On Wed, 27 Jan 2010 20:00:04 -0800, jk wrote:

Ned Simmons wrote:



310.4 Conductors in Parallel.
(A) General. Aluminum, copper-clad aluminum, or copper
conductors of size 1/0 AWG and larger, comprising each
phase, polarity, neutral, orgrounded circuit conductor shall
be permitted to be connected in parallel (electrically joined
at both ends).

I read that to mean you can run 1/0 and larger conductors in parallel
if they're run together. 12 ga romex daisy-chained around the
perimeter of a room with both ends tied to the breaker would be (1)
too small and (2) not "within the same raceway, auxiliary gutter,
cabletray, cablebus assembly, trench, cable, or cord."


The question is, is a "ring" really parallel conductors. I could
argue it either way.

jk

There's something in the NEC to the effect of (and it's 1:25 AM so
I'm not looking it up now...) 'Parallel conductors on the same
circuit must be the same length and conductor construction (AL or CU)
and wire gauge and insulation temperature class, etc.' so the currents
are evenly balanced between the individual conductors.

On a ring circuit, there can only be one point where this is even
remotely possible.

And with low voltages (up to ~600V) it is normally done with two or
more conduits in parallel following the same routing, each with A, B,
C phases, Neutral and Safety Ground. Then you pre-cut the wires
before pulling (and trim slack evenly between them all) so they are
the exact same length (within a foot or so) to keep the resistance
balanced.

That way the magnetic forces inside each conduit mostly cancel
themselves out - when you get a short circuit at the far end the
conduits are far less likely to try to leap away from each other, rip
themselves off the walls or the ceiling or the Unistrut racking, and
make an even bigger mess.

-- Bruce --