Warehouse Sub-board Voltage Drop and Cable Selection
A 400 V three-phase sub-board supplies a new workshop 62 m cable-route away from the main switchboard. Connected load is 55 A per phase at 0.85 power factor, with a design allowance that the load may grow to 70 A. The electrician proposes 25 mm² four-core copper XLPE in free air on a cable tray, but the tray passes through a 5 m section where it is bundled with six other loaded circuits. Ambient temperature in the plant room is 40 °C. The client complains that existing equipment at the far end already dims when a large welder starts.
1. Explain how you would determine whether the proposed cable is adequate, identifying the derating factors that apply and the order in which you apply them.
Read the worked answer
Start from the design current. The welder start is a transient, so size for the 70 A growth figure rather than 55 A, but check motor-start voltage dip separately.
Apply derating in a defensible order:
- Ambient temperature - correct the tabulated current-carrying capacity for 40 °C using the XLPE correction factor (roughly 0.91).
- Grouping - the 5 m bundled section governs. Seven loaded circuits grouped means a substantial grouping factor; the 5 m length is not automatically exempt, so check the relevant installation rules before ignoring it.
- Installation method - free air on tray is favourable, but confirm the tabulated rating matches the actual method.
Then check voltage drop on the 62 m run at 70 A. For 25 mm² copper, the mV/A/m value is low, but at 70 A over 62 m the drop can approach or exceed the 5% limit for a sub-main feeding further circuits. Remember the drop is cumulative: the sub-main plus the final sub-circuit must both fit within the allowance.
Finally, check protective device coordination - the cable's derated capacity must be at least the device rating, and the device must clear a fault within the disconnection time. If voltage drop fails, go up a size rather than accepting a non-compliant installation.
2. The customer asks whether simply upgrading the protective device to a higher rating will fix the dimming. How do you respond?
Read the worked answer
No - and it would make things worse in a different way. Dimming on welder start is a voltage drop symptom driven by cable impedance and starting current, not by the breaker rating. A larger breaker does not reduce impedance; it only allows more current before tripping.
Worse, increasing the device rating may exceed the cable's derated current-carrying capacity, removing overload protection and breaching the requirement that the device protect the conductor.
The real fixes are:
- Increase conductor size to cut resistance and reactance, reducing drop under start current.
- Shorten the route if practicable.
- Reduce starting current - soft start or a different welder connection.
- Check the actual drop with a load test and compare against the limit.
Explain to the customer that the dimming is a design limitation of the existing cable, and that correcting it means addressing impedance, not protection settings.