Substation Tie breakers

[weressl]

Hi everyone,

I have a problem that I haven't encountered before. I've been doing MV substations for a little bit now, but one thing I've never seen is an undersized tie breaker.

I'm doing a Waste Water Treatment Plan upgrade. I may need to add another unit substation. There is one MV outdoor substation that feeds 4 unit substations, which are all located indoors. Every tie bus breaker (480V) in these substations is sized for half of the bus rating. I've always used fully rated tie breakers, but can't seem to find a reason to provide a tie half the size. The bus is rated for 2500A at 480V, the secondary breakers are rated at 2500A, but the tie bus breaker is rated 1200A.

What do you think? :?

480V is not MV, so I am not sure why you're bringing that issue in and so it becomes ambigous what your question pretains to.

There was a trend to do the half size breaker. The logic is that in a perfect world each half is sized to run =/< than half the load of the supply capacity on either mains, be it a transformer or other source. So when one of the source's is lost the tie-breaker closes and carries the 'half' of the available single main, so the tie breaker is sized to tha load.

Benefit is that you are only paying for a smaller breaker. Hopefully it is the same frame size as your feeder breakers, so you have a 'spare' frame, which brings us to the argument as why have a full size breaker in the tie position? Even though you do not need a full size breaker functionally in the tie position, it can provide you with a spare main, if one fails.

The load balancing requires more finesse. Example, we have A/B motors all over and it means that A AND B will never run at the same time for any appriceable time. Given the above scenario, A and B would be on different half of the buss and each bus load could vary above or below the 50% load, but the totalwould not exceed the 100%. Obviously half sized tie would not work in that scheme.