# ? sub panel wired from main breaker ?

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• posted on December 16, 2003, 6:39 am

service.
than
and
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Yes. The only place that neutral and ground are allowed to be the same is at the main disconnect. In your case, the main disconnect is your 100A main panel. Adding a separate panel downstream of that disconnect requires that neutral and ground be separated.

100A
protecting
than 100 amps? How do you

In your case (a subpanel), the wires feeding the panel must be sized per the breaker protecting it. So no, you can't reduce the size of the wires unless you install a breaker smaller than 100A in your main panel and feed the subpanel through it. Logical sizes would be 60A, and 40A for 60 degree C rated installations (70A, and 50A for 75C rated installations). Since this subpanel will be so close to your main panel, I'd just go with #4 copper and let the existing main breaker be your disconnect.
In a residential setting, there is no specific load for a receptacle. If there is something fastened in place that will connect to this circuit, you count that at its nameplate value. If there is a certain amount of square footage to be served by this circuit, then it is 3 VA per square foot. When adding up circuits based on square footage, you count the first 3000 VA at 100% and then count the remainder at 35%. Motors are calculated per the motor horsepower current listed in NEC 430.6 and then 25% is added only to the largest motor. Some other specific circuits have code minimum values -- the two kitchen and one laundry small appliance circuits are calculated at 1500 VA each, and these numbers can be added into the VA number that gets the 100%/35% demand factor applied.
-- Mark Kent, WA

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• posted on December 16, 2003, 1:59 pm
Mark or Sue wrote: > In your case (a subpanel), the wires feeding the panel must be sized per the breaker protecting it.

I thought the original post said a 200A main breaker/disconnect.
Bob

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• posted on December 17, 2003, 2:56 am

your
close
disconnect.
The original thread said 200A, but then the thread got intermixed with questions. The way I read Art's question was that he has a 100A main panel with a sub next to it. If that main panel is greater than 100A, then he needs a breaker <= 100 A feeding that sub panel.
There may be an exception using the tap rules, but even if so I wouldn't use it in a residence -- people have a bad habit of sticking breakers in a panel without calculating feeder or service loads.
-- Mark Kent, WA

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• posted on December 14, 2003, 5:46 pm

Ok, I'm not the first person to be confused by the NEC. <G>
IMHO, if you have more than 6-8 circuits in a sub-panel, it should have a single breaker, in it, which shuts down that whole sub-panel.
Feeding a sub-panel from a breaker, gives the ability to completely kill that sub-panel, making it much easier and safer to work in.

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• posted on December 14, 2003, 10:04 pm

sub
its
All subpanels are protected by a breaker, it just isn't in that panel it is in the parent panel.

You get that capability in the parent panel breaker feeding the subpanel. There is no rule preventing you from having a breaker at each end, but you have to buy two large breaker this way instead of only one. In a setting where you can't control access to the parent panel (no lock out tag out or people/kids ignore signs) then maybe you'd want a main breaker in that subpanel.
-- Mark Kent, WA

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• posted on December 15, 2003, 11:24 pm

One also needs to spend a buck or two to get the little gizmo that clamps in the breaker that is being used as the main for that panel.
IMHO, it is worth it. Can you tell I don't like having to walk between panels? <G>