Main Panel Breaker Sizing in relation to Installed Feeder Breakers 2

[GFS666]

Greetings All,
I have a question regarding Main Panel Breaker Sizing in relation to Installed Feeder Breakers. I have been told by a technician I trust that the installed feeder breaker sizes have to add up to equal or less than the rating on the Main Panel breaker. (Assumption: Main panel breakers and feeder breakers are the same # of phases. I.E. 3 phase breakers for everything)

For example, a 200 Amp 3 phase breaker will support ten 20 amp 3 phase breakers, or any multitude of 3 phase breakers as long as the total feeder (downstream) breaker sizes add up to equal or less than 200 amps.

Question: Is this sizing philosophy correct? And if it is (or is not correct), can anyone point me to the relevant electrical code section that spells this out in clear black and white?

My apologies for the "simpleness" of the question. I'm still learning things about electrical distribution systems.

Thank you for your time,

Gordon

 

[DanDel]

Sorry, the sizing philosophy is incorrect. There used to be a limit to 42 circuits per panel (this limit is not in the NEC anymore, BTW), but there was never any limit to the size of the C/Bs. You commonly see a 200A-main C/B in a 200A panel with 42 15A or even 20A C/Bs.

The only NEC limitation for number of overcurrent devices in a panel is based on the manufacturer's design. (See NEC 408.54) This only means that you can't put more than 42 C/Bs in a 42-circuit panel.

 

[waross]

Not even close.
There are too many factors to consider when sizing feeders and feeder breakers to give a quick answer. The sum of the ratings of the breakers in the panel is not one of the factors that is considered.
Consider a panel feeding small motors. The sum of the ratings of the individual breakers may be several times the rating of the feeder breaker.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I agree with DanDel. There is no relationship between the size or quantity of the feeder breakers and the size of the main breaker, except that the feeder breaker can't be larger than the main (I'm not even sure about that right now).

The NEC requires that the breakers be large enough to carry the load. I can have a 200 A main and 5-100 A feeders. The NEC simply doesn't care about what the sum of the feeder breaker sizes are. As long as the 200 A feeder can carry the maximum expected load and all the conductors are sized correctly, this is an acceptable installation.

 

[EEJaime]

Gentlemen,

I once had a rookie city plan check staffer tell me that my feeders were undersized because my panel had 42 circuits in it and at 20 Amps each that meant I needed 840 Amps of feeder capacity if the panel were fully loaded!

When I stopped laughing, (to myself of course, it never pays to burn bridges at your AHJ- you never know when you'll run into them down the line), I went through the sizing process with the young lady, and explained the logic. Good thing, she ended up staying in the department and my plan check submittals went through as smooth as glass for many years.

Regards,
EEJaime

 

[mcgyvr]

EEJaime,
Sad thing is that a UL project engineer told me that same thing just last year when we sent one of our fuse panels through UL certification. Boy did he feel stupid when we had to bring his boss into a formal meeting to discuss the 'many' issues we had with his "interpretations" of the standards.
He supposedly identified 20 or so issues and after the meeting we had 0 open issues and a listed panel.

 

[jraef]

People love to boil down complex issues into "sound bites". I wonder where they get that from?

Here is a decent treatise on the complexity of it all, I have found it very useful in explaining it, or at least explaining how complex it is, to the lay people I sometimes have to deal with.

http://www.nfpa.org/assets/files//PDF/necdigest/CodeIssues072704.pdf

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[EEJaime]

mcgyvr,
Now that is a little scary!

jraef,
Thanks, nice concise paper.

Regards,
EEJaime