| Andrew Thomas 2005-06-17, 11:30 pm |
| I assume the NEC requires than any split-phase subpanel (or main panel,
for that matter) be protected by a common-trip, two pole breaker.
Certainly that's how I've always done it. But now I have a situation
where if I had my druthers, I might not. So it begs the question: what's
the real reason behind this requirement, if requirement it be? Certainly
I can see two arguments in favor of this practice. One being that, well,
it's convention: and we all expect that by throwing a single "main"
breaker or sub-panel feeder breaker handle, the panel is de-energized.
Likewise if one side of the house goes dead (assuming we _didn't_ use
common trip) and the branch breakers don't seem responsive, the less
cautious among us would feel at liberty to rip the face off the panel
and go poking around in there assuming both lines were cut off. OK,
maybe that's only one reason. And who would go poking around rather than
flip the main breaker(s) anyway.
But is this really all there is to it? Seems like kind of a weak reason,
especially where subpanels are concerned. Me, I'd just as soon wire a
pair of "night lights", one to L1 and one to L2, near my subpanel so
that if one "side" popped, I'd still have light enough to see the
breakers. In my duplex, each of the main panels has such a light and
it's a very nice feature: especially given the fact that the breaker I
usually pop just happens to be the one wired to my night light.
By extending the same principle, and recognizing that darkness
(especially complete darkness) is a real hazard, I'm wondering: why
should I lose the whole house just 'cause one side is overloaded?
Is there an exception in the NEC for panels/closets which can only be
accessed by "authorized personelle"? Not that it applies to the case in
mind, but I'm curious.
Ultimately the solution is to ensure the main breakers are of such
ampacity as to never trip: but the case in mind is band-edge and not
practical (affordably speaking) to fix. As it stands I've never popped
the main or subpanel breaker but my usage is soon to increase somewhat
and the calcs indicate it might be expected to start happening, once in
a while.
-Andrew
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