I'm the guy who was asking about the arc welder/table saw wiring in
the "Another 220V question" thread.
In working through the various solutions there I noticed that I've run
out of space in my main load center. Not space for breakers--there's
about six vacant slots--but space in terms of holes in the outside to
feed wires/conduit/EMT/AC through. One side is unusable due to the
proximity of the meter and the service end box; the other side is
difficult because there's a door there; and the bottom is way away
from the action (the load center is in the cellar and all the wiring
goes up). I'm left with the small area on top where the electricians
have taken the best knockouts with their 3/4 and 1'' EMT.
It seems that a solution might be to move the action away from the
load center to a position on a joist about three feet in front and
above it. Not a sub-panel--no breakers--but ganged together deep 4
inch boxes which would be connected to the load center by 1" or even
2" EMT. The cable from the outlets, light fittings, etc would run to
the ganged center where each would be connected to its own wire(s)
coming from the load center through the 2" EMT.
Now I understand that there's a requirement to derate wire when more
than [some number] share the same EMT/conduit so maybe they have to be
spread over (say) 3 * 3/4 EMT and 1 * 1" EMT. Further that there's a
limit to the number of connections in a box but I also seem to
remember that pass-through's don't count. Would this constitute a
pass-through? Might not get to this anyway--my rule of thumb says that
if it looks crowded put in another box and I've always been well under
the permitted number of connections.
Is this a solution worth considering? Is there an easy way to work out
the derating requirement? (E.g. for six feet (the three feet distance
doubled for luck <g>) and 12 ga you can put 3 hots and 3 neutrals in a
1/2 EMT; 6 hots and 6 neutrals in 3/4; and 12 hots and 12 neutrals in
1" all without any derating. Just in case anyone gets the wrong idea:
these are a guess at what the numbers might be. Can someone tell me
what the real numbers are?)
Go out the bottom anyway, neatly loop down, over, up whichever side is
easier then to the receptacle. And extra 3-6 feet of wire's not a lot.
Alternately use one or two dual cable clamps to bring pairs of 15A or 20A
circuit into a single knockout. You could free up several 1/2" KO's that
way, less messy then a big offset nipple (via a big or concentric KO to a
deep 4-11/16" box (or bigger or even a wire trough, a 4" would be too
small). Note too concentric KO's are no good for grounding, a bare ground
wire will be needed to jumper the nipple.
See the current (2002) NEC for derating tables. Alternately hire an
electrician to do it for you.
I don't know if this will work for you, but there is such a thing as duplex
connectors for BX cable as well as Romex cable. They allow two cables to be
attached with one connector. They are a little bigger than standard
connectors, but if you line them up nicely you can free up some holes.
Also, many new standard romex connectors are approved for more than one
cable. You could double up some Romexes in one connector.
Many factors come into play with derating wires. Load diversity, length of
pipe, circuit size, etc. Instead of ganging a bunch of boxes together you
could use a length of wiring trough above the panel. It comes in different
sizes (4" x 4", 6" x 6", 8" x 8", 12" x12") and different lengths which can
be cut. From the trough run a few conduits down to the panel. As long as
the conduits are 24" or less, no derating is necessary.
I don't have my codebook with me so I cannot give you the exact code
reference for derating, but perhaps someone else can.
The other advice given was good. You could install a 6x6 junction box and
connect it to your panel via the largest knockout you've got (hopefully a
1.5" or 2" one). Your wire fill is still regulated in all conduits or
nipples, and cables like romex will count a lot against fill. For ampacity
derating, use individual wires with a "HH" in them (e.g. THHN), or use romex
with a -B suffix (e.g. NM-B). These are all rated for 90C, and can tolerate
a 70% derating and still be used like you nomally use them (#14 on 15A, #12
on 20A). If using #10 wire, it won't quite make a 70% derating and still do
30A. So be careful with that. Neutrals in