Soft Start Feeder

[mls1]

The question has come up as to how to size the feeder cables to a motor circuit with a soft starter per the NEC. It would seem that article 430.122 would apply and that, with a bypass circuit, the feeder needs to be sized by 125% of the soft start rating or the motor FLA, whichever is higher. But the language of 430.122 says "adjustable speed drive" rather than just "power conversion equipment". Some soft starts work much like a cycloconverter so in that respect they could be qualified as "adjustable speed" but most are just a voltage chopper so the language would seem to not apply. But the logic for sizing based on the device rating does make sense to me. Is there any CLEAR guidance on if 430.122 applies to soft starts or anywhere else that soft start feeder sizing is defined?

 

[jraef]

430.122 does NOT apply to soft starters. There is a direct connection from line to load, there is no conversion taking place. your concern for the option that appears to be a cyclic inverter is unfounded, that option cannot be used continuously and only at severely reduced capacity, ie 7 to 15%. So normal rules for motor starter circuits apply to soft starters.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[mls1]

For a classic soft starter with bypass I would concur but these units do not use a separate contactor, they use a solid state switch, and the current rating of the soft start is the continuous current rating. Obviously, during starting and braking the current is much higher. For a 250Hp soft start the listed continuous current rating of the soft start is 317A but the motor FLA per NEC table is 302A. At 125% we could use 500 kcmil cable for the motor but we would need to bump up to 600 kcmil for the starter cable if it were applied to that rating.

Also, while Part X is titled "Adjustable Speed Drive Systems" it describes in multiple locations "Power Conversion Equipment" for which an active solid state switching device such as these soft starters would appear to apply. At the very least, the logic seems consistent that if the device is capable of drawing a higher current continuously it should be fed by a circuit sized on that basis. But the NEC language, in my opinion, is ambiguous on this topic.

 

[LionelHutz]

A soft-starter is not an adjustable speed drive or power conversion equipment. The standard motor branch feeder rules apply the same as any mechanical starter.