transformer ocd question

[orlandosvt]

I have a customer who has a 200 amp single phase 120/240 service who wants to add 3 480 volt machines, 1 rated at 71 amps, 1 at 48 amps, and 1 at 46.5 amps, he has a 112.5 kva transformer with 120/208 primary and 277/480 secondary's he wants me to install for the equipment, I have several questions regarding this installation, do I have to feed the transformer at its kva rating or the actual load connected to it, I want to build a new 400 amp 3 phase service leaving the existing 200 amp single phase disconnect and panel as is, and add a 200 amp 3 phase disconnect to feed the transformer covering the load rather than with a 400 to cover the kva rating, is this code compliant? I will install a 200 amp mcb panel on the secondary side to feed the machines. Thanks

 

[augie47]

In regard to your need to feed the transformer at it's full rating, the answer is no, however, if my math is correct, the 112.5 kva is undersized for your load as it is.
I also don't think it would be advisable to feed a 208 transformer with a 240 volt supply.
I would also question your 200 amp MB secondary panel IF any of these loads have a significant motor factor.
It appears to me to be "back to the drawing board".

 

[kingpb]

Exactly how is that you plan on generating 3-phase out of a single phase supply?

 

[david luchini]

Exactly how is that you plan on generating 3-phase out of a single phase supply?

I read that he is going to install a new 3 phase service and leave the single phase service in place.

I want to build a new 400 amp 3 phase service leaving the existing 200 amp single phase disconnect and panel as is

I don't know why he wouldn't just make the new service 480V.

 

[kwired]

I think he intends to install a 480 volt service and supply the existing single phase from the transformer mentioned, but wants to leave it pretty much intact, so he basically will not be using one of the phases of the secondary to supply any load.

I think he should find a way to balance the load on this transformer myself, and likely can use a smaller transformer if he does so.

Otherwise why not just install a single phase transformer to supply the existing load, unless the existing load is near full 200 amps, he may only need a 25 or 37.5 kVA transformer.

 

[gar]

131024-0925 EDT

The original post is totally confusing. The service is apparently single phase. At 240 V you might classify it as having 200 * 240 = 48 kVA capability. But is there really a 50 kVA transformer on the pole. At 120 V the capability is possibly 24 kVA, but still possibly pole transformer limited because you do not want all the load on 1/2 of the secondary.

Are the referenced loads single phase or three phase? Big difference in current and total kVA. Later there is a discussion of three phase. How does this relate?

If one fed a single phase transformer of 120 V @ 112 kVA rating (1000 A) from a 200 A and 120 V portion of a 200 A panel, there is a total disparity. Just the magnetizing current of such a transformer would be large.

Furthermore you may not want to backfeed a standard stepdown transformer because of the turns ratio.

Then as augie47 pointed out 240 into 208 is a problem.

Nothing seems to correlate. The original post needs clarification.

.

 

[Jraef]

...
Nothing seems to correlate. The original post needs clarification.

Lots of it too...

My interpretation:
1) has 112.5kVA 240V single phase transformer (maybe utility pole?), feeding a 200A Main Service panel for 120/240V existing loads.
2) wants to install a NEW 400A 240V 3 phase service to provide power for the new machines, PLUS take over feeding the exiting 120/240V loads.
3) plans on re-using the existing service panel as a (now) sub panel to the new 400A 240V 3 phase service, wants to feed it off of the new 200A disconnect
4) plans on a new 200A 3 phase service panel for the new 3 phase service as well
5) wants to know if that is OK

Missing:
A) where the 480V is going to come from.

My questions:
a) why a 200A disconnect and worry about the 6 hand rule issues with the other service panel, why not just put in a 400A service panel and feed the old service with a 200A feeder breaker in it? Cleaner that way.
b) where IS that 480V going to come from? If you were planning on stepping it up, you might have to over size the boost transformer to allow for any motor starting capacity. Why not just get a 200A 480V service drop and tap off to transform DOWN to 240V single phase for the existing service? Fewer issues with motor starting inrush and having to over sizing the boost transformer.

 

[kwired]

After reading OP again ...

I have a customer who has a 200 amp single phase 120/240 service who wants to add 3 480 volt machines, 1 rated at 71 amps, 1 at 48 amps, and 1 at 46.5 amps, he has a 112.5 kva transformer with 120/208 primary and 277/480 secondary's he wants me to install for the equipment, I have several questions regarding this installation, do I have to feed the transformer at its kva rating or the actual load connected to it, I want to build a new 400 amp 3 phase service leaving the existing 200 amp single phase disconnect and panel as is, and add a 200 amp 3 phase disconnect to feed the transformer covering the load rather than with a 400 to cover the kva rating, is this code compliant? I will install a 200 amp mcb panel on the secondary side to feed the machines. Thanks

I think he has existing 200 amp single phase service, needs to supply some 480 volt equipment (likely three phase equipment). Customer has a transformer (probably sized larger than necessary) that can step up from 208 to 480/277. He wants to install a new three phase service (not sure if it will be 208 or 240 volt) and supply the existing single phase from that new service and an additional 200 amp supply to the transformer, even though it is able to handle 312 amps and would need a 400 amp switch if using full capacity.

 

[david luchini]

After reading OP again ... I think he has existing 200 amp single phase service, needs to supply some 480 volt equipment (likely three phase equipment). Customer has a transformer (probably sized larger than necessary) that can step up from 208 to 480/277. He wants to install a new three phase service (not sure if it will be 208 or 240 volt) and supply the existing single phase from that new service and an additional 200 amp supply to the transformer, even though it is able to handle 312 amps and would need a 400 amp switch if using full capacity.

I think you are close...

If the 480V equipment is all 3 ph, then the customers transformer is too small. If the equipment is all single ph, the customers transformer would be OK.

It appears he wants to leave the existing single phase service as is.

leaving the existing 200 amp single phase disconnect and panel as is

Whether the 480V load is single phase or 3 phase, feeding the step-up transformer with a 200A feeder will not be sufficient for the load.

 

[kwired]

If the 480V equipment is all 3 ph, then the customers transformer is too small. If the equipment is all single ph, the customers transformer would be OK.

Good catch - that transformer secondary is going to be good for 135 amps, his wanting to feed it with a 200 amp supply could still be too little.


I think he would be wiser to install a 480 volt service and transform to supply the existing 120/240.

 

[kingpb]

Y'all sure do a lot of speculating...

 

[david luchini]

Y'all sure do a lot of speculating...

Not a lot, reallly. The OP said he had an existing 200A single phase service that he wanted to leave as is.


He wants to install a new 400A 3 phase service and install a 208-480/277 step-up transformer with a 200A, MCB panel on the 480V secondary to supply the new 480V equipment.