Help solve a debate between my twin brother and I (both electrical engineers) read message below for details

[PE (always learning)]

My twin brother and I are both electrical engineers and sometimes get into some fierce arguments when it comes to design.
Today we had an argument on primary breaker sizing for a 75 kVA transformer. The transformer was feeding a secondary 200 amp main circuit breaker panel at 120/208V, 3 phase, 4 wire and the primary was 480V, 3 phase and he used a 125 amp breaker (which is fine, but read on)

He told me that it is industry standard to size the overcurrent protection on the primary at 125 amps in this scenario to which I disagreed with. I told him that a 100 amp or even a 90 amp breaker would be fine in this scenario based on table 450.3(B). 450.3(B) clearly states that the 250% is a maximum overcurrent protection for transformers 1000 volts and less, but does not state minimum protection.

I know that having a higher primary breaker size is always nice in a scenario like this to avoid tripping from transformer inrush, but I have looked at the actual transformer inrush charts for many transformers and the real values are always well below the breaker trip curve values in the instantaneous region.

Again, I go back to his argument about using a 125 amp breaker as industry standard to which I say no. I say using a 100 amp breaker would be just fine and would probably save you money as usually going from 100 amps to a 125 amp breaker leads to using a sub fed breaker which costs more money.

Anyway, let me know your thoughts so we can know who wins.

 

[Eddie702]

I think either one meets code. I think most would protect it at 125 amps.

 

[PE (always learning)]

I think either one meets code. I think most would protect it at 125 amps.

I used to think that way until I saw the actual trip curves and the real inrush points plotted by the manufacturers. The transformer inrush is honestly typically well below a normal 90 amp breaker.

 

[Little Bill]

My twin brother and I are both electrical engineers

I bet you two were "womb" mates!

 

[hillbilly1]

Usually a 200 amp frame breaker has an adjustable magnetic trip. I’ve seen quite a few incidents where they would trip if the transformer was backed up by generator. The breaker would trip when transferred back to utility. Turning up the magnetic trip usually fixed the problem. 100 amp frames usually don’t have that adjustment.

 

[jaggedben]

Sorry but where's the actual disagreement? He says 125% is industry standard. You say it doesn't need to be industry standard because a lower rating can work. These aren't actually contradictory, you could both be right.

 

[jim dungar]

In the old days, most molded case breakers were limited to 100A frames so this was a very common size for feeding 75kVA transformers. I know it was my standard design size.

 

[PE (always learning)]

Sorry but where's the actual disagreement? He says 125% is industry standard. You say it doesn't need to be industry standard because a lower rating can work. These aren't actually contradictory, you could both be right.

haha, yea I guess we are just arguing about nothing, he just didn't want to admit that I thought 100 amps would be a better idea because you wouldn't force the contractor into upgrading to sub feed breaker at 125 amps, which cost more.

 

[Fred B]

Generally Must protect wire at a given at no more max current rate of wire used.
Biggest issue I've seen, is all component parts not providing adequate SCCR for potential provided by the transformer AFC.

You can size wire to maximum OCD at panel or reduce OCD for a reduced wire size, Either will protect the wire.
A 125A panel can have a 125A OCD or a 60A OCD doesn't matter, you just can put in a 150A OCD into the 125A max panel (of course if upstream protection provided at 125A the 150A doesn't mater, but not the case under discussion) it's about protecting the wire feeding the equipment.

 

[Fred B]

One issue I see could be if using the 100A OCD rather than the 125A would be if the loads are greater than the 100A would allow without tripping.

 

[JoeStillman]

The default inrush point on SKM is 1200% @ 0.10 seconds. I was hard-pressed to find a 100A MCCB with instantaneous to the right of that. Where did you get these manufacturer's "transformer inrush charts"?

 

[Johnhall30]

I will typically size the primary of a 75kva XFMR at 125 Amps

 

[charlie b]

Let me see if I understand the argument. He said, "The sky is blue." You replied, "No, you are wrong. The grass is green." Do I have that right?

1. I agree that 125 amps is industry standard. I've never seen any other size used for this application.

2. I also agree that a 100 amp would "likely" be successful. I say "likely" because we, as engineers, generally have very limited control over the exact makes and models of components that the contractor purchases based on our drawings and specifications.

