15KVA Transformer

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Dexie123

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Question....

Elevator machine room. 480v/277 to 208/120 transformer and it's 15kva.

I have the primary amperage at 18amps plus 125% and the secondary at 42amps plus 125% which puts you at 52amps.

Can the secondary terminate to a 100amp panel? This is all in the same room and within 10' or so of each other. In this situation the primary is protected from OCPD of 30amps and 10 wire and the secondary is loaded with #2's to 100amp main disco....
 
450.3B secondary protection not required if primary is at 125%. question is, if he provides an ocpd, must it meet the table. or to put another way, why not protect according to the table, otherwise just use MLO panel.
 
That's exactly what I'm asking nakulak.

The primary is the protection. The secondary 100amp I believe is just a switch but the panel is a 42 circuit. I suppose as long as you don't exceed the calculated load but doesn't seem right to me....
 
nakulak said:
450.3B secondary protection not required if primary is at 125%. .
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That only applies when you have a 2-wire primary 2-wire secondary, or a 3-wire to 3-wire.Otherwise you protect the secondary at 125%.
 
RUWired said:
That only applies when you have a 2-wire primary 2-wire secondary, or a 3-wire to 3-wire.Otherwise you protect the secondary at 125%.
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No, you do not have to protect the secondary of transformer.

You will have to protect the conductors.
 
brian john said:
I assume this is 3 phase 480 to 208/120?
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Yes it's 3 phrase

Iwire the conductors are protected for their ampacity.

But does it need to terminate in a 60amp OCPD or is 100amp ok?
 
One thing I never understand in reading posts or the code is how do u protect the conductor without protecting the transformer? If you put a OCPD in to protect the conductors, wouldn't it protect the transformer, or vise versa? I realize there is much more to protecting a transformer than what the code requires, and if the transformer is protected correctly, will this normally protect the conductors?
 
rdelauter said:
... and if the transformer is protected correctly, will this normally protect the conductors?
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In general, no, the transfomer protection will not protect the conductors. You just have to remeber that the rules in 450 are only for the protection of the transformer itself and you have to look to 240 for the rules to protect the conductors.
 
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