On 2008-05-26, Aaron Fude wrote:

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?

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
called subpanels.

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
achieved.

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
feeder.

Yours, Wayne