Primary Transformer undersized breaker question

[Shocking personality]

75 KVA 480V 3 phase primary to 120/208V 3 phase secondary transformer... Can you put the primary side on a 480v three phase 50A breaker? I believe the code lets me put it on a 125A but they want to use an existing breaker. I cant find where you are not able to use a smaller breaker, you just obviously will not get the full potential out of the transformer before it trips.. is there anything else this changes down stream? the secondary side still has a 200A main breaker panel on it. the load on the 200A panel will be about 35 amps. they went big on design and now are wanting to cut cost.

 

[ron]

With the FLA at 90 for the primary, I doubt the 50A breaker will stay closed when you try due to inrush, but nothing stopping you from trying as long as you are above the calculated load.

 

[MD Automation]

Just as a comparison...

We have a small number of 34 KVA Step Up transformers in our shops, 208 -> 480 VAC. Protected on the primary side by a 50A breaker. Oddly enough, in terms of magnetizing inrush current this is similar to your situation. Our 34 KVA Step Up is rated 94 FLA on our 208 primary and your 75 KVA Step Down is 90 FLA on your 480 primary. Our 50A breaker has always held on inrush. We help ourselves by having no load on the secondary when we energize these transformers.

Hope this helps.

 

[Jraef]

Just as a comparison...

We have a small number of 34 KVA Step Up transformers in our shops, 208 -> 480 VAC. Protected on the primary side by a 50A breaker. Oddly enough, in terms of magnetizing inrush current this is similar to your situation. Our 34 KVA Step Up is rated 94 FLA on our 208 primary and your 75 KVA Step Down is 90 FLA on your 480 primary. Our 50A breaker has always held on inrush. We help ourselves by having no load on the secondary when we energize these transformers.

Hope this helps.

It seems you are comparing primary with secondary currents. They are not the same.

In a 75kVA transformer, the maximum FLA of the transformer ON THE 480V SIDE is 90A, so a 50A breaker will trip unless you keep the load on the transformer under 5/9ths (56%) of the transformer's capacity. But even if you do, the magnetizing inrush current of that transformer, which is INDEPENDENT of load on the secondary, could be as high as 900A. A typical thermal mag circuit breaker has a mag trip setting of 10x the thermal rating, so a 50A breaker will have 500A mag trips (maybe less, never more). 900A inrush, 500A trip setting, 'taint gonna work.

 

[MD Automation]

Hi Jraef,

I was not considering any secondary currents in my comparison, only line side primary currents. I was just trying to describe a situation I have that might be similar to the setup the OP was asking about – in terms of inrush currents and primary breaker sizes.

I use a Step Up 34 KVA with a 208 VAC primary. Rated FLA is 94 amps on my 208 volt primary.

OP has Step Down 75 KVA with a 480 VAC primary. Rated FLA is 90 amps on his 480 volt primary.

So despite their different ratings, the inrush currents to magnetize each transformer I thought would be in the same ballpark. And my primary is protected by a 50A breaker, like the OP was asking about. And I have had good luck with my 50A breaker holding at startup (it has never tripped on me). And maybe it’s just luck – I know you roll the dice with residual flux and the phase angles when you re-energize in terms of brief core saturation.

As you point out, that breaker is certainly undersized in terms of the full rated load of each transformer, but that’s OK as long as the load on the secondary is low enough. OP indicates his secondary load is 35 amps (which is ~ 15 amps on his primary). So well under his 50A protection.

1000 pardons if I have gotten any of this wrong and am way off base. I will be the first to admit I have been wrong before!

 

[paulengr]

Older thermal magnetic breakers are tolerant of inrush of a cycle or two. So no matter what the ratings are they will frequently ride through at least the first hard cycle.

Newer electronic breakers will usually trip on the first half cycle. So often when replacing old breakers the new one trips intermittently or needs a 1-2 cycle delay on “instantaneous”.