Feeder Calculation Part 2

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karagory1

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I have a 100% rated 800-Amp CB in the Main Distribution Panel feeding a Power Panel with a 80% rated 1000-Amp main CB. The feeder is using wire that has a maximum current capability at 75 degrees C of 760 amps (as best I can tell they used the incorrect column when they initially sized the wire). Both of the CB's are rated to handle 75 degree C rated wire. (I didn't design system so please don't blame me.)

One of many questions that I have is it OK to run the feeder at 760 amps continuously? Does the 80% rating of the 1000-Amp CB mean that ANY wire connected to it can only be run at 80% of the wire current rating continuously in spite of the value of the current capability of the wire being less than 80% of the 1000-Amp rating (800 amp max continuous) of the CB? If in a different feeder I supply a 1000-Amp CB with only 200 amp wire (from a corresponding 200-Amp CB) could I only run it at 160 amps continuously?

Any other thoughts on this feeder would be greatly appreciated.
 
Re: Feeder Calculation Part 2

It sounds like the feeder circuit is parrallel 500Kcmil conductors, which is a very common practice. You are permitted to "round up" to the next standard (see 240.6) siza OCPD up to and including 800 amps (see 240.4(B)). The problem that people may run into is 230.42 and also 215.2 which, in a nutshell, both say that the feeder or service conductors (feeders in your case) must be sized to handle to the computed load. If you have a computed load over 760 amps, you have a problem, otherwise, I can see nothing wrong with this as you describe it.


In your other scenario you have not provided overcurrnet protection for your conductors, so as far as I can tell your question about the 200A wire on an 800 amp breaker is a moot one.

Learn section 240.4 well. It really is one of the most important sections of the code, in my opinion. Hope this helps, :)
 
Re: Feeder Calculation Part 2

You have surmised my initial scenario exactly. I came to the same conclusion as you. Unfortunately, the addition of the standard rated (80%) 1000-Amp CB in the problem is causing me some difficulties. Hence, I was hoping that my second scenario would provide me with some additional insight into my original question.

I apologize for not stating my second scenario more clearly.

A 100% rated 200-Amp CB using 200 amp continuously rated wire feeding a 1000-Amp standard rated (80%) CB. Is the wire de-rated to 160 amps because it is connected to an 80% rated CB even if this CB is 1000-Amp?
 
Re: Feeder Calculation Part 2

Conductor does not have an 80% rating. The ampacity of 2-500 kcm thhn is 380 amps at 75C for a total of 760 amps. See table 310.16 75C col.
The conductor can handle the 760 amps continuously
assuming the terminations are rated for 75C.
The 80% factor is applied to the breaker and not the conductor.
If in a different feeder I supply a 1000-Amp CB with only 200 amp wire (from a corresponding 200-Amp CB) could I only run it at 160 amps continuously?
Click to expand...
If the breaker is not rated for continuous load
then the max is 80% continuous or 160 amps. The
derating if for the breaker and not the conductor.

[ March 29, 2004, 04:04 PM: Message edited by: bob ]
 
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