3. I have nothing to offer on the question of relative costs.

4. If you called for a 90 amp breaker, you would be getting the blame for what will almost certainly be occasional (if not downright numerous) unnecessary trips.

5. A PE's job includes protecting the client's interests. That does include costs, but it also includes convenience (e.g., future operational and maintenance considerations).

 

[PE (always learning)]

The default inrush point on SKM is 1200% @ 0.10 seconds. I was hard-pressed to find a 100A MCCB with instantaneous to the right of that. Where did you get these manufacturer's "transformer inrush charts"?

I will show you, the 1200% is completely off when comparing most 75 kva transformer inrush to the charts. give me a min to pull up the siemens charts, I have a meeting

 

[ron]

During the design stage, when I have no idea which of the 3 or 4 manufacturers will be chosen to supply the project with equipment, it is not worth the headache associated with a potential for inrush trips to be tighter than 125% of FLA (I round up from there). Why take the chance with transformer manufacturers trying to design to DOE 2016 efficiencies (and inevitably DOE 2024 - Distribution Transformer Efficiency and Supply Chain Reliability Act of 2024) that they may be higher than 12xFLA or the breaker that is installed is slightly more sensitive than the TCC.

 

[jim dungar]

I will show you, the 1200% is completely off when comparing most 75 kva transformer inrush to the charts. give me a min to pull up the siemens charts, I have a meeting

I agree.
The SKM default is most appropriate for larger, say >500kVA, distribution transformers than it is for small panel board size ones.

 

[ptonsparky]

The advantage of the 90 or 100 would be cost. Once it was energized you may never have an inrush issue again. Well until that power outage.

 

[electrofelon]

I say using a 100 amp breaker would be just fine and would probably save you money as usually going from 100 amps to a 125 amp breaker leads to using a sub fed breaker which costs more money.

I think you are off base on this. I am pretty sure every manufacturer has a 125A three pole panelboard breaker. For siemens its an NGB or a 3VA41. For a square D NF panel its an EDB. For GE its a TEY Not real familiar with eaton but Im sure its the same for them. I didnt price anything out, maybe there is a big price jump but since its the same frame I wouldnt think it would be that much more.

 

[PE (always learning)]

Gentlemen,

Please reference the following:

1) Siemens Series J Typical Performance Data for an Aluminum 3 phase dry type K1 without electrostatic shield transformer at 150 degree temp rise. They have produced charts with the absolute peak inrush and the practical max inrush in amps. I have clouded them for reference. SKM typically assumes that the inrush is 12 times the FLA of the primary current which would put you at 90.317 amps x 12 = 1083.815 amps of inrush current. Now compare that to the absolute peak inrush of the 75 kVA transformer produced by Siemens, which clearly shows 578 amps of maximum. This is half as much as what you would normally assume for inrush. The practical inrush is even less at 193 amps.

2) Reference a time current curve plot I've created using Siemens absolute peak inrush vs 90A breakers of several different manufacturer's.
You will see that they all clear the inrush with no issue. Again, this is at 90 amps!

Just food for thought....

 

[PE (always learning)]

Gentlemen,

Please reference the following:

1) Siemens Series J Typical Performance Data for an Aluminum 3 phase dry type K1 without electrostatic shield transformer at 150 degree temp rise. They have produced charts with the absolute peak inrush and the practical max inrush in amps. I have clouded them for reference. SKM typically assumes that the inrush is 12 times the FLA of the primary current which would put you at 90.317 amps x 12 = 1083.815 amps of inrush current. Now compare that to the absolute peak inrush of the 75 kVA transformer produced by Siemens, which clearly shows 578 amps of maximum. This is half as much as what you would normally assume for inrush. The practical inrush is even less at 193 amps.

2) Reference a time current curve plot I've created using Siemens absolute peak inrush vs 90A breakers of several different manufacturer's.
You will see that they all clear the inrush with no issue. Again, this is at 90 amps!

Just food for thought....

Not only that, but the sign wave would have to be at the peak when they energize for them to have 578 amps, which again is well below what the 90 amp breakers show on their curves.

Finally, let's say that they did trip the primary breaker upon energizing with some other manufacturer's transformer somehow and it was caused by inrush. Couldn't they just reset it and hope that it doesn't energize when the sign wave is at it's peak.