NEC Table 450.3(B) - Transformer Primary OCPD Size

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designer82

Senior Member
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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
 
2023 NEC
death900 said........

My rebuttal follows.

See T.240.6(A).

Maximum is when a calculated answer about a fuse or circuit breaker protection (OCPD) falls in between the standard ratings as found in Section 240.6 (A) and T. 240.6(A), and if your load is less than 800 amps it could be permissable to round up to the next standard rating as found in the Table 240.6(A).

There may be certain times when you cannot round up because NEC prohibits it, thus it is a violation of NEC and an incorrect answer on his exam.

The standard amperes ratings for fuses AND inverse circuit breakers are found T.240.6((A). These are what are used for state electrical exams here in Texas when selecting an OCPD, (fuse OR circuit breaker).

I cannot tell my students to always round up or not. It could result in an incorrect answer on their exam.
At the end of the day, it could cause a student to fail his exam.

So as an instructor I must explain to my students what the Code permits or doesn't permit about selecting an OCPD.

240.6 permits additional sizes to be 1,3,6, and 601 amps, and non standard sizes ratings shall be permitted.

I have inherited my "madness" as result of our Code book in as much as it can be difficult to explain to a student novice apprentice electrician preparing to take on perhaps, his most challenging event and that maybe, passing the state electrical exam.

The electrical exam questions do not revolve around actual cost to any particular job; only what the Code prescribes on the exam.

With all due respect, the reader may have already formed his own opinion of his version of Code interpretation, so be it.

I have found my madness and it is found between the several hundred pages of the 2023 NEC and I always include the Code section and table to the best of my ability based on the question.

Now, if I may go and bury myself deep into the 2023 NEC and a bottle of aspirin's.

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...
Depending on whether conditions and related notes to the table apply, maximum can be higher than what the table says
 
Other than for step-up transformers, I don't think the maximum OCPD is necessary. I don't know of too many scenarios where 125% time-delay fuse inverse-time breaker would be a problem, outside of step-up transformers.
 
Tip DS said:
Other than for step-up transformers, I don't think the maximum OCPD is necessary. I don't know of too many scenarios where 125% time-delay fuse inverse-time breaker would be a problem, outside of step-up transformers.
Click to expand...
Step up transformers or reverse fed transformers?
 
kwired said:
Step up transformers or reverse fed transformers?
Click to expand...
I dont believe step up transformers necessarily have higher inrush, but reverse fed typically will. I reverse feed 25 KVA MV transformers (usually 2400 or 7200) when doing a step up step down and have not had those trip a 200A breaker, but thats about 190%. The most inrushy transformer I have delt with is a single phase 25KVA dry type, 240->240. IT has gone out of range on my meter which goes up to 1000A. But even that is fine on 100A RK5's
 
electrofelon said:
I dont believe step up transformers necessarily have higher inrush, but reverse fed typically will. I reverse feed 25 KVA MV transformers (usually 2400 or 7200) when doing a step up step down and have not had those trip a 200A breaker, but thats about 190%. The most inrushy transformer I have delt with is a single phase 25KVA dry type, 240->240. IT has gone out of range on my meter which goes up to 1000A. But even that is fine on 100A RK5's
Click to expand...
That's why I asked. My understanding is if it is designed as a step up it will have the primary winding closer to the core where a back fed transformer you are supplying the designed secondary winding and it is further from the core. Actual transformer design can factor in but I think is pretty common to wind the intended primary closer to the core and intended secondary afterwards leaving it further from the core regardless which one is intended to be the higher voltage.
 
kwired said:
...where a back fed transformer you are supplying the designed secondary winding and it is further from the core...
Click to expand...
I think that's backward. On a typical step-down transformer, the secondary is closer to the core, leading to lower impedance. That lower impedance leads to a higher inrush when back-fed.
 
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