277/480v to 120/208v Help me

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precise8128

precise8128

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Memphis, TN
I have a project I am working on and I could use some help on determining what size KVA transformer I need to order. Here is what I got:

A 400A Fused disco (parallel fed)using 4/0 AL going to a MLO 277v/480v delta Panel, I have 1) a compressor 480v 65A 50hp 2) a compressor 480v 65A 50horse Power 3) a dryer 480v 30A 4) a dust machine 480v 30A 7 hp 5) A crane that is 11.5 A and the lighting that will be 277v 20Amp

Then a 100 amp breaker that will feed a 3phase Transformer 277/480 to 120/208 v 100 amp panel that will only have receptacles on 2 breakers 20 amp.

I need help determining the KVA of the transformer I need.

I can provide more info if needed, Please help me work this out on paper thanks


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All the other stuff was smoke and mirrors until you got to
precise8128 said:
Then a 100 amp breaker that will feed a 3phase Transformer 277/480 to 120/208 v 100 amp panel that will only have receptacles on 2 breakers 20 amp.
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Do you want to size it based on the load expected from the (2) 20A circuits of receptacles or the 100A source breaker or the 100A 120/208V panel? Your choice.
 
ron said:
All the other stuff was smoke and mirrors until you got to


Do you want to size it based on the load expected from the (2) 20A circuits of receptacles or the 100A source breaker or the 100A 120/208V panel?
Click to expand...

Just maybe 50% of the 100A 120/208 Panel
More than likely there will not be any additional circuits added in the 100 A panel


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In that case, you could use a single-phase transformer (480 to 120/240 or 277 to 120/240) and panel. Two 20a circuits is a max of 4800va. To have double that capacity, you would still only need a 10Kva unit.
 
LarryFine said:
In that case, you could use a single-phase transformer (480 to 120/240 or 277 to 120/240) and panel. Two 20a circuits is a max of 4800va. To have double that capacity, you would still only need a 10Kva unit.
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Thanks alot


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I know I'm an A hole but I don't think you're qualified to be installing what you're installing if you're asking such a question
 
LarryFine said:
In that case, you could use a single-phase transformer (480 to 120/240 or 277 to 120/240) and panel. Two 20a circuits is a max of 4800va. To have double that capacity, you would still only need a 10Kva unit.
Click to expand...

If he goes that route, with a single phase 480 volt primary 10 KVA transformer, the max primary amps will be ~15, or 25ish with 277 V... He cannot protect that with a 100-amp breaker.

To the original poster, read up on Section 450.3(B) for determining your primary and or secondary ocpd sizes. Also read the code sections regarding disconnects and where they need to go, as well as working space. And make sure your transformer is suitable for the location.

If all you need is 2 20amp 120v circuits, a 5 KVA transformer is sufficient as you cannot continuously load those circuits beyond 4 KVA anyway (2*120*20*0.8=3840). A 480/120-240 single phase transformer of that size along with appropriately-sized fused disconnects will be markedly cheaper than a much larger 3 phase unit that would give you 50 amps at 208 volts on the secondary, which if I did the math in my head right would be 15-20 KVA.
 
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farmantenna said:
I know I'm an A hole but I don't think you're qualified to be installing what you're installing if you're asking such a question
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He may very well know how to install it, just not sure on the math.

would pointing out that the Forum rules prohibit questioning members qualifications make me a bottom side entry way as well?
 
farmantenna said:
I know I'm an A hole but I don't think you're qualified to be installing what you're installing if you're asking such a question
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I can assure you I am fully qualified, just need some clarification and socialize with some professionals who does this work everyday. I’m more of a resi guy, but I am dabbing more and more in the commercial game these days.


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electrofelon said:
Actually transformer sizing is a bit tricky. The code doesn't say how they are to be sized, so I can understand some confusion. Then you have 240.21 (C), 450.3 (b), and 408.36 that all need to be met.
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Absolutely I can definitely install the equipment just trying to fully understand the engineering side.


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