Switchgear vs. MCC

[MD88]

Hello,

I'm trying to figure out the best way (cheapest, while still safe) to do the following thing:

I have 5kV coming in to a room, and I need 1 5kV motor starter and a 4160/480V transformer for other loads. Now, the classic way to do this is to get 5kV switchgear, feeding the transformer and the starter. So I'd need an main breaker and 2 feeder breakers in the switchgear, plus an MCC with a 5kV starter.

But I was thinking, what about having the 5kV feed right into a 5kV MCC with a main breaker, 1 starter and a feeder to the transformer. It seems like it'd be quite a bit cheaper, take up less room and work just fine! I have a feeling there must be a reason why this wouldn't work, but please explain! Thanks

 

[rcwilson]

5 kV MCC as Switchgear

5 kV MCC as Switchgear

That's a common way of doing it.

Use a latched contactor to feed the transformer. It is the same as a motor starter but it is mechanically latched so it doesn't drop out on loss of power. The fuses protect for short circuits, a relay trips the contactor to provide overload and ground fault protection. Note that you are not using a motor starter to feed a transformer even though they look almost identical and have the same parts.

Depending on your load, you may be able to use a fused disconnect switch for the incoming main breaker or even an 800A latched contactor.

If you want to really cut costs, use the feeder breaker back at the 5 kV switchgear as the main. You can run a close and trip circuit from the room back to trip the feeder breaker if someone wants control in teh room. Lockout is a little more complicated because the electricain has to walk back to the gear to place the lock, but it is still safe.