Secondary Protection of Utility transformer

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mull982

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This question is sort of along the lines of an existing post I have on secondary transformer protection however this one is focused on a utility owned transformer.

A utility padmount 500kVA transformer with has a 480V secondary feeding the main panel to building. The secondary rated current of this transformer is about 600A, however the buildings main panel and main breaker are rated 1200A. Although this 1200A is within the 250% rating of the secondary OCPD allowed by table 450.3(A) I dont know that this adequately protects the secondary of the transformer.

In the case of utility owened transformers is any consideration really given to protecting the secondary of the transformer since it is utility owned? Since it is a utility owned transformer does this even fall under the guidelines of the NEC?

Would you set the secondary OCPD to 600A to protect the transformer, or leave it at 1200A and let the utility worry about transformer protection?
 
Unless the utility says otherwise, I would not waste any effort to worry about their equipment at all. It is their responsibility and not yours. JMNSHO.

having said that, in a very technical sense, whether the utility owns the transformer or not does not actually matter as to whether it is covered by the NEC or not. I don't know how it is these days, but at one time it was not unusual for utilities to lease equipment like transformers to a facility that was covered in the NEC as well as supplying equipment for the same facility that was outside the scope of the NEC.
 
The utility conductors do not fall under the NEC. The FLA of the transformer is 602 amps. Forget it.
 
bob said:
The utility conductors do not fall under the NEC. The FLA of the transformer is 602 amps. Forget it.
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I notice that all of the load calculations for the building are based off of 1200A and cables between transformer and main panel are rated for 1200A. Would you set the main breaker for 1200A and not worry about it, since it would be the utilities problem?
 
mull982 said:
I notice that all of the load calculations for the building are based off of 1200A and cables between transformer and main panel are rated for 1200A. Would you set the main breaker for 1200A and not worry about it, since it would be the utilities problem?
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I'd size it for 1200 amps. Like others said, don't worry about the utilities transfromer - let the utility do that.

The utilities routinely undersize the transformers for the NEC load calculation. But they have the experience to do that, and they are the ones that have to replace the xformer if it goes bad or is undersized.

They could easily change it out for a larger transformer without telling anyone, so you don't want to size your stuff based on the utility transformer size.
 
Our XF's are sized with a diversity factor anywhere from 1.5-3.0, depending on what is being served.
Also the Bay-o-Net in many of the PMT's today do a fair job of protecting the whole XF.
Like others have stated, if the XF goes, it is the utilities resources being used for replacement.
 
Utility Transformer

Utility Transformer

Need to understand, your 1200amp switchboard, is based on National Electrical Code minimum Requirements....when you provide your electrical load letter to the Utility, they have their Engineering Department, and will analyze your service based on their collective data throughout the years...they do not have to comply with NEC....for all they care, they can provide you 300kva transformer or service...if their transformer fails, they can easily replace it with a larger one...so don't worry about it....
 
The only thing I would worry about is voltage regulation. If the actual load is closer to the switchboard rating, then the Xfmr will be overloaded. As others said, who cares from a protection standpoint, i.e. not your problem, except, the voltage could also be low at the switchboard, which could be your problem depending on your tenant/operation.

During start-up, check the voltage under normal operating load at the switchboard, if it's low, have the utility use the transformer taps to get voltage closer to nominal.
 
mull982 said:
... The secondary rated current of this transformer is about 600A, however the buildings main panel and main breaker are rated 1200A. Although this 1200A is within the 250% rating of the secondary OCPD allowed by table 450.3(A) I dont know that this adequately protects the secondary of the transformer. ...
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The building main panel and main breaker are not present for the protection of the transformer. Table 450.3(A) does not apply to the building breaker.
 
Yea, they size the transformer on what they project the actual average load will be, not for the maximum. I put in a 1200 amp single phase service on a store once (the owner did not want the expense of three phase, along with being charged a higher kw rate for commercial power), the POCO put up the largest pole mount transformer they had in the yard, and about a month later, it self destructed because they overfused it by so much. They brought in an even bigger one, and it's been there for 25 years now. The full load on the service during the peak summer months was around 650 amps per leg.
 
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