125amp feeder breaker in a 100amp main breaker panel

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Ravenvalor

Ravenvalor

Senior Member
Location
Triad region of NC
Occupation
Electrician
Hello Friends,

Is it permissible by the NEC to install a 125amp panel feeder breaker in a 125amp mainlug service panel that is back fed with a 100amp main breaker?

Thanks,
 
ptonsparky said:
You have my permission. I would make at least $50 more on just the breaker. Might as well put in the bigger wire to.

Why do you want to do this?

Your buss tabs may only be rated at 100 amps.
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Hey Ptonsparky,

Thanks for the help. The Siemens panel is rated 125amps so I know the buss tabs are rated the same.
The reason that I am doing this is because I want to put a sub panel in the basement for lighting and receptacles.
 
I would just come off the main lugs and feed my sub. No advantage of the additional breaker. Back feed another 100 if you want but remember to size the wires correctly. They may be larger than your service wires.

Generally we take a 60 amp feeder to another panel. Usually more than adequate for general lighting & recepts.

Do a load calc if you are worrried.
 
I am trying to understand your install. You have a 125 amp main lug panel that you want to add a main breaker to that is 125 amps? This panel is then fed from a 100 amp breaker??? Why? I know most 200 amp panel will only allow a 125 amp breaker on the buss but not sure a 125 amp panel will allow that.
 
ptonsparky said:
I would just come off the main lugs and feed my sub. No advantage of the additional breaker. Back feed another 100 if you want but remember to size the wires correctly. They may be larger than your service wires.

Generally we take a 60 amp feeder to another panel. Usually more than adequate for general lighting & recepts.

Do a load calc if you are worrried.
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I have decided to design the job with a 100amp breaker that will supply a 1/0 SER AL cable which in turn will feed a subpanel in the home. You are right 60A or 100A is large enough for this home.

Thanks,
 
ptonsparky said:
Over a roof? Thru an attic?
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Even then ....

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.

(3) Individual Dwelling Unit or Mobile Home Conductors.
Feeder conductors for individual dwelling units or
mobile homes need not be larger than service conductors.
Paragraph 310.15(B)(6) shall be permitted to be used for
conductor size.
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It says what it says and does not say what it doesn't. :grin:
 
iwire said:
Even then ....




It says what it says and does not say what it doesn't. :grin:
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I agree:grin:

Does make me rethink putting a 70 amp breaker on a a set of 4s to another panel in the house. Might as well be another 100 or in the OPs case, none at all.

The unattached garage/shop would require a #2 cu feeder for the 100 amp panel or is that associated with the dwelling unit?
 
iwire said:
Feeders never have to be larger than the service conductors.
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Not when applying exceptions allowed by 310.15(B)(6). This has been debated many times on this forum. If the feeder is not carrying the entire dwelling load then it may have to be larger than the service conductors supplying it.


As far as the 125 amp breaker in the panel that only has 100 amp overcurrent device I see no problem with it. The max overcurrent protection of any conductor or device is not exceeded. This is no different than putting a 100 amp main breaker panel in a detached garage because you need a main and it cost less to just use a main breaker panel but you are supplying it with only a 60 amp feeder.

If the feeder supplied by 125 amp breaker draws more than 100 amps the breaker protecting the panel is going to trip.

I would not put in the 125 amp breaker unless for some reason you already have one. It is likely to be more expensive than a 100 amp breaker.
 
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