# 3 Phase Transformer load calculation



## TOOL_5150 (Aug 27, 2007)

I have a 45kva 208v Delta to 480v/277 Wye transformer. I know the load on the secondary is going to be 34A @ 480v. How do I calculate the current draw on the 208v primary side?

Also, there is only one thing hooked to this transformer, a heat pump, can I fuse it for the one load, or do I have to fuse it as if its at 100%, 45kva?

Thanks!


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## RePhase277 (Feb 5, 2008)

TOOL_5150 said:


> I have a 45kva 208v Delta to 480v/277 Wye transformer. I know the load on the secondary is going to be 34A @ 480v. How do I calculate the current draw on the 208v primary side?
> 
> Also, there is only one thing hooked to this transformer, a heat pump, can I fuse it for the one load, or do I have to fuse it as if its at 100%, 45kva?
> 
> Thanks!


34 x 480 x 1.732 = Load kVA = 28.266 kVA

28266 / 208 / 1.732 = Primary amps = 78.46 amps

You can fuse it for the load, but the conductors ahead of the fuses should be sized to the full rating. I'm not sure if that's a rule, but it's how I would do it.


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## frenchelectrican (Mar 15, 2007)

Rephase277 have correct answer on that.,,


But make sure you pay attetion to the nameplate of that heatpump unit to read the max fuse / breaker size.

there is some leeway on conductor size if this is the only load it is serveing .,, 

Look up in Art 430 and Art 440 it will mention very clear on that. 

make sure you use the correct fuse or breaker on primary side but just be aware of inrush current when you first kick it on ., ( the inrush current intended to be higher in step up mode )


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## TOOL_5150 (Aug 27, 2007)

Am I right saying the primary load on this trans if it was at 100% would be 124.91 Amps? [45000 / 208 / 1.732]

Right now the 208V circuit I will feed this transformer is a 125A feed. I think I will fuse the primary at 125A

The plate from the heat pump reads:
Max fuse / HACR Breaker - 45A
Minimum circuit ampacity - 33.9A


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## telsa (May 22, 2015)

The typical dry type.

Transformers are ALWAYS calculated as if the transformer is ultimately going to be totally loaded out.

No-one EVER assumes that they're the last fella to see the project.


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## brian john (Mar 11, 2007)

You need to check the maximum HP your transformer can handle at close to 100% loaded as you state, the inrush may exceed the instantaneous load the transformer can support


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## telsa (May 22, 2015)

TOOL_5150 said:


> I have a 45kva 208v Delta to 480v/277 Wye transformer. I know the load on the secondary is going to be 34A @ 480v. How do I calculate the current draw on the 208v primary side?
> 
> Also, there is only one thing hooked to this transformer, a heat pump, can I fuse it for the one load, or do I have to fuse it as if its at 100%, 45kva?
> 
> Thanks!


Pull your answer from the Ugly's tables.

Such an answer is NEVER calculated any other way.


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