What breaker for 112.5 KVA 3Ph Transformer?

I'm installing a turning center that draws 90A 480V 3ph. I have 240V service, and I have purchased a 112.5 KVA 480/240 transformer. I'm thinking I could feed the transformer with the max of 250A in order to draw more than the 90A from it if I need to run other 480V devices in the future. To do that, do I feed it with a 250A breaker, or does the breaker need to be higher capacity than that to meet the 80% of max capacity figure? Are my numbers right on the input current?


What size wire do I need here? it will run part way in conduit, and part way in an attic, both to the transformer and to the machine. The transformer is on a concrete pad outside the shop. Is PVC conduit acceptable, or should it be metal conduit?

Since a transformer will have inrush current when power is applied, the breaker needs to be sized for the transformer, not the load, otherwise the breaker would most likely trip when you try to close it.

112.5kVA / 240V / sq root 3 = 271A.

If you chose an 80% rated breaker then you would need a 350A breaker (271A x 1.25 = 338A...350A is the next standard size I believe).

If you can find a 100% rated breaker then a 275A would work (not sure if that is a standard size though), or maybe 300A.

Either way I believe you will end up with a 400A frame breaker, as the standard frame sizes are 225, 250, and 400.

Not sure on the wire size...

John

Thanks John. Anyone else have advice?

Output load directly effects input current. Mr. Kessler is currently dis-allowing me to open my code book to spout the laws to you, but I would say you can go down in size. (from a 350a breaker) The field on something like that would likely not be able to nock over a 20a breaker, as long there is no secondary load. However once loaded everything changes (don't flip the breaker with all the motors in run). Does your turning center actually draw 90a or is that the name plate on a multi motor assembly? Either way 90a at 480 is ruffly 75kw * 1.25 still puts you at less than 100kva, unless your PF is wacked?. If I remember I'll dig through the code on that, but using my x-ray vision looking right through the first 2/3's of the book, out in the barn, I bet you can use a 250a. Seriously though, are you going to be pulling 90a?


Doug S.

Edit: A thought... ... ... You shouldn't be doing that sorta crap yourself. FWIW, that's some juice. Get a Sparky.

Yes, it pulls 90A. The nameplate says 90A FLI. I wired it up temporarily to a smaller transformer and put an amprobe on it, and as the spindle accelerates and decelerates it goes right to 90A, even without a chuck on it.

I have other transformers here, I'm used to the breaker occasionally tripping as it is switched on because of the inrush current.

I'm planning on mounting everything and pulling the wires, I intend to have my electrician connect it to the service. I was quoted $5800 to install it all, that's more than I want to spend to save some time.

Sorry for the delay in response, been busy and forgot about this one.

You can go up to a 350 with primary protection only, ((112.5k / 240 / 1.732) * 1.25 = 338 then up to next size aval.) With secondary protection you could go bigger, but I'm guessing you don't want to.

After thinking about it, I'd go with the 350, adjustable trip on the load side. This leaves you room to add something else to the x-fmr.
100 fused disconnect on the load side.


My 2¢

Doug S.

Thank you Doug S. I'm going to look for a 300A, because my service is only 400A, I doubt I'll need or be able to use more. What size wire will that need?

350's w/ a #4 ground.(sized at 75c)
Do have a sparky check your stuff out before you fork out the cash for wire. Most places don't let you return cuts.
Also care should be taken when working with wire that size, I know it's not gigantic, but fighting that stuff into today's microscopic enclosures can easily weaken/strip insulation. If you do do it your self, triple check the xfmr connections prior to turn on. They don't let smoke out quietly!

Mud.
Our Carrier units are limited by software to 21A on 460 and protected by 32A breakers. They almost never trip. So if its legal I'd multiply by 5 for 105 amp limit, 160(150) breaker. A 300A breaker as mentioned previously won't be cheap.
Average running current is under 16A each if that helps.

John