You would have to check your local codes, but we have three separate
service entrances (main panels).
Our electrical meter is located on a pole out at the road. Then the power
comes down to a junction box behind our pump house. Separate lines then run
to individual service entrances at the pump house, garage, and our house.
Each building has it's own main breaker, grounding electrodes, etc.
Completely seperate and isolated from each other (other than the incoming
We built the pump house first, then our garage, and finally the house. So
we wouldn't have been able to run a subpanel off the house anyway.
However, I did run a small 30A circuit off the house panel to a small
subpanel in our storage shed.
So how are the service entrance conductors sized? If the ampacity of
the service conductors is less than the sum of the ratings of the main
breakers on the three main panels, then the only thing protecting the
service conductors from overcurrent is the statistical unlikelihood of
all three services drawing maximum power simultaneously?
The NEC doesn't require the main service conductors to be equal or greater
than the sum of the up to six main disconnects. It does however require the
calculated load to be less than the size of the service conductors
So nothing protects the service conductors from overcurrent. This
surprises me, but I guess if the main service conductors overheated
due to overcurrent, they are far less likely to start a fire in an
enclosed space compared to a branch circuit or feeder. I suppose that
is also the the reason that the distance the service conductors can
travel within a building before the main disconnect is restricted.
It's surprising to me as well. Usually the NEC seems to overprotect wiring
and equipment. Every time I, as an electrical contractor need to add a load
to one of these types of services, I have to do a load calculation, just to
be sure that the homeowners and electricians before me, didn't overload the
I installed a 200A panel in our garage, and another 200A panel in our
house. Each panel is supplied with three 4/0 aluminum conductors that
connect to the power companies junction box. Our pump house only has two
breakers, 20A for the pump, and 15A for the lights. It has been many years
since I hooked that up, but I think I used three 1/0 copper conductors for
The cable between the junction boxes and the transformer at the road was
provided and installed by the power company. I wasn't here when they hooked
us up, so I don't know what kind of cable they have on their side. When we
hooked up our house, I asked the power guy about the cable, and he made a
comment that their cable was a little undersized to be powering all three
buildings. But he said that was done routinely a few years ago before they
started running load calculations. He said it wasn't a big problem, since
we were unlikely to max out all panels at once, but we may notice dimming
lights if I was powering up equipment in the garage while the heaters were
running in the house or something.
But, in the 15 years or so we have lived here, I've never noticed any
problems with dimming lights, or any other signs of too much current draw.
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