NEC Table 450.3(B) - Transformer Primary OCPD Size

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designer82

Senior Member
Location
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Table 450.3(B) says "Maximum" rating or setting of OCPD

I have a 45kVA transformer (480/277V). Primary rated current would be 54.126 Amp

I have primary and secondary protection in my setup so 250% for primary and 125% for secondary per the table.

Question is... does it HAVE to be 250% as in 54.126 * 2.5 = 135.3 A resulting in 150A breaker for the primary?
Or can it be less, say 100 Amp?
I'm thinking less is acceptable since Table 450.3(B) says "Maximum".
What's throwing me off is that the NEC handbook examples are using the maximum for the example problems.

Thanks
 
Note that one option, and likely the most selected, is 125% so any value between 125 and 250 is acceptable provided the secondary protection is selected according to the Table.
 
The issue is the inrush that occurs when the transformer is energized. I've heard that the value is maximized with temperature. The colder the unit, the higher the inrush. It's true that 250% is the maximum, but then you have to upsize the feeder cable to match the 250% breaker. The engineering firm I work for has standardized on 175% for the primary as a reasonable compromise.
 
You can go however low your overcurrent protection device will still hold while energizing.

I've done temp power for construction a few times with transformer that happened to be available but was larger than needed and put on relatively small overcurrent device for the primary compared to what would be typical if it were used at closer to it's rating.

In rush is going to be limited by conductor resistance, not just the circuit for said transformer but everything between winding and the source, though often that final run has potential to be the most limiting section involved.
 
designer82 said:
Table 450.3(B) says "Maximum" rating or setting of OCPD

I have a 45kVA transformer (480/277V). Primary rated current would be 54.126 Amp

I have primary and secondary protection in my setup so 250% for primary and 125% for secondary per the table.

Question is... does it HAVE to be 250% as in 54.126 * 2.5 = 135.3 A resulting in 150A breaker for the primary?
Or can it be less, say 100 Amp?
I'm thinking less is acceptable since Table 450.3(B) says "Maximum".
What's throwing me off is that the NEC handbook examples are using the maximum for the example problems.

Thanks
Click to expand...
Maximum is maximum. the 135.3A you calculated would mean the maximum breaker size is 135A. If you use a 150AF breaker thats adjustable, you would need to set it to the first value that is equal to or less than the 135A.

Also i come from a consulting firm that basically told all their employees to always use the maximum for motors and transformer breaker sizing. Probably to eliminate any liability on their end for not doing proper coordination.

At the end of the day, the actual breaker size can be less than 250%. It is a maximum after all. I have found that some small transformers TCC curves did not work with a larger breaker size that was closer to the 250%. I was used to doing 150A breakers to 75kVA transformers, and had to dial down further to 125A.

You should find some kind of method for the madness so to speak. If someone asks you why you picked that size, try to make sure you have a reason why you picked it.
 
designer82 said:
Table 450.3(B) says "Maximum" rating or setting of OCPD

I have a 45kVA transformer (480/277V). Primary rated current would be 54.126 Amp

I have primary and secondary protection in my setup so 250% for primary and 125% for secondary per the table.

Question is... does it HAVE to be 250% as in 54.126 * 2.5 = 135.3 A resulting in 150A breaker for the primary?
Or can it be less, say 100 Amp?
I'm thinking less is acceptable since Table 450.3(B) says "Maximum".
What's throwing me off is that the NEC handbook examples are using the maximum for the example problems.

Thanks
Click to expand...
2023 NEC Texas
Yes, means yes, unless you have a Foot Note number 1 in your box or row.

Foot note number one allows you to round up.

You don't have to round up, but it is permitted. Read footnote number 1 and notice the last 3 words of that sentence, shall be permitted.

Check out 90.5 (A) (B) Mandatory Rules, Permissive Rules, and ........"

Thanks for reading
Comments accepted
TX+MASTER#4544
 
death900 said:
Maximum is maximum. the 135.3A you calculated would mean the maximum breaker size is 135A. If you use a 150AF breaker thats adjustable, you would need to set it to the first value that is equal to or less than the 135A.

Also i come from a consulting firm that basically told all their employees to always use the maximum for motors and transformer breaker sizing. Probably to eliminate any liability on their end for not doing proper coordination.

At the end of the day, the actual breaker size can be less than 250%. It is a maximum after all. I have found that some small transformers TCC curves did not work with a larger breaker size that was closer to the 250%. I was used to doing 150A breakers to 75kVA transformers, and had to dial down further to 125A.

You should find some kind of method for the madness so to speak. If someone asks you why you picked that size, try to make sure you have a reason why you picked it.
Click to expand...
I think that generally, there is excessive concern placed on this issue. A few points:

1. I noticed you used "breaker" a lot in your post. An RK5 fuse is a much better ocpd for this application in many instances. For example, at 1000A, a 100A RK5 takes at the very least three times longer to blow then a standard plug on 100A breaker. That is a recent application I had, I have not dug into the comparison of larger frame breakers with highest adjustable settings and how they compare.

2. Depending on the client and application, so what if the transformer trips on startup sometimes? Unless the transformer is being turned on and off frequently of course.

3. I would be very surprised to see a fused application at about 125% fail

4. Sometimes the cost to go to 250% from 125% would be large and that may be a factor in the decision
 
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