While installing a ground rod(s) is not required if you use the ground
(bare) wire from the house, I'd highly suggest you install at least
one rod. The main reason is that if lightning hits near that shed,
the force will have to travel back to your main panel in the house,
and do damage in there. It's not that hard or costly to drive a
ground rod and connect it. You could save your computer and other
stuff in the house if lightning hits near the shed.
You should NOT be using the neutral for grounding. But the ground bar
for your box, or considering that you probably have 5 at most bare
wires, you could just wirenut them together including the one from the
house. But if you use a gound rod, that #6 wire would be hard to
wirenut into the bundle. Ground bars are only a few $$.
Why did you use a 12-2 wire for FOUR 20A circuits. If you have not
yet buried the cable, I'd replace it with at least 10-2, or preferably
10-3, which would give you 220 availablity. Otherwise, you are
running a 20A circuit to four 20A breakers. Rather pointless. You
may as well have just relied on the house breaker and skipped the
breaker box in the shed.
Thanks for the advice, everybody. I gather that there was really no need
to install a breaker box in the outbuilding. I was wondering about that
since the main panel in the house would offer circuit protection. So when
exactly would you need to install a subpanel or additional box in a
You put a subpanel (service panel) in the outbuilding when you *really*
want more than one circuit out there. A building can only have one
electric circuit coming in (with a few very specific and odd
exceptions.) So you run a *big* circuit into the building and then use
the panel to split it up into multiple small circuits.
In your case, maybe you would run 10/3 cable with a 2-pole (240v) 30A
breaker in the panel at the house, then have several 15A or 20A breakers
in the garage panel, and have the capability of later adding a 20A 240V
breaker for an air compressor or 3500W electric heater without needing
to add any new wire from the house. Another good thing about that is,
if you overload a circuit, the smaller breaker in the outbuilding
usually trips rather than the big one in the house.
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