Sizing conductors and overcurrent protection on both the primary and secondary of a 45 KVA transformer

[Dennis Alwon]

So what would you use for the overcurrent protective device? 125% or like Texie stated

 

[winnie]

Thank you that is extremely helpful. It will be for a specific piece of equipment that will be turned on and off quite often but it is a very short supply run and there are no other Upstream Transformers. Now if I can only find a 200-amp breaker to bolt in the 400 amp panel I feel confident then will be sufficient

Is the 400A panel the service panel from the utility? What is the available short circuit current at the service?

But will the _transformer_ be turned on and off frequently? Or will the transformer be turned on and then the load turned on and off frequently?

-Jon

 

[Terry J Loiacano]

Yes before hundred amp panel is from utility, and correct and saying the Transformer will be loaded periodically not actually turned on and off. Do not know the available short circuit current

 

[winnie]

As you describe things, you have a pretty good risk of the breaker tripping on inrush. The larger the breaker the lower this chance. Unfortunately I believe that the largest breaker you can get for your panel is 150A, which _might_ be enough.

Tripping on inrush is a probabilistic thing, meaning that if you try once and the breaker trips, you might try again and it will hold. A breaker that trips once on inrush every 10 times you energize the transformer would be _horrible_ if you are energizing the transformer once per day, but totally acceptable if you are energizing the transformer once per utility power outage.

There are tricks for energizing transformers on breakers that are 'too small', but they require control circuits to 'precharge' the transformer through a resistor while the secondary load is disconnected, before you get to that stage you would probably consider replacing the 400A panel.

-Jon

 

[augie47]

Lots a variables.. and budget might be an unmentioned one.
If expense is a consideration you need to compare the cost of the 150 amp breaker (saying thats the max for your panel) to the alternative to adding more gear. Weigh that against the consequences if the breaker trips on inrush say after a power interruption.
Since it's marginal the decision comes down to "this might trip" vs "for ***$ more, we can significantly reduce the possibility of it tripping". Most folks I deal with would want to try the 150 amp.

 

[petersonra]

Lemme try:
Primary current = 45,000/(1.732X208) = 124.9A. 250% of that is = 2.5 X 124.9 = 312.3, therefore, use a 350A breaker!
Secondary current = 45,000/(1.732X 480)= 54.1A. 125% of that is = 67.6A, therefore, use a 70A breaker!

The next standard size provision does not apply to the primary.

 

[augie47]

The next standard size provision does not apply to the primary.

 

[Terry J Loiacano]

Yes 150 is the largest for that bolt in panel, but it is 22kva....will that help at all with the in rush current on start up

 

[petersonra]

Yes 150 is the largest for that bolt in panel, but it is 22kva....will that help at all with the in rush current on start up

Did you mean 22kaic? If so, the answer is no.

 

[Terry J Loiacano]

Yes

 

[electrofelon]

A few thoughts regarding getting a 200A OCPD for that panelboard: Have you talked to you sq D gear guy about options? I am not that familiar with SQ D panelboards, but it would think a 400A PB could take a 200A frame device. You might need some different parts or "fingers" than is currently installed (and enough real estate). Another probably simpler option is add some feed thru lugs and go to a safety switch usinga tap rule.

 

[texie]

Yes 150 is the largest for that bolt in panel, but it is 22kva....will that help at all with the in rush current on start up

The AIC rating of a breaker or panel has nothing to do with how much fault current there actually is.

 

[petersonra]

A few thoughts regarding getting a 200A OCPD for that panelboard: Have you talked to you sq D gear guy about options? I am not that familiar with SQ D panelboards, but it would think a 400A PB could take a 200A frame device. You might need some different parts or "fingers" than is currently installed (and enough real estate). Another probably simpler option is add some feed thru lugs and go to a safety switch usinga tap rule.

if you did that you could pretty much use anyone's enclosed CB. Might be nice to have a MCCB with an adjustable IT setting to feed the transformer.

 

[Terry J Loiacano]

Thank you for all your help guys, I finally was able to get into the enclosure adjacent to the 400 amp panel today and it's hey 400 amp fused disconnect oh, I believe I'm going to double tap the bottom side and feed another 200-amp fused disconnect