Voltage Drop Calculator per BS 7671 for Industrial Installations
Industrial voltage drop calculations under BS 7671 are governed by Regulation 525.1, with a maximum of 5% for power circuits per Appendix 4, Table 4Ab. Long cable runs to remote plant equipment, high motor starting currents, and cable tray installations with grouping derating factors make industrial voltage drop assessment critical for plant reliability.
Quick Reference Table
| BS 7671 Voltage Drop Data — Industrial — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Power circuit limit | 5% (20 V at 400 V three-phase) | Appendix 4, Table 4Ab |
| Copper SWA mV/A/m values | Tabulated for armoured cables | Table 4D4B |
| Aluminium conductor mV/A/m | Higher values per CSA than copper equivalents | Table 4H4B |
| Permissible limits | Measured at terminals of current-using equipment | Regulation 525.1 |
| Motor starting exclusion | Transient voltage dip during starting not subject to steady-state limits | Regulation 525.1 Note |
How to Calculate Voltage Drop for Industrial Installations
- 1
Identify the heaviest motor load and cable route
In industrial installations, the worst-case voltage drop is usually the longest cable run feeding the largest motor. Measure the route length along cable trays, through risers, and across the plant to the motor control centre or direct-on-line starter.
- 2
Determine full-load current and starting current
Use the motor nameplate full-load current (FLC) for steady-state voltage drop. Note the starting current (typically 6-8× FLC for DOL) — while BS 7671 excludes starting transients from the 5% limit, excessive starting voltage dip can cause nuisance tripping of contactors.
- 3
Select cable type and find mV/A/m value
For industrial installations, steel-wire armoured (SWA) cable is standard. Look up Table 4D4B for copper SWA or Table 4H4B for aluminium SWA. Use the three-phase column for three-phase motor circuits.
- 4
Calculate steady-state voltage drop
Apply VD = (mV/A/m × Ib × L) / 1000 using the motor full-load current. For a 400 V three-phase supply, the 5% limit gives 20 V maximum. Include the sub-main contribution if the MCC is remote from the main switchboard.
- 5
Check motor starting voltage dip
While not a BS 7671 compliance issue, calculate the voltage at motor terminals during starting: V_start = V_supply − (mV/A/m × I_start × L / 1000). Motors typically need at least 80% of nominal voltage to develop adequate starting torque.
- 6
Verify and document
Confirm the total steady-state voltage drop is within 5% per Table 4Ab. Record the calculation with cable reference, route length, and mV/A/m value used. For critical process motors, apply a safety margin of 1-2%.
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Calculate Voltage Drop NowBS 7671 vs IEC 60364 Cable Sizing Comparison
| Parameter | BS 7671 | IEC 60364 |
|---|---|---|
| Scope | UK & derivatives | International (adopted by 60+ countries) |
| Voltage drop limit | 3% lighting / 5% other | 4% lighting / 5% other (typical) |
| Reference ambient temp | 30°C (air), 20°C (ground) | 30°C (air), 20°C (ground) |
| Installation methods | Reference Methods A-G (Appendix 4) | Reference Methods A-G (Table B.52.1) |
| Grouping factors | Table C.3 (BS specific) | Table B.52.17 (international) |
| Disconnection time (230V) | 0.4s final / 5s distribution | 0.4s final / 5s distribution |