On Fri, 30 Aug 2013 14:48:36 +0000 (UTC), HerHusband
wrote:

I have a sub-panel in my greenhouse that is fed by an incoming wire
from the main house panel. The incoming wire is directly feeding a
30 amp breaker, therefore I can disconnect the power to the panel
by simply tripping the breaker therein. (Also within the panel are
two additional 15 amp breakers, one for lights and the other for a
single outlet.
I've always liked the convenience of the "breaker input" instead of
hard wiring the input directly to the panel. Does this method meet
code? Any hazards? Any down-side?

If I remember correctly, code "requires" a breaker in the main panel to
feed the sub panel. Your 30 amp breaker protects the wiring that runs
from the main panel out to the sub panel in your green house. If you were
ever to cut the wire with a shovel or something, that breaker is what
would prevent the whole system from shorting out.

There are exceptions but that's certainly the way I'd do it. The
issue wasn't feeding the sub through a breaker in the main, though.
The issue was feeding the sub *backwards* through a breaker in the sub
(with a breaker also in the main). This normal.

The 15 amp breakers in your green house protect the wiring for your
individual light and outlet circuits.

Your configuration is normal, and I have the exact same setup powering a
sub panel in my shed.

However, unlike the main panel, the neutral bus should be isolated from
the ground bus in the sub panel. You should also have an additional
ground rod installed out at the greenhouse to suppliment the ground at
the subpanel.

This is perfectly normal (I do it all the time). Electricity
doesn't care about the direction it "flows" through the breaker.
To be completely precise it's not flowing either direction. It's
vibrating back a forth a tiny bit.
The energy is "flowing" one direction

The US power grid uses "alternating current" (AC). The electrons flow
from positive to negative, then reverse to flow backward from negative to
positive (rising and falling in a sine wave). In the US the current
alternates back and forth like this 60 times a second (60 hz). So
technically, the electricity is flowing in both directions.

True but irrelevant. There is a "source" and a "sink". The breaker
can't tell the difference, so it doesn't know it's being back-fed
(which is the subject under discussion).

From a practical standpoint you have a supply (the power grid, generator,
etc.), and a load (lights, heaters, radios, computers, etc.). Without the
load, current isn't flowing either direction.

Huh?