Transformer sizing

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Tyler1931

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Hello all,

Looking to see if I am selecting a transformer correctly.

We are wanting to use a 208 3 phase service into a transformer to step up to a 480 3 phase output.

We want to run a 100A 480v machine off this transformer.

From using an online transformer calculator and sizing chart it seems a 112.5KVA step up transformer should work.

Am I correct in thinking this? Also, would the transformer require a fused disconnect in line before the transformer?

Thanks for any input!;

Sent from my moto g(7) supra using Tapatalk
 
112.5 would be the correct choice in most situations (a large motor might make a difference)
No primary disconnect is required AT the trannsformer IF the out of sightydisconenct is lockable
 
augie47 said:
112.5 would be the correct choice in most situations (a large motor might make a difference)
No primary disconnect is required AT the trannsformer IF the out of sightydisconenct is lockable
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How do you arrive at 112.5 kva?
 
It's hard to say without knowing more about this machine, but 112.5 MAY be oversized making 75 KVA a better choice. As a rule of thumb, I would be surprised if that machine Drew more than 2/3 of its stated amps. 112.5 is certainly "safe" but the best analysis would take into account load losses, no load losses, duty cycle of the machine and efficiency at different percent loadings of the transformer.
 
Have you given thought to the size of OCPD required on the primary of this transformer, and compared it to the existing service size?

Remember that transformers often have significant 'inrush' current and you can easily end up in a corner case where the OCPD needed to reliably 'start' the transformer is greater than the service size, even if the service can support the actual connected load.

See https://forums.mikeholt.com/threads/3ph-transformer-blows-breaker-on-power-up.2564416/ for a thread on the nightmare that this can be. (Note: often there is no problem. I've personally used a 75 kVA step up transformer with OCPD sized for 50 kVA continuous loading and not had a problem.)

This may be another reason to suggest using a slightly smaller rather than slightly larger transformer.
 
You need to investigate the 480V machine. Is it one motor with 100A FLA? Multiple smaller motors and/or non-motor loads adding up to 100A? How are the motors (if any) started? What else is going to need to be powered?

You cannot approach this blindly. For example even if you go with the 112.5kVA transformer, if it is a single motor, that may not be big enough to handle the starting current without creating a deep voltage drop. If it is across-the-line and you use a 480V delta transformer, you will need to corner ground the 480V side or add ground fault monitoring. But if it is on a VFD, it might be fine at 112.5kVA, but you may need to make sure the 480V side is a Wye with a grounded X0 terminal. Too many details to consider without investigation.
 
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