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.
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
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
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.
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.
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
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
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