effect of using soft starter on cable size 1

[usamaegypt]

Dear all,
I would like to know whether using soft starter in starting a motor could decrease the cable size rather than the case of starting the motor DOL since the starting current will be limeted and not 7 times the nominal current.
Best regards.

 

[dpc]

Not in general. Normally, the motor feeder is sized based on the motor full load current (125% of FLA per the NEC). Unless the feeder had been oversized beyond that for some reason related to motor starting, changing the starting type, won't change the feeder size.

 

[Marke]

Hello usamaegypt

Cable size is usually selected on one of two criterea.
1] Thermal.
2] Voltage drop.

The thermal rating of the cable determines the maximum continuous current that the cable can carry. The thermal effect of starting a motor is very small and the effect of using a soft starter instead of DOL is essentially the same. The soft starter will result in a lower start current for a longer time.

If, in addition to the thermal selection of the cable, it has been increased in size to reduce the voltage drop, then there is a possibility to reduce the size due to the reduced voltage drop, however remeber that the soft starter operates by phase controlled SCRs. Although the average start current is lower, (perhaps 50 - 60%) the peak current is not much lower(75 - 80%). The SCRs conduct a high current for a shorter time to get a reduced average.

Best regards,

Mark Empson

http://www.lmphotonics.com