Transformer primary Breaker

[Electriman]

I have a 30 kVA lighting transformer 480v/120-208V that will feed only a 5 kVA lighting loads.
The secondary breaker is sized 100 A but since my load is limited and I have only a few options(spare breaker) I was wondering if I can use a 30 A breaker (the highest existing breaker) to protect the primary feed of the transformer; even though, the secondary is rated 100 A. Is there any code requirement that limits me?

 

[david luchini]

Yes you can. There is no minimum size requirement for protecting a transformer. Though if you make it too small, tripping on inrush could be a problem. The breaker must also be large enough to carry the load, but that's not an issue in your example.

 

[don_resqcapt19]

I have a 30 kVA lighting transformer 480v/120-208V that will feed only a 5 kVA lighting loads.
The secondary breaker is sized 100 A but since my load is limited and I have only a few options(spare breaker) I was wondering if I can use a 30 A breaker (the highest existing breaker) to protect the primary feed of the transformer; even though, the secondary is rated 100 A. Is there any code requirement that limits me?

No code requirement, but the inrush when you energize the transformer may trip the 30 amp breaker.

 

[iwire]

I have a 30 kVA lighting transformer 480v/120-208V that will feed only a 5 kVA lighting loads.
The secondary breaker is sized 100 A but since my load is limited and I have only a few options(spare breaker) I was wondering if I can use a 30 A breaker (the highest existing breaker) to protect the primary feed of the transformer; even though, the secondary is rated 100 A. Is there any code requirement that limits me?

The NEC will allow you to do that but you might have trouble trying to initially power up the transformer. The breaker might trip.

 

[under8ed]

I have pushed the limits of inrush on temporary transformers, found it is often not a problem until available primary current nears half of normal, (for ones near that size anyway). The BIG difference though is that I am using time delay fuses, and that there was no downside if it didn't work the first time. Believing this to be a 3 phase transformer; you are only asking to reduce full primary amps by 6, and I would expect this to power up Ok. You however; are the one to make the call as to what's at stake if the breaker does not hold.

 

[Pierre C Belarge]

Are you saying you want to use an existing spare circuit breaker rated at 30A in the primary supply?

Is this a MCCB?
If so, what is the condition of the existing breaker? Has it ever been through any fault conditions? Is it dirty? How long has it been sitting there - with no actuation of the switching mechanism? Do you know if it will operate properly? - under fault conditions?
To me, the cost a new breaker cannot be a deal breaker here.

In some jurisdictions, there are restrictions to using existing equipment (building codes) that would not permit the use of existing equipment.

Just thinking out loud......

 

[Electriman]

Are you saying you want to use an existing spare circuit breaker rated at 30A in the primary supply?

Is this a MCCB?
If so, what is the condition of the existing breaker? Has it ever been through any fault conditions? Is it dirty? How long has it been sitting there - with no actuation of the switching mechanism? Do you know if it will operate properly? - under fault conditions?
To me, the cost a new breaker cannot be a deal breaker here.

In some jurisdictions, there are restrictions to using existing equipment (building codes) that would not permit the use of existing equipment.

Just thinking out loud......

Yes it is a MCCB and it is functional it has never been used however it is a 30 year old breaker.