Transformer Protection - NEC 450.3(B)

[GreenNGold]

I was hoping someone could help me interpret Table 450.3(B) - Note 1 in the NEC.

The note states: "Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted."

What does it mean when they say: "a higher rating that does not exceed the next higher standard rating shall be permitted"?

I am looking at an example of a 12,470V/7200V - 277/480V, 300KVA transformer. The secondary current is 360.84 Amps (300KVA /(1.73*480)). If there is primary and secondary protection 360.84 * 1.25 = 451 Amps. Why do we chose 450A as the secondary protection rather than 500A?

When Note 1 is called out, how do we know to go to the next higher or next lower standard value?

Thank you.

 

[knight1fox3]

Out of curiosity, where did the example problem come from? Because according to the code, a 500A over-current protection device would be permitted. Looking at this from a practical application standpoint, since the 125% is so close to the standard rating of 450A, I probably would have chosen that as well since 500A protection would be more costly. But that wasn't the question. I believe the example itself is questionable and open to interpretation.

"a higher rating that does not exceed the next higher standard rating shall be permitted" basically means you wouldn't be able to select a size directly after the very next size up. So for instance, say you had to provide protection for 130A. Since 130A is not a standard size, the "next higher standard" would be 150A. You would not choose 125A. The "next next higher standard" would be 175A which would also not be permitted. Here's some more info from Mike Holt's website that could provide additional insight for you. HTH

http://www.mikeholt.com/technical.php?id=EE/unformatted/conductorsize-protection&type=u&title=Conductor%20Sizing%20and%20Protection%20%2810-11-2K%29

 

[GreenNGold]

The problem is from some class notes I am borrowing from a friend to study for the PE.

That is also how I interpreted it, so I am glad we are on the same page. Thanks for clearly that up for me.

 

[GreenNGold]

I have another question for this problem.

The primary current is 13.89A ( 300 kVA / (1.73 * 12470)). Since there is primary and secondary protection, 13.89 * 2.5 = 34.725A. Are we allowed to round up to 35A (which is what is shown as the answer), or would you round down to the lower standard size? Note 1 under table 450.3(B) is not called out for this case so IMO the answer should be 30A.

What do you guys think?

 

[Ship Wreck PE]

In your question, you ask about table 450.3B, and the problem has a 12,470 volt transformer. Table 450.3B is for under 600 volts?? Which table are you using?

 

[GreenNGold]

You are correct. Silly mistake on my part.

Thanks.