600 volts or less dry type 3 phase transformers

[Pat@leict.com]

whats is the maximum overcurrent protection allowed on a 45 KVA transfromer for the primary and secondary.

 

[charlie b]

Is this a sample test or homework question? How about giving us your answer first, and we'll tell you what we think.

By the way, the answer will depend on the voltage levels on both the primary and secondary. Since you didn't tell us those numbers, we couldn't give you an answer anyway.

 

[raider1]

Welcome to the forum.

I agree with Charlie,

If this is a homework or test question let us know what you have come up with and we will help you get the answer.

Chris

 

[jim dungar]

The answer will also depend on if the primary device is protecting both the primary and secondary or if there are separate primary and secondary devices.

The NEC section, 450, that applies to transformers uses %FLA so the answer you are looking for may not be in actual amps.

 

[Pat@leict.com]

Well here is the deal, I have been having conversations with an Engineer who is feeding a 45 KVA Transformer 480v/208v (Aluminum windings) with an 100 amp primary feed and secondary with an 150 amp load MCB panel board.

Based on what I can tell is the maximum line side 54 Amps and load side is 125 amps. He is telling me that he can oversize the primary side by 125% and oversize the secondary side at 175%. I have reviewed the code and Table 450.3(B) which I found in autotransformers section states that 250%. I think he is mistaken using this chart. Just asking for some help?

 

[Pat@leict.com]

No homework or test making sure the building does'nt burn down..

 

[charlie b]

Thanks for clarifying the intent of the question. Please forgive our having questioned your intent. You would be surprised how often we get students who want us to do their homework for them.

You have the primary and secondary currents right. You can size the primary protection at 250% of the rated 54 amps. So the selected size of 100 amps is acceptable. You can size the secondary protection at 125% (not 175%) of the rated 125 amps. So the selected size of 150 amps is acceptable. It should be noted, however, that the table you cited does not apply to autotransformers, and your application is not an autotransformer.

 

[jim dungar]

The rules are:
Primary protection sized at no more than 125%FLA, then no secondary protection required, therefore secondary device can be any size, even 400%. But, the primary device can be up to 250% FLA if the secondary device is not more than 125%.

So, based on your information your sizing is correct.,
100A primary device is more than 125% of 54A but less than 250%, so secondary device is required and limited to 156A (you can not round up to the next breaker).

 

[Pat@leict.com]

Thank you guys, i learned something

 

[Pat@leict.com]

Transformers

Transformers

do you know whee this is located in the code??

 

[jim dungar]

do you know whee this is located in the code??

section 450.3(b)