panel ampacity vs breaker sizes


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Old 03-26-08, 10:35 PM
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panel ampacity vs breaker sizes

Howdy again to the experts:

I'm getting confused by different answers from different people (or maybe it is just my foggy brain).

Planning on putting in a new main OSD of 200A.
I have two MLO "sub" panels--125A for the residence and 100A for an attached shop.

Some folks say that I can't have two disconnects (125 and 100) on a 200A service.

But someone mentioned that the breaker total could be greater, but I'm not sure if this requires a "main" 200A disconnect and then the two MLO disconnects are down stream from the Main.

What is the legal means for me to have two MLO panel disconnects (125A and 100A) on a 200A service?

A. Just two disconnects of 125A and 100A,
or
B. 200A Main breaker and then two MLO disconnects
after it,
or
C. something you gurus suggest that I haven't thought of.

Thanks for all your help and advice to us with less knowledge.

Cheers,
s/Mike
 
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Old 03-26-08, 10:45 PM
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Im sure in a moment or two, a "Pro" will answer this for you.

However... From a legal standpoint... Anything that requires more than 6 throws to completely sever power requires its own disconnect. Some inspectors will even go as far as requiring this for any box that holds more than 6 breakers, not necessarily how many are installed.

If your subs are 125 and 100, How else would you feed them ?

Just for reference... ADDING the SUM of the breakers means absolutely Nothing to your Amperage capacity. It is based solely on the "Overall " load. Just because a breaker is marked 20 amp, doesnt mean it flows 20 amps at all times.
 
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Old 03-27-08, 03:41 AM
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Is this breaker box do have 200A Main breaker or not ??

but if this breaker box right below of the meter ??

and what it got in there now and any chance can you get the model number from the breaker box ?? the reason why because i can tell by model number to see where you stand


however with i did see 125 and 100 amp breaker in 200 amp box but this get little tricky due the rating of each breaker it can NOT be on each other like 100A on #1,3 slot and 125A breaker on #2,4 slot due the busbar tab " fingers" rating

if any possiblty if you can get a photo of it. It will really do help a bit with this.

Merci,Marc
 
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Old 03-27-08, 08:03 AM
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Originally Posted by billybard
But someone mentioned that the breaker total could be greater, but I'm not sure if this requires a "main" 200A disconnect and then the two MLO disconnects are down stream from the Main.
MLO "main lugs only" panels don't have rated disconnects. I think you're mixing some terms that makes it hard to follow what you actually have. Instead of "disconnect" tells us which panels have which breakers.

Usually it would not be a problem to have a 200A meter/main combo panel outside with 200A main breaker. This panel can then have a 100A breaker feeding the shop subpanel and a 125A breaker feeding the house subpanel. As Marc said though, you need to read the manufacturer spec sheet carefully to make sure that the panel stabs can handle the 125A and 100A. As long as both shop and house are part of same structure, neither of those subpanels needs a main breaker so they can be type MLO. Both house and shop subpanels need a four wire feeder with isolated ground and neutral buses.
 
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Old 03-27-08, 08:47 AM
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Presuming that there are 200 amp Service Entrance Conductors terminating on a 200 outdoor Service Dis-connect in the form of a 200 amp MCB, the question ( as I perceive it ) is how to extend two sets of Feeder Conductors from the "Load" lugs of the 200 amp MCB.

The 100/125 amp Feeder Conductors could "Tap" off 200 amp conductors, the tap connections made inside a WP trough set under the 200 amp MCB. The Tap/Feeder conductors would have to terminate on breakers in the MLO panels, the purpose of the breakers to limit the maximum current to the ampacity of the Feeder Conductors.

I'm inclined to guess that you already have two MLO panels "on hand" , and you want to make use of them. Am I correct?
 
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Old 03-27-08, 02:48 PM
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HI guys:

Thanks for the responses.

Guess a little clarification is in order.

My house, built in 1974 has an existing 125A meter/breaker combo outside the house. I have two subpanels being fed from the main breaker (both the 125A house sub and the 100A Shop sub). However, the existing install is a no-no, since both the 125A feed and the 100A feed are both under the same lug of the 125A disconnect, and the grounds and neutrals are not split apart in the subs.

I am in the process of remodeling and have to move the OSD.
Thought that I would upgrade to 200A while I am at it.

Thinking of adding a new 200A meter/breaker combo--particularly the Milbank U5168-XTL-200 which comes with a 200A MB and has room for two more 2-pole breakers.
(The spec sheets imply that only the smaller 100A model is limited to the MB capacity.)

Yes, I am aware that both subs need to be rewired with the 4-wire SER (really 3-wire with ground) and also separate the neutrals and grounds in both panels.

My main question was whether I really needed the 200A MB to be legal with the 125A and 100A disconnects for the two subs.

Quickie add on: does the 100A subpanel need #1 AL or #2 AL (short run)????

Hope this was some help.

Thanks for your advice.

Cheers,
s/Mike
 
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Old 03-27-08, 03:05 PM
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Originally Posted by billybard
My main question was whether I really needed the 200A MB to be legal with the 125A and 100A disconnects for the two subs.
The only way to know for certain is to perform a "demand load calculation" on each of the subpanels and add together to get the total service size. You can google to get a procedure for this calc or many home wiring books cover the calc too. My gut says that the heavy up to 200A is appropriate for this situation since you're redoing the service anyway. Do you have a lot of electric appliances? Heat, A/C, spa, welder, range, etc?

Quickie add on: does the 100A subpanel need #1 AL or #2 AL (short run)????
Under the NEC 2005 and earlier, most jurisdictions allow #2 aluminum SER for 100A.

Under NEC 2008, #1 aluminum is required.
 
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Old 03-27-08, 04:55 PM
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The best move is do the load demand caluacations then you can see where you stand and you will end up going with 200 amp when you replace the meter/breaker combo which it will get more common those day anyway.

now just a little correction { Ibpooks } you got it pretty much right on the target but check with the NEC again for feeder on #2 Al it only rated for 90 amp but if used for service entrance then yes it will take full 100'er

However the 2008 code they fix that mess so if you are on 08 code you have no choice but go with either #1AL or #2 CU so that the big differance there.

some inspectors will nail on that spot with this one but some will say it is ok

Merci, Marc
 
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Old 03-27-08, 10:10 PM
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Thanks again, guys:

Ah ha; I got caught without being aware of the changes with NEC 2008.

I now understand why I was getting different answers for the 100A feed with AL!

Yep, already did the load calcs (88A for the 125 MLO and about 43A for the 100A MLO)--so guess I am OK.

Still wondering if I have to bother with the expense of the MAIN 200A breaker or not. ???????
 
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Old 03-27-08, 10:21 PM
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Originally Posted by billybard
Thanks again, guys:

Ah ha; I got caught without being aware of the changes with NEC 2008.

I now understand why I was getting different answers for the 100A feed with AL!

Yep, already did the load calcs (88A for the 125 MLO and about 43A for the 100A MLO)--so guess I am OK.

Still wondering if I have to bother with the expense of the MAIN 200A breaker or not. ???????

88 amp that is pretty close to the borderline but you have some room there allright

but however senice you mention you are in California you are still on 05 code cycle for now they will adopt the 08 code much later on the time but let get back to the topic for a min right now you have all in one box right ??

if any possible you can take a photo of it it will really help me and few other electrician here to able steer you in the right direction ??

[ you can use the photobucket or other site and paste the url or hmtl link here]

Merci, Marc
 
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Old 03-28-08, 07:46 AM
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Originally Posted by billybard
Yep, already did the load calcs (88A for the 125 MLO and about 43A for the 100A MLO)--so guess I am OK.
88A + 43A = 131A

I'd say this is justification for upgrade to at least 150A service. There is not a big price jump between 150A and 200A, so it's probably a good idea to go to 200A if you want any room for expansion.

Still wondering if I have to bother with the expense of the MAIN 200A breaker or not. ???????
Yes, I think you do. It also will probably require a new meter base rated at 200A.
 
 

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