
Source: BS 7671:2018+A4:2026, Appendix 4, Section 6.4, Table 4Ab. Volt figures are the percentages applied to a 400 V nominal line voltage.
Table 4Ab attaches a footnote to row (b): the voltage drop within each final circuit should not exceed the values given in row (a). A generator-fed or transformer-fed installation therefore gets more headroom across the distribution path as a whole, but each individual final circuit is still held to 3% and 5%.
Long wiring systems. Table 4Ab permits the percentages above to be increased by 0.005% per metre of wiring system beyond 100 m, provided the increase is not greater than 0.5%. The cap is reached at 200 m, where the 5% power limit becomes 5.5%. Beyond that length the allowance does not grow any further.
Motor starting. Regulation 525.203 accepts a greater voltage drop during motor starting periods and for other equipment with high inrush currents — but only where it is verified that the voltage variations stay within the limits in the relevant product standard, or the manufacturer’s recommendations where no product standard exists.
Harmonics. Appendix 4, Section 6.4 states that the calculated voltage drop should include any effects due to harmonic currents. On a board full of switched-mode supplies that is not a formality.
The limit applies across the whole path, so where sub-mains sit between the origin and the load their voltage drops are added together and the total is what must comply.
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The square root of 3, about 1.732, runs through three-phase calculations. It comes from the geometry of three sinusoids displaced by 120 degrees: in a star-connected system the line voltage is root 3 times the phase voltage, which is where 230 V × 1.732 = 400 V comes from.
The question that brings most people to this page is whether they need to apply it themselves. They do not. Appendix 4, Section 6 states that for three-phase circuits the tabulated mV/A/m values relate to the line voltage and balanced conditions are assumed, so the factor is baked into the number in the table.
Do not multiply the result by root 3 as well. If you do, your answer comes out 73% high and you will oversize the cable for no reason.
It helps to see where the difference comes from. A single-phase circuit drops voltage in two conductors, the line and the return, so the drop is 2 × I × R × L. A balanced three-phase circuit carries no neutral current, and the line-to-line drop works out at √3 × I × R × L.
The ratio between them is therefore √3 / 2, or about 0.866. A tabulated three-phase value is roughly 87% of the single-phase value for the same cable — not a third of it, and not the single-phase value divided by root 3. Above 16 mm², where reactance is tabulated separately, the relationship is no longer a simple ratio.
Voltage drop in a three-phase circuit is the loss of potential along the conductors as current flows from the supply to the load. Every cable has resistance, and at larger sizes reactance too. When current passes through that impedance, part of the supply voltage is consumed by the cable rather than delivered to the equipment.
Three line conductors each carry current displaced by 120 degrees, and the reference voltage is the line-to-line value of 400 V rather than the 230 V phase-to-neutral value. That is why the same 5% limit gives 20 V on a three-phase power circuit but only 11.5 V on a single-phase one.
Getting it right matters most on the jobs where the runs are long: cable sizing for sub-mains feeding distribution boards, motor circuits, three-phase EV chargers and large power supplies on commercial and industrial installations.
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A balanced load draws equal current on all three phases at the same power factor — three-phase motors, three-phase heaters with equal elements, 22 kW EV chargers. Because the tabulated three-phase values assume exactly this, one calculation covers the circuit.
An unbalanced load draws different currents on each phase. That is the normal state of a three-phase board feeding single-phase final circuits; perfect balance is close to impossible in practice.
For a sub-main feeding a three-phase board, design for the worst-case phase. Where the circuit loading is known, check the highest individual phase current against the single-phase values. Where the loading is genuinely expected to be near balanced, the three-phase calculation is the right one.
Where a three-phase circuit supplies a high proportion of switched-mode power supplies or LED drivers, the third harmonic currents from each phase add in the neutral instead of cancelling. Appendix 4, Section 5.6 notes that the rating factors in Section 5.5 take account of the heating effect of the third harmonic in the neutral as well as in each line conductor. The neutral can end up carrying more current than any line conductor, which affects conductor sizing and the drop along the neutral, and the balanced / unbalanced framing above does not capture it. Where third harmonic content is high enough that the neutral becomes the basis of sizing, Regulation 431.2.3 also requires overcurrent detection for the neutral conductor.
Each example uses the three-phase a.c. column and checks the result against the Table 4Ab limit for a public low voltage supply. Confirm the mV/A/m figure against the printed table for your own cable before relying on it.
A 4-core 25 mm² copper XLPE SWA cable runs 55 metres from the main switchboard to a sub-distribution board. The balanced design current is 75 A per phase. The three-phase value taken from Table 4E4B is 1.50 mV/A/m. At 25 mm² this is an impedance value, so the answer is on the pessimistic side.
VD = 1.50 × 75 × 55 / 1000 = 6.19 V (1.55% of 400 V)
Pass — well inside the 20 V limit, leaving 13.81 V of budget for the final circuits downstream.
A three-phase motor draws 42 A at full load, supplied by a 10 mm² 4-core copper PVC SWA cable over 80 metres. Table 4D4B gives a three-phase value of 3.8 mV/A/m.
VD = 3.8 × 42 × 80 / 1000 = 12.77 V (3.19% of 400 V)
Pass against the 20 V steady-state limit. If this motor sits downstream of a sub-main, add that drop too. The starting dip is a separate check under Regulation 525.203.
A three-phase lighting circuit serves a warehouse at a balanced 14 A per phase over 65 metres, wired in 4 mm² 4-core copper PVC. Table 4D2B gives a three-phase value of 9.5 mV/A/m.
VD = 9.5 × 14 × 65 / 1000 = 8.65 V (2.16% of 400 V)
Pass against the 12 V lighting limit. At 90 metres the same circuit gives 11.97 V, or 2.99% — still inside 3%, but with nothing left over.
A three-phase sub-main of 35 mm² SWA runs 40 m at 100 A with a three-phase value of 1.05 mV/A/m. A final circuit from that board — 6 mm² singles in trunking, 25 m, 28 A, three-phase value 6.4 mV/A/m — serves a three-phase heater.
Sub-main: 1.05 × 100 × 40 / 1000 = 4.20 V
Final circuit: 6.4 × 28 × 25 / 1000 = 4.48 V
Total = 8.68 V (2.17% of 400 V)
Pass — and the total is what Regulation 525.202 asks for, since it measures from the origin of the installation.
Indicative three-phase values for copper conductors, for orientation only. Voltage drop figures live in the B-suffix Appendix 4 tables, and the right table depends on insulation and construction: 4D2B multicore non-armoured 70 °C thermoplastic, 4D4B multicore armoured 70 °C thermoplastic, 4E4B multicore armoured 90 °C thermosetting.
| Size | mV/A/m (three-phase) | Cable | Table |
|---|---|---|---|
| 4 mm² | 9.5 | 4-core PVC, non-armoured | 4D2B |
| 6 mm² | 6.4 | 4-core PVC, non-armoured | 4D2B |
| 10 mm² | 3.8 | 4-core PVC, non-armoured | 4D2B |
| 16 mm² | 2.4 | 4-core PVC SWA | 4D4B |
| 25 mm² | 1.50 | 4-core XLPE SWA | 4E4B |
| 35 mm² | 1.05 | 4-core XLPE SWA | 4E4B |
| 50 mm² | 0.78 | 4-core XLPE SWA | 4E4B |
| 70 mm² | 0.55 | 4-core XLPE SWA | 4E4B |
| 95 mm² | 0.41 | 4-core XLPE SWA | 4E4B |
Read the figure for your own job from the printed BS 7671:2018+A4:2026 table. The value changes with insulation type, conductor material and construction, and from 25 mm² upwards the table gives an impedance value alongside separate resistive and reactive components rather than a single number.
Appendix 4, Section 6 states that for single-core armoured cables the tabulated voltage drop values apply where the armour is bonded to earth at both ends. Bond at one end only and the tabulated values no longer hold. Multicore SWA, 3-core and 4-core, is not affected by this.
Six steps to a compliant three-phase voltage drop figure using the BS 7671 Appendix 4 tables.
Determine the design current (Ib) per phase in amperes, the run length (L) in metres from the distribution board to the furthest point, whether the load is balanced or unbalanced, and the circuit type (lighting or power).
Identify the cable construction (armoured, singles in conduit, multicore), conductor material (copper or aluminium), insulation type (70 degrees C thermoplastic or 90 degrees C thermosetting), and the number of cores — 3-core or 4-core for three-phase.
Open the correct Appendix 4 voltage drop table. The B-suffix tables from 4D1B to 4J4B carry the voltage drop figures. Find the row for your cross-sectional area and read the three-phase a.c. column, not the single-phase column. Above 16 mm2 the table gives an impedance value plus separate resistive and reactive components.
Calculate VD = mV/A/m (three-phase) x Ib x L / 1000. The result is the line-to-line voltage drop in volts, because the tabulated three-phase values relate to the line voltage. For a balanced load, Ib is the current per phase.
On a public low voltage supply, compare against 20 V (5% of 400 V) for power or 12 V (3% of 400 V) for lighting. If a sub-main feeds the board, add its drop to the final circuit drop — Regulation 525.202 covers the whole path from the origin.
Appendix 4, Sections 6.1 and 6.2 allow correction for operating temperature and load power factor. The temperature factor Ct applies only where the protective device is not a BS 3036 fuse and the ambient temperature is at least 30 degrees C, and above 16 mm2 it is applied to the resistive component alone.
Purpose-built for UK electricians working on three-phase commercial and industrial installations. Faster and more accurate than manual table look-ups.
The BS 7671 Appendix 4 voltage drop tables are built in. Select your cable type and the correct three-phase value is applied automatically.
Switch between balanced three-phase calculations and per-phase unbalanced calculations. Enter a single current or individual phase currents.
Supports SWA, singles in trunking and conduit, XLPE, multicore, and MI cables. Copper and aluminium conductors. Three-core and four-core configurations.
Instant colour-coded pass/fail against the correct limit — 20 V (5%) for power or 12 V (3%) for lighting on a 400 V three-phase public supply.
Works out the longest permissible run for your chosen cable size and load before the voltage drop limit is exceeded.
Calculations follow the current edition of the wiring regulations, including Amendment 4. Values taken from the published Appendix 4 tables.
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