Transformer fed from MCC starter bucket

[Designer69]

There is a spare MCC starter bucket and the intention is to use it to power a dry-type 30 KVA transformer.

Is this bad engineering practice? I noticed in the one-line that all other MCC starter buckets feed motors. Other similar transformers are fed from plain breaker buckets. That's why I am asking the question because this seems to be non-typical.

Thank You

 

[Jraef]

Maybe, but probably not. Most MCC starter buckets that use circuit breakers are going to use MCP type (officially "IT" breakers for Instantaneous Only per the NEC). Article 430.52 allows for their use only as part of a factory built and tested combination motor starter because the motor starter will by definition provide running overload protection in the form of an overload relay that replaces the thermal current detection component missing in the breaker. But because that unit is UL listed as a motor starter, you cannot use it as a feeder, which is what a transformer requires. A motor starter allows for the thermal element to be a adjustable or replaceable by virtue of the "heater elements", feeders must have fixed trips*.

But IF your MCC was built with a Thermal-Mag breaker instead of an MCP (rare), or your MCC bucket uses a fused disconnect instead of a breaker and you size the fuses properly, the OL relay and contactor portion become irrelevant and you can use that starter bucket as a feeder. It's done quite a bit when feeding lighting circuits with a lighting contactor, but again in that case, the breaker becomes a Thermal-Mag version.

*(if using thermal elements, solid state feeder breakers are an allowable exception)