IN RUSH RATING FROM TRANSFORMER TRIPPING MAIN AT SES

[rgoodwinelectric]

I've installed a transformer and when I've tried to power it on it trips the primary breaker and also the fuses at the SES. Not sure if this had something to do with the in-rush current so I requested a cut sheet from Federal Pacific (attached). When I powered the transformer on I had no load applied and the main and all branch circuit breakers were off in the secondary panel. I'm certain the install and wiring are correct, but am unsure of the math of the in-rush current here and if the main fuses can handle it. I've attached a spec sheet for the fuses also (JDT 200 Amp). Any help is greatly appreciated on this one.

Thanks!

 

[Phil Corso]

RGoodwineElectric..

The key is to determine if the breaker will withstand the inrush for a specific time! A good start would be, say 0.1 sec!

Regards, Phil corso

 

[templdl]

I've installed a transformer and when I've tried to power it on it trips the primary breaker and also the fuses at the SES. Not sure if this had something to do with the in-rush current so I requested a cut sheet from Federal Pacific (attached). When I powered the transformer on I had no load applied and the main and all branch circuit breakers were off in the secondary panel. I'm certain the install and wiring are correct, but am unsure of the math of the in-rush current here and if the main fuses can handle it. I've attached a spec sheet for the fuses also (JDT 200 Amp). Any help is greatly appreciated on this one.

Thanks!

I reread your post a couple of times and you have included no information about the name plate rating of the transformer such as kvs, pri and sec voltages, dry of liquid cooled, is it energy efficient, does ithave a k factor rating? The only thing that I would goes at iais that it is a dry type.
Knowing the KVA and the price and sec voltages would at least give some direction as to what the sec OCPD rating would be and the kva woud provide some direction as to what the magnetizing inrush current may be which then could be compared to the mag pickup of the project. OCPD is..

 

[topgone]

I reread your post a couple of times and you have included no information about the name plate rating of the transformer such as kvs, pri and sec voltages, dry of liquid cooled, is it energy efficient, does ithave a k factor rating? The only thing that I would goes at iais that it is a dry type.
Knowing the KVA and the price and sec voltages would at least give some direction as to what the sec OCPD rating would be and the kva woud provide some direction as to what the magnetizing inrush current may be which then could be compared to the mag pickup of the project. OCPD is..

The OP posted an attached pdf and I guess here's what you wanted:

1 Phase, 60 Hz, 50 KVA
208x104-120/240
150 C Rise, Alum
10/10 kV BIL, +2, -2 x 2.5% taps​

 

[GoldDigger]

Make sure that the transformer is being used in the "forward" direction. Otherwise the initial current may be far higher than expected.

 

[templdl]

The OP posted an attached pdf and I guess here's what you wanted:

1 Phase, 60 Hz, 50 KVA
208x104-120/240
150 C Rise, Alum
10/10 kV BIL, +2, -2 x 2.5% taps​

Thanks for pointing this out.I did a study regarding transformer inrushes as requested of me by one of my DTDT marketing managers to compare then to the mag. pickups of what the pri. OCPDs would be. It looks like the price. FLA is 208.3a. When a sec. OCPD is required which is is this case the OCPD. Per NEC art 450 table 450.3(B) the pri. OCPD can be sized up to 250% of 208.3a or 520 75a and as such a 500a rated breaker is allowed. This breaker would probably have an adjustable mag pickup of 5- 10x the breaker rating or 2500- 5000a with a +-20% calibration range making the max setting to be up to 6000a.
For a 150degC rated transformer of std efficiency the expected inrush would be about 11x the pri FLA or 208.3x11= 2296.8 and I would suggest a 300a rated OCPD with the mag set to the 10x position should work just fine. But you may consider a 250a rated breaker which would keep you in the 250 frame size for a max mag pick up of 2500aa which I think would cover you. The next lower frame fra!me size is 150a which is not even reasonable to consider.
Remember that the pri. OCPD does not protect the transformer as the sec. OCPD provides that. The pri. OCPD takes the transformer off line should there be a transformer failure.
Also remember that studng transformer inrushes in not an exact science as some would expect it to be, that is those who were expecting exact values because they are not. Your goal is to anticipate where the values may be such that you can stay as for away from those values as practical. Remember that even the calibration of the allowable mag pickup of the breaker will vary. You just want to make darned sure you stay away from trying to "fine tune" the protecting because that isn't going to happen.