75 KVA transformer

[Gategator37]

I know the typical industry installation is to size the primary OCPD of a 75 KVA transformer at 125 amps and the Secondary at 300 amps. I currently have a set of drawings that is showing the Primary OCPD being sized at 90 amps and the Secondary at 200 amps. It looks like the designer sized these breakers and conductors to the load it will be serving rather than the size of the transformer. Is the designer ok by doing this, or should it be done at 125 amps and 300 amps.

Thanks in advance.

 

[Gategator37]

Sorry, left a big peice of information out. The Primary is 480v Delta and Secondary is 208v Wye.

 

[iwire]

The 90 may work fine or it may trip due to inrush when energized.

 

[raider1]

The designer is well within his element to design a 75 Kva transformer with a 90 amp primary overcurrent protective device and a 200 amp secondary overcurrent device. Where the secondary is protected at a lower rating as well as the primary I doubt that there would be an inrush issue with the transformer.

Chris

 

[iwire]

Where the secondary is protected at a lower rating as well as the primary I doubt that there would be an inrush issue with the transformer.

The inrush is there even with the secondary open.

 

[raider1]

The inrush is there even with the secondary open.

I agree, what I was trying to get at (and doing a poor job of it) is that most likely the secondary load is lighter if the designer went with smaller breakers on the secondary side.

Chris

 

[jim dungar]

I know the typical industry installation is to size the primary OCPD of a 75 KVA transformer at 125 amps and the Secondary at 300 amps.

This is not an industry standard, although it may be a regional one.

I currently have a set of drawings that is showing the Primary OCPD being sized at 90 amps and the Secondary at 200 amps.

This is by far and away the typical installation, for the past +30 years) in Wisconsin, although 100A on the primary and 225A on the secondary is becoming a close second.

 

[iwire]

This is by far and away the typical installation, for the past +30 years) in Wisconsin, although 100A on the primary and 225A on the secondary is becoming a close second.

Here if an EC has designed it you will typically find a 100 on the primary and a 225 on the secondary.

If an EE has designed it it will be a 125 - 150 on the primary and 225 or 300 on the secondary.

 

[Ragin Cajun]

Personally I tend to size the primary protection above the 125% figure to assure it will not trip on inrush, which is greater with the new energy saving transformers. I don't go any less than 150A on the primary for a 75 KVA transformer, but I often put a 225A on the primary and use 300A on the secondary.

Note that 125% on the primary is 113A so using a 100A breaker in inviting trip on inrush.

RC

 

[radiopet]

One little point ( simply to add to the other comments ) is remember that the heading of table 450.3(B) says "Maximum Rating or Setting of Overcurrent Protection for Transformers" and thats what it means. So typically, I would OCPD the primary at 125A and the secondary OCPD at 250A.....thats my typical preference ( as if you care...lol )

So if so designed it can be less than the Maximum....but as others stated you have the issue of inrush when you go smaller. The engineer should know the loads he/she is dealing with and it should be on the drawings.

 

[pfalcon]

*sigh*

Primary
Line current : 90A
125% : 113A nsu 125A
Maximum : 250A

Recommendation for primary on a transformer is to start at 125% to avoid nuisance trips during inrush. The inrush is there even with no load attached and is relative to the transformer size. DO NOT REDUCE the primary OCPD based on the load. OCPD adjustments should be based on the transformer inrush characteristics. There is no minimum requirement except that it trips.

Secondary
Maximum line current : 208A
125% : 260A nsu 300A

Recommendation for secondary on a transformer is 125% of load amperage. Adjust up or down based on inrush characteristics of the load.