I have a detached garage with a subpanel in it that is fed underground
from the main panel in the house. When the electrician wired this up,
I mentioned that some day in the future, I might want to have a light
on the front of the garage that I could control from the house if I
wanted to. Since at the time there was already a trench dug from the
house to the garage, he said why don't I throw in a 12/3 UF cable (so
it could be a three way switch in the house and garage) and leave the
ends hanging in the garage and the house for that potential future use.
So now it's 5 years later, and I still haven't hooked up that cable to
a light in the garage, but now I'm wondering what's the correct way to
wire this up. As I have since come to understand, you can have only
one electrical feed to an outbuilding, so if I have the feed to the
subpanel in the garage and this other cable coming in also, it sounds
to me like that counts as two feeds, and would not be allowed. On the
other hand, could I wire it such that the light on the front of the
garage is fed from the subpanel in the garage, and then use that 12/3
feeder as a switch leg back to the house? In that case the garage
would have just the one feeder. So would this be the corect way to do
this? Or maybe it doesn't matter which end the power comes from?
This is not considered a second feed because you will be using the power
from the garage to feed the switch in the house. You are not sending power
anywhere just turning the power on. Check the web for a three way wiring
diagram, this should get you started. Or try to fin my logic below
That call is up to the Authority Having Jurisdiction (AHJ) which is the
codes way of saying the local inspection authority. Using the cable as
a switch leg back to the hose would always be acceptable and would
render the question moot.
The other approach that is perfectly valid is to take advantage of the
code exception for emergency power and run it supplied from the house
with a receptacle just inside the out building. Then if you have to
work on the out buildings wiring you power your trouble light and tools
from the emergency outlet.
A third approach is to power a post light with a weather proof three way
switch. In that way the circuit never reaches the out building.
I cannot speak for your AHJ but most would not be concerned about the
power to an outside light as long as the switch were in a completely
separate box or a divided box so that when the out building's Building
Disconnecting Means is open there will be no power in the outbuildings
wiring system. Some would require that the three way switch be labeled
as powered from the house. Others would require that it be mounted on
the exterior of the building so that all access to the lights wiring
would be from outside the building.
I have always preferred the emergency circuit approach for it's obvious
advantages for having power while trouble shooting the rest of the
buildings wiring. Emergency circuits must be kept entirely separate
from other wiring. If the circuit enters a non emergency box the box
must be fully divided so that the two circuits cannot effect each other.
A four inch square by one and one half inch deep box with a plaster
ring and a box divider fill that bill quite nicely as would completely
"This alternating current stuff is just a fad. It is much too dangerous
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