Sub-panel feds thru 100 amp breaker?

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ngd4130

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Help me understand and if there is a NEC code reference, can you direct me?
The new main panel installed is most probably rated for 200 amp. However, they only installed a 100 amp main breaker. I don't believe there is a problem with that part of the installation. FYI - It may have up to 40 spaces (most not being used).
Next, there is a sub-panel (100 amp rated, less capacity) installed on the other side of this one-car attached garage. This sub-panel is a main lug panel and they used a 100 amp branch circuit breaker to feed it from the main panel.
I don't believe this is a good practice to install this way but is there a code reference prohibiting it? Thanks for your thoughts!
 
What part do you consider to be “not good practice”?

Seems strange to leave the main panel empty, but nothing to me suggest a problem.
 
This is a very common installation. Not a code issue and IMO not really a bad practice either.
 
retirede said:
What part do you consider to be “not good practice”?

Seems strange to leave the main panel empty, but nothing to me suggest a problem.
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The 100 amp sub-panel circuit breaker is in the same panel as the 100 amp main breaker panel. How many 100 amp sub-panel circuit breakers can be installed in this main panel before it is an issue or code violation? Just doesn't seem right.
 
ngd4130 said:
The 100 amp sub-panel circuit breaker is in the same panel as the 100 amp main breaker panel. How many 100 amp sub-panel circuit breakers can be installed in this main panel before it is an issue or code violation? Just doesn't seem right.
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How many? As many as will fit. Why would it make a difference if it was a 125 - 200 main?
What is the calculated load?
 
ngd4130 said:
Help me understand and if there is a NEC code reference, can you direct me?
The new main panel installed is most probably rated for 200 amp. However, they only installed a 100 amp main breaker. I don't believe there is a problem with that part of the installation. FYI - It may have up to 40 spaces (most not being used).
Next, there is a sub-panel (100 amp rated, less capacity) installed on the other side of this one-car attached garage. This sub-panel is a main lug panel and they used a 100 amp branch circuit breaker to feed it from the main panel.
I don't believe this is a good practice to install this way but is there a code reference prohibiting it? Thanks for your thoughts!
Click to expand...
I don't see anything wrong with this. An attached garage with its own panel does not require any kind of disconnecting means at the panel or in the garage.

I'm not sure why you think this is not a good practice. Perhaps you could indicate why you think this is not a good practice. I normally don't use a whole lot of panel boards at all because I do industrial controls but when I do use them I often use one with a main because it's generally cheaper to buy a panel board with the main already in it so I might feed a panel board that has 20 breakers in it and a 100 amp main with a 30 or 40 amp breaker. Do you think that is not a good practice? If so why?
 
It's up to the electrician to determine the load (total) before adding anymore load. The subpanel may have just been added to make circuit runs easier. No additional loads, just divided between the two panels.
 
BTW, sometimes panelboards can be bought with several 20 Amp breakers already installed. I don't normally need any 20 A breakers so rather than remove them and throw them away, I just leave them there and don't use them for anything. Less labor than removing them. Is this a "not good" practice in your mind?
 
Some may be missing the point.
"How many 100 amp sub-panel circuit breakers can be installed in this main panel (protected by a 100 amp main breaker) before it is an issue or code violation?"
Is it one? If it is two, is that too much? This has nothing to do with panel locations, 20 amp breakers or anything else.
 
The answer is as many as will fit.
Someone should have done a load calculation. That load can be split into however many 100 amp subs that you want. Convenience or you have them left over from another project. It could be split up into branch circuits originating in the SE panel or at each sub. Doesn't matter,
 
ngd4130 said:
Some may be missing the point.
"How many 100 amp sub-panel circuit breakers can be installed in this main panel (protected by a 100 amp main breaker) before it is an issue or code violation?"
Is it one? If it is two, is that too much? This has nothing to do with panel locations, 20 amp breakers or anything else.
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The code does not care as long as the calculated load is satisfied. You can break up your loads so every breaker is only carrying 1 Amp if you want to and have 100 breakers total.
 
The answer is that the number of 100A subpanel circuits installed in a main panel is _irrelevant_ to code compliance.

Code requires that the calculated load on the main panel be less than a it is rated for. This has nothing to do with the number or size of breakers installed in this panel.

NEC has an article dedicated to 'load calculations'. The total of the loads connected to that 100A panel needs to calculate out as less than 100A.

If you chose to run a 100A feeder to each room in the house, with a subpanel for each room, and an individual 20A circuit to each receptacle...you might end up with 10 100A feeders coming off of this main panel...but as long as the NEC calculations for the load of this home comes to less than 100A you would be kosher.

-Jon
 
ngd4130 said:
Some may be missing the point.
"How many 100 amp sub-panel circuit breakers can be installed in this main panel (protected by a 100 amp main breaker) before it is an issue or code violation?"
Is it one? If it is two, is that too much? This has nothing to do with panel locations, 20 amp breakers or anything else.
Click to expand...

No issue or violation as long as the 100A main panel is sufficient per the load calc.
 
ngd4130 said:
Some may be missing the point.
"How many 100 amp sub-panel circuit breakers can be installed in this main panel (protected by a 100 amp main breaker) before it is an issue or code violation?"
Is it one? If it is two, is that too much? This has nothing to do with panel locations, 20 amp breakers or anything else.
Click to expand...
Just one more point: Add up the handle rating of all the breakers in a typical 200 amp panel - you'll get way over 200. No problem.
 
Thanks everyone for your input. It is great to have this resource to bounce questions off others. I actually agree with the conclusion that the load calculation for the main is the final word. Just looking for any other angles.
 
No compliance issue here. Remember the NEC protects installed wiring systems. From a practical standpoint, we're looking at conductor ratings and OCPD ahead. So, assuming no VD or derating adjustments, lets say #3 cu from 100A distribution breaker to first panel, call it "A", a 100A MB panelboard. Then you install (4) 100A breakers in panel A, feeding (4) 100A MLO (or MB) subs or loads, call them "B". Whatever. So now, all parts of this system are protected and individually or in combination, they cannot overload any part of the system. If any one of the B subs crest its feeder OCPD limit, it will open. If 3 of the B subs are at 40A ea. then their combined load will open the main at the first panel, A, or at the main distribution 100A breaker. All this is basic layout. Now the factors from 408.30 and 408.36 come into play, where we began to input limiting calcs into the system, But keep in mind, the feeder circuit ampacity (incudes it's OCPD) cannot exceed the panel rating and OCPD's cannot exceed their feeders.
 
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