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Motor cable sizing

Whatever the circuit (motor, lighting, other circuit) the software you use is made to check following criteria for cable sizing:
  • the cross section area has to support the design current taking into account derating factors due to temperature and installation method,
  • limit voltage drop under 8% at the motor level (8% is recommended by installation standard like IEC60364),
  • the cable thermal withstand is also checked to ensure that cable will not be damage under short-circuit condition,
  • at last but not the least the indirect contact protection has to be ensure till end of the circuit.

That's the reason to minimize the cable's section, because it not, the Earth Fault Current should be enough to trip the protection installed at the head of this circuit, so, the Software will choose the necessary section to have an Earth Fault Current bigger than the setting of "Magnetic Protection or Short-time Protection". Noting that this current should be at least "20%" more than the setting, because the IEC 60947-2 "define the maximum tolerance's percentage of this protection by" +-20% ".

If the distance is "400m", the smaller section that can be used is "Single-Core, 95 mm2 for phases and also PE", where the Earth Faults current at the end of cable is around " 1010 A ", for that the maximum setting of Magnetic or Short-time Protection should be smaller than " 800A ".

By the way, if the setting of this protection is " 800A ", it will be better to be in the safe side to use the following Single-Core cables :
- " 2 x 70 mm2 " for Phases
- " 1 x 95 mm2 " for PE
as the Earth Faults current at the end of these cables will be around " 1200 A "

At the same time, I'm not sure that " 35 mm2 " is enough even with differential protection, as the voltage drop will be more than necessary value. Noting that the voltage drop of " 95 mm2 " for this distance, even without adding that from the source till the switchboard is:
- 2,42%: for normal running
- 13.1%: during start-up
So with smaller section, the voltage drop during start-up will be more that 15%.

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