I'm confused about the very essense of a sub panel: is it just a
glorified junction box or can it actually increase the amount of
current that enters your house? For example, if I want to increase the
amount of current from 100amp to 150amps can I accomplish that with a
"buffalobill's" short and sweet answer is correct.
What it does,basically, is gives you extra spaces for more breakers,so you
can run additional circuits. More or less an extension off of the main
The only way to increase the amount of
current that enters your house , or better said, the amount of current
to use in your house, is a service up grade.
New main panel, larger wire going to and coming from the electric meter, and
often times larger wire from the power company's transformer.
I don't think it is quite that simple. Suppose you have 200 amp
service, and a panel that will accommodate only eight breakers, each
of which is occupied by a 15 amp breaker. Then the only way to access
the other 80 amps of capacity you already "have" is via a subpanel. It
doesn't really increase the capacity you have available, but it allows
you to access it. Adjust the numbers if you like, and argue about how
mis-designed my hypothetical situation might be, but the principle is
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First off adding up the ratings of the installed branch circuit Over
Current Protective Devices is not how you calculate how much of an
existing services ampacity is in use.
Second there are a hell of a lot of main lug only panels that have only
eight slots in them but the smallest breaker would have to be thirty
ampere for the panel to pass electrical inspection as service equipment.
It's important to realize that "sub panel is not a term that you will
find in the US NEC. A panel can be supplied by service entry
conductors, a feeder, the secondary conductors of a transformer, or a
local power source. Since the term itself is undefined it is difficult
to give you a good answer. I have converted multiple panel
installations to a larger service size by using a new main lug only
panel installed as service equipment to control the feeders that supply
each lighting and appliance or power panel but I doubt that is what you
had in mind.
"This alternating current stuff is just a fad. It is much too dangerous
A panel is just a box to hold circuit breakers (overcurrent protection
devices, or OCPDs). The panel that holds the main shutoff for the
electrical service is called the main panel, and other panels are
You can have all your breakers in the main panel if that is convenient
and you have enough space. Or you can use a subpanel if you need more
space for the breakers or it is more convenient. For example, if you
have 8 parallel circuits all running to your kitchen at the opposite
end of the house, it might be more convenient to have a subpanel near
the kitchen, run a single larger feeder circuit to the subpanel, and
then originate your kitchen circuits at the subpanel.
The other thing about panels is that the calculated demand of the
supplied loads from the panel should be less than the rating of the
feeder supplying the panel. So in the case of the main panel of a
house, the total demand of the house should be less than the rating of
your service conductors. Nothing you put downstream of the service
conductors can increase the rating of those service conductors; the
presence or absence of subpanels is immaterial.
Lastly, note that it is perfectly fine for the sum of the ratings of
the breakers in a panel to exceed the rating of the feeder for the
panel. Each of these ratings is a maximum and will seldom be
For example, if you have 6 circuits rated at 20 amps in a panel
supplied by a 100 amp feeder, then you could run 5 of those circuits
at their full load of 20 amps as long as you like. If all 6 circuits
were at full load, then eventually the 100 amp breaker protecting the
feeder should trip (it won't be immediate). But in practice almost
never will all 6 of those circuits be at their full load--you might be
able to service a whole house with 40 circuits from a single 100 amp
Expanding on Whayne's point, a good use of a sub panel; is to reduce
voltage drop problems. If you have a big house it might be worthwhile
to run a feeder to the far end and serve those loads from another
panel. If you are not running all the circuits at a level stressing
the feeder your voltage drop will be minimal. "Diversity" usually
assures that does not happen.
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