Size-down to find OCPD in primary protection of transformer. Why?

[Andres Arias]

Hi all.
I have the following problem:
A three phase trx installed in a supervised location, is rated 2.5MVA 13.8kV/2.4kV Z=7.5%. Fuse is used in the primary side. What is the maximum rating allowed for transformer OCPD?

The solution uses Table 450.3A NEC, specifically column 4 (Fuse rating for Primary protection over 1000V), last row: 300% of transformer rated current.
S = 3*V*Iprim Iprim = (2.5E6*sqrt(3))/(3*13800) --> Iprim = 104.59A.
Then applying the 300% as per Table 450.3A, we have 313.77A.
What I don´t understand is, as per Art 240.6 why we have to size-down to find the correct OCPD for the primary of transformer? This is, OCPD=300, and not OCPD=350A.

 

[wwhitney]

The short answer is "240.4 allowances on conductor protection don't apply to the 450.3 requirements on transformer protection." They are two different things.

However, assuming your arithmetic is correct, I would say there is an error in the problem answer. Given the parameters specified in the problem, there are three lines in Table 450.3(A) that could be applied. The "Any Location, 6% - 10% Impedance" line, the "Supervised Locations, Any Impedance" line, and the "Supervised Locations, 6%-10% Impedance" line.

If we choose the last line, then the Primary fuse is limited to 300% by the table. Note 1 of the table allows rounding up the OCPD in some cases, but this particular entry in the table does not refer to Note 1, so we don't get to round up in this case. This is the answer you were given.

However, just because the last line most specifically fits the problem situation, that doesn't preclude us from choosing one of the other lines. The first of the 3 options permits a Primary fuse of 300%, with Note 1 applying. So by choosing the first applicable line, we will have the option to round up, and the correct answer for maximum allowed primary fuse is 350A, not 300A.

Cheers, Wayne

P.S. Note that choosing the second line in Table 450.3(A) does come with a slight penalty on the secondary side. In that on the secondary side we now have a maximum circuit breaker / fuse of 250% / 225%, with Note 1 applying. While in the last line of the table, we would be at 250% / 225% with Note 5 applying. And Note 5 says that for certain transformers, secondary protection may be omitted.

So if you have a transformer where Note 5 applies and want to take advantage of it, you are forced to use the last line of the table, and you don't get to use Note 1. Of course, in a supervised location you could also omit secondary protection by using the 3rd line of the table, but then your primary fuse is limited to 250% with Note 1 applying. The last line allowance of 300% without Note 1 may be higher than 250% with Note 1. I'm unclear on that, as Note 1 refers to "commercially available" ratings or settings for OCPD above 1000V, and I have no idea what ratings or settings are commercially available.

 

[Andres Arias]

Thank you for the reply Wayne. See below the complete problem from the book. Actually the trx is in a supervised location. It is clear for me the answer in 9.1i, but in 9.1h it is not.

 

[wwhitney]

Thank you for the reply Wayne. See below the complete problem from the book. Actually the trx is in a supervised location. It is clear for me the answer in 9.1i, but in 9.1h it is not.

The book is wrong. Clearly "any location" includes "supervised locations". So you can use the "any location" lines if preferred, meaning 350A is still allowed in supervised locations. And since 9.1(h) asked for the "maximum allowed", 350A is the correct answer to both questions.

Cheers, Wayne

 

[Andres Arias]

Thank you Sir. Appreciated it.