100a main panel, 100a subfeed, NEC legal ?

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dnem

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The situation is a residential 100 amp main breaker panel with 24 slots. All slots have breakers of various sizes. Breaker #23 & #24 is a 2 pole 100 amp breaker feeding a 24 slot main lug only subpanel.

Does the code allow a 100a main panel to feed a 100a subpanel if there are other loads in the main panel ?

David
 
finnegan said:
Assuming that the total load is within the limits of the service, what is possibly poor about the design?
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I said possible. You said assume (ing)!

You are assuming it is done right, I did not.
 
So you have no problem with this type of installation so long as it is done to code. I only ask becuase I have done this several times and if there is an inherent problem to this, I want to know what not to do. Thanks.
 
I have also done this.

The only problems I see are the customers perception of what is really there.
They will see a 100A main breaker and assume they can load the panel to 100Amsp. Then they see a 100A subfeed break and assume they can load the other panel to 100A. Can't happen!

Now comes along the HVAC guy who (incorrectly, improperly) installs the A/C circuit in the sub panel. Now almost every time the A/C starts, it trips the main or sub breaker.

Just a scenario. :D
 
We can't do today's work in fear that tomorrow's worker will do something unsafe. We can't build in extra safety margin, just to protect the owner from the unsafe work of tomorrow's worker. We are allowed, and I believe we should feel free, to presume that tomorrow's worker will look at the situation, see the existing configuration, and make any additions in a code-compliant manner.

A 100 amp panel with a 100 amp breaker feeding a 100 amp sub-panel is not a code violation. I think we all agree here. To that I will add that I don't think it a bad design. If it were my house, or a house I was designing for a friend, I would use a "better" design, but I still won't call this design "bad."
 
Tshea wrote:
"Now comes along the HVAC guy who (incorrectly, improperly) installs the A/C circuit in the sub panel. Now almost every time the A/C starts, it trips the main or sub breaker."

This train of thought would put every contractor out of business.

Charlie wrote:
"We can't do today's work in fear that tomorrow's worker will do something unsafe. We can't build in extra safety margin, just to protect the owner from the unsafe work of tomorrow's worker. We are allowed, and I believe we should feel free, to presume that tomorrow's worker will look at the situation, see the existing configuration, and make any additions in a code-compliant manner."

This train of thought is what an experienced contractor will use to help him get through the everyday grueling work schedule to stay competitive.


If someone (anyone doing electrical work) does not know how to install electrical installations to be safe and compliant, they should not be doing the work... yet we all know that does not stop many.
 
Pierre C Belarge said:
If someone (anyone doing electrical work) does not know how to install electrical installations to be safe and compliant, they should not be doing the work... yet we all know that does not stop many.
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No truer words have been (said) written!

Amen!
 
Maybe its just me, but when I see something like this I don't think "bad design" because I usually figure that the sub panel was added AFTER the first panel go full. If I went to a new job and saw both panels going in right from the start, then I would start thinking "thats not how I'd do it"
 
This situation is no different than a panel that has feed-thru lugs. We install them all the time. Mostly we install them in new buildings where the engineer needs more than 42 breaker spaces for a panel. We feed the second panel from the first with feed-thru lugs. The feed-thru lugs are cheaper than a breaker and they do not use up breaker spaces.
 
What if the first panel has 42 circuits and is rated 200 amps, and the average breaker is drawing 3 amps. Yet the building needs 3 more circuits... do we double up on some breaker slots? No, we install a subpanel. Remember in most installations there is diversity of loads, and those 42 circuits may not all be drawing current at the same time... the second panel may not be such a bad idea, especially since the jobs today are so competitive price wise.
 
My answer is this. Just because you are installing a 100 amp sub panel doesn't mean you need to feed it with 100 amps.

What should be done is do a load calc on the main panel and find what you have left and then feed the sub panel accordingly.

i.e. 100 amp main panel using 40 amps then feed sub panel with 60 amps and you should have no problems.
 
Re-read the OP. 24 space 100A in residence. 100A branch breaker feeding a 24 space 100 (125A) MLO panel.

I would probably have used a 200Amp 40 space panel to start with. That's why I say poor design. Now if the HO is cheap adding the sub panel is the way to go!

The more I think about it I agree it's like a feed thru panel (comm/ind). The feed thru panels don't have a XXXAmp breaker protecting the second panel. This resi one does.

We can come up with a lot of "what ifs."

I don't see any code violation.
 
As far as a homeowner or whoever overloading a service that looks like 200 amp but is actually 100 amp, this is not a safety issue. His HVAC trips the main circuit a bunch of times, he calls an EC who tells him to upgrade his service. No one gets hurt.
 
Thanks everyone for the input !

At the IAEI meeting tonight I brought up this same question. One reply was that I should check the listing inside the panel. It was this persons thought that a 100a main breaker panel would have a listing in the panel that would restrict the size of the branch circuit/feeder breakers to something less than 100a.

I have yet to find one person that has referenced a code article that would indicate that the installation was not allowed by NEC.

I'll post the result of the inspection of the instructions inside the panel.

David
 
There was no breaker amperage limit listed on the labeling in the panel for branch circuit or feeder breakers installed. We are accepting the 100a subfeeder breaker in the 100a main because the main is rated for 100amps and there is no calc for this residence that shows that it will be pulling more than 100a.

David
 
This is a design issue, and the NEC is not a design document. Following the NEC results in a safe installation , not one that is necessarily efficient or adequate. There is nothing unsafe about this installation as long as everything is properly protected against overcurrent. In order to judge its adequacy we would have to know the loads being served.
 
OK Here's some food for thought...
SAme situation only this time instead of a 100Amp branch breaker, a 125A branch breaker is installed. Assume all wire sizes are correct. Is this a violation??
 
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