On 5/25/13 8:24 PM, Patrick Greenlee wrote:
H
Note: Two conduits does not imply two failures to get a short since the
wires will be insulated.
actually 4 failures would be needed. But, in general, the code requires
three layers between YOU and the conductor. Wall, raceway, wire
insulation. Romex (NM) is allowed inside a wall because it has two
layers: the jacket and the conductor insulation. But, it's only allowed
inside the wall because there's no mechanical protection, like there is
with a raceway/conduit.
I think the concern is not so much in the conduit itself, but in the
junction box to which the conduit attaches. At that point, you'll have
splices or connections, and they don't want your wire nut to fall off
and the end of the live wire to poke into another wire nut.
They DO make weird looking enclosures with dividers that split the end
of the conduit, presumably to meet the need for mixed conductors in the
conduit for some odd-ball application, but keeping the joins in separate
areas.
You would need both wires to lose their
insulation plus both conduits to fail and then a miracle migration of
the wires so they could touch.
Shovel or backhoe cuts are the sort of thing they're thinking of. run
both line voltage and low voltage in the same PVC conduit and sticking a
pickaxe, metal stake, or metal shovel blade through both simultaneously
isn't all that unrealistic. Yeah, two conduits side by side are about
the same, but you draw the line somewhere. Imagine someone driving a
ground rod. May hit one, but won't hit both.
Of course if you are a member of the
drain hole camp and insist that bugs will sneeze into the conduit with
sufficient force to get past the RTV then wire migration say during an
earthquake could be a problem if the tower is still standing.
Disclaimer: I acknowledge that some folks have had water ingress into
their conduit, that it is not a laughing matter, and that minor
misinterpretations of the laws of physics are the likely root of the
mistaken ideas propagated post water incident/disaster. ;) ;)
Problems arising from construction or landscape activity seems to be
where most of the stuff about "pacakging" arises. The size of the
bonding wire from one ground to another, for instance, is not driven by
current carrying, but by mechanical strength. Ditto for wire antennas.
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