Transformer Connection

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MichaelW

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I have a service which is 480V three phase with one conductor grounded, no neutral.
This is primarily going to be used for motor loads.
I have installed a transformer: 480V to 208/120V Wye. (45KVA)
My problem is that when the transformer is energized, with no load, it trips the main, which is adjustable.
I don't know if that ground fault is being picked up because XO is bonded on the transformer. Any suggestions / solutions? Thanks and God bless!
Michael
 
Re: Transformer Connection

What size breaker is feeding the transformer? Do you have a single main device on the secondary of the transformer?

It is not uncommon for an unloaded transformer to trip a primary protective device sized at 125% of full load amps.

Do not confuse your primary grounded pahse connection with the secondary grounded wye connnection. They do not affect each other.
 
Re: Transformer Connection

If he has a 70a breaker with a 150degC transformer he shouldn't have a problem. There may be a problem with a 115, 80degC, and K-factor transformers which often times are on the ragged edge of a 70a breaker with 700a mag calibration.
If he is using a 70a rated breaker the panel may also limit the frame rating of the breaker to more than 100a if one tries to opt for that 250% rule should sec. protection be provided.
With 480v, 75kva transformers people commonly back them into a corner when they don't plan properly and try to feed a transformer rated at 90fla from a panel that you can't install over 100a rated branch breakers in. At least there is some head room with a 45kva transformer.
 
Re: Transformer Connection

Michael,
Please keep all of the question discussion material on the open forum. That way eveyone can participate.

In your PM you said:
Thanks for the reply.
The transformer is feeding a load center, used mainly for lighting and convieniece outlets.
It is protected on the line side with a 60A breaker and on the load side with a 100A main on the load center. Nothing has been energized on the load center. In fact, nothing has been energized on the whole building until now. After 35 years in this business this old dog is always ready to learn new tricks, but I also may be overthinking it.
The main on the service panelboard, the one that is tripping, is 200A.
Click to expand...
Does your 200A have an adjustable magnetic trip setting? If so what is it set at? I would expect the transformer inrush to be about 650A but it depends on if it is an energy effcient design
Does the 200A breaker trip when the transformer is energized with a load connected?
Have you tested the transformer for faults?
 
Re: Transformer Connection

Actually, I just found out that the trip module in my main was bad. That's where I've been. Will let you know what the outcome is.
Now I can stop beating myself over the head! Thank you all for the input.
God bless!
Michael
Sorry about answering through the private messsage.
This is my first time on here and I was not familiar with the protocal.
 
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