T450.3 (A) and T450.3 (B)

[finhead]

T450.3(A)
Can someone explain the correlation between the "Transformer Rated Impedance" column and the two "Primary Protection Over 600 Volts" columns?
For example: A transformer in a supervised location with rated impedance of 5% can fall under the "Any" impedance of row three, or the "Not more than 6%" impedance of row four.

T450.3(B)
The last sentence of Note 2 states,"If both breakers and fuses are utilizied as the overcurrent device, the total of the device ratings shall ot exceed that allowed for fuses."

The problem is, this table does not distinguish between fuses and circiut breakers.
Appreciate the help
Brian Dolan
Palos Park, IL

 

[charlie b]

Re: T450.3 (A) and T450.3 (B)

Originally posted by finhead: T450.3(A): Can someone explain the correlation between the "Transformer Rated Impedance" column and the two "Primary Protection Over 600 Volts" columns? For example: A transformer in a supervised location with rated impedance of 5% can fall under the "Any" impedance of row three, or the "Not more than 6%" impedance of row four.

Yes, it falls into both rows. That means that you can use a primary breaker rated at 300% or one rated at 600%. Which do you want to use? Since you are talking about a 5% impedance, you can use either one you want. But keep in mind that "any" also includes, for example a 7% impedance. If you have a transformer with a 7% impedance, you can?t use a 600% rated breaker.

T450.3(B): The problem is, this table does not distinguish between fuses and circuit breakers.

Good catch. I looked through earlier versions of the code. As of 1996, Table 450.3(A) was split into two tables, one for unsupervised locations and one for supervised locations. In 1999, these two were combined into a single table. Also in 1999, a separate table was created for transformers under 600 volts. I think (I can only guess) that they copied Note 2 from the ?A? table, without noticing that it does not precisely apply (because, as you pointed out, the ?B? table does not distinguish breakers from fuses).

Do you want 15 minutes of fame? You can submit a proposal for a change to the 2008 code. You can start by posting the suggestion on this Forum, under the topic area ?Proposals for the 2008 NEC,? and see if you get any comments on your proposal.

 

[steve66]

Re: T450.3 (A) and T450.3 (B)

Can someone explain the correlation between the "Transformer Rated Impedance" column and the two "Primary Protection Over 600 Volts" columns?

You can use the least restrictive row. So if you know the impedence will be 5%, you may use the "Not more than 6%" row and size the primary OCP at up to 600% of the rated current.

It took me a minute to understand why the OCP limits go down as impedence goes up. I think I just figured out that the reason is to make sure the transformer is able to open the overcurrent device during a fault. For example, if the impedence is greater than 6%, a short on the secondary may not be able to open a 600% primary fuse.

Steve

Steve.

 

[steve66]

Re: T450.3 (A) and T450.3 (B)

I agree with Charlie that the wording of note #2 isn't quite up to date (or it just wasn't worded well to start with). But I think the intent is clear. If you have a transformer with a 500 A secondary, and you use the 125% row, that gives a limit of 625A for the secondary protection.

That means you can have a 600A fuse, or a 600A breaker. Or you can have two 300A fuses, or two 300A breakers. Or any other combination that adds up to 625 or less.

Note 2 is basically saying that if you use fuses and breakers, just add the values like they were all fuses. So a 300A fuse and a 300A breaker is OK. So is two 100A fuses, and two 200A breakers. But a 400A fuse and a 300A breaker would add up to 700 A, and that would not be OK.

Steve

 

[charlie b]

Re: T450.3 (A) and T450.3 (B)

I agree that that is the intended meaning of the last sentence in Note 2. But I believe you could delete this last sentence, and not change the requirement, nor would you confuse anyone concerning its intent. The second sentence says all that needs to be said.