# 42 circuit limitation question



## JohnJ0906 (Jan 22, 2007)

It looks like that exception is the same as the '05 408.36(A) 

I think this is the wording that got rid of the old "split-bus" panels. No longer could a panel have 6 "mains", 2 of which were protecting lower portions of the panel.


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## JohnJ0906 (Jan 22, 2007)

http://www.electriciantalk.com/showthread.php?t=1481

A previous thread on split-bus panels.


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## nap (Dec 26, 2007)

Ah! the old split bus panels. Luckily as I do very little resi, I have not run into them much as they do not seem to be used in the commercial/industrial equipment I deal with.

as usual when having problems with the code, I put too much thought into it.

So, then if I understand the application correctly, if there is a split bus panel, it is still limited to 42 circuits but other than that, the limitation has been removed.

but if split bus panels are no longer made, why the inclusion in this manner?


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## cmec (Feb 11, 2008)

Split buss panels are only allowed for replacement porpuses.

Split buss panels have been illegal for commercial work since I started in 
1975

The amount of breakers and bussing arrangement is an manufacture issue not a in field issue, I hear guys on another foreum who think that means they can twin up a whole panel.The whole panel is a ul listed assembly for regular and or twins as bussing allows ,so I think this is so the mfr can build a main distribution panelboard with more than 42 circuits,In todays world lots of times 42 circuits isnt enough, before 08 they come out with 2 mains and 2 sets of busses or 2 enclosures one fed thru the next .The way I understand it the 42 circuit rule came because some fire marshall thought a panel with more than 42 circuits started a fire and went to nfpa /nec code making panel and started a proposal ,kinda dont think theres any technical merrit to it.

My take on exception number two , mlo panels can only be 42 circuit

The best example of two main circuit breakers is how semens parellels 2 100 amp breakers and ties all the handles together on some of their 200 amp load centers


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## nap (Dec 26, 2007)

but it says for panels protected by two main breakers or fuses. How would this apply to mlo panels. Johns explanation seems to fit better but I still wonder why the need for the explanation and exception though.




I understand the incorrect application of tandem breakers. Nothing has changed concerning that. That was and still is a manufacturers limitation and cannot be deviated from.

and all that the removal of the 42 circuit limitation would allow is for the manufacturer to build a bigger panel.


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## goose134 (Nov 12, 2007)

> but it says for panels protected by two main breakers or fuses. How would this apply to mlo panels.


I'm not saying it would be right, but picture switchgear that feeds a distribution panel, that feeds a couple of panels on a tap. That would not conform to the overcurrent limit (2 devices totalling the rating of the fed panel), but it would give you two devices before the MLO.


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## leland (Dec 28, 2007)

---Split buss panels have been illegal for commercial work since I started in 
1975----

We hve them all over the place, MLO, Emergency on top(6 ckts).
Normal on the bottom.
Old Campus.


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## cmec (Feb 11, 2008)

I could be all wet about this and somone, a member should call nfpa for an interpretation but here goes, If a mfr wants to make a listed mdp or a distribution panelboard over 42 circuits if the main breaker is two breakers parelled and tied together like I described in semans loadcenter 200, 400. 800 amps etc and puts subfeed lugs of equal ampacity as the main and buss or feeder in the 1st enclosure to feed the second enclosure and it is mlo it is limited to 42 circuts


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## cmec (Feb 11, 2008)

leland said:


> ---Split buss panels have been illegal for commercial work since I started in
> 1975----
> 
> We hve them all over the place, MLO, Emergency on top(6 ckts).
> ...


 
I looked it up,the 1971,1975 AND 1978 NEC article 384-16 exception #2 limits them to residential use, dosent mean they wernt used for commercial :cowboy: some people ignored or didnt understand ,or didnt want to understand the code.



Guess it depends what shade of gray your glasses can see.


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## leland (Dec 28, 2007)

Well the campus started building in 1865, many changes since then.
Yes some of the active systems are still from the 50s' and into the 60's.

Still wide open slate boards etc. behind closed doors, some are even locked!!!:no:


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## gilbequick (Oct 6, 2007)

cmec said:


> I could be all wet about this and somone, a member should call nfpa for an interpretation but here goes, If a mfr wants to make a listed mdp or a distribution panelboard over 42 circuits if the main breaker is two breakers parelled and tied together like I described in semans loadcenter 200, 400. 800 amps etc and puts subfeed lugs of equal ampacity as the main and buss or feeder in the 1st enclosure to feed the second enclosure and it is mlo it is limited to 42 circuts


This is a question I've thought about for a while and can't come up with a code reference for. 

So what if I were to use a main lug, feed through panel and use the feed through lugs on the bottom of the panel to feed another panel. Say it's on a 200A main and I pull 4/0 AL to the first sub panel. Out of the bottom lugs I run 4/0 AL to the next panel that's literally right next to (side by side) the other panel. Both panels are now protected by the same 200A main breaker. Is this code compliant??


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## JohnJ0906 (Jan 22, 2007)

gilbequick said:


> This is a question I've thought about for a while and can't come up with a code reference for.
> 
> So what if I were to use a main lug, feed through panel and use the feed through lugs on the bottom of the panel to feed another panel. Say it's on a 200A main and I pull 4/0 AL to the first sub panel. Out of the bottom lugs I run 4/0 AL to the next panel that's literally right next to (side by side) the other panel. Both panels are now protected by the same 200A main breaker. Is this code compliant??


Yes, it is done often in commercial... which means the 4/0 AL might not be suitable.


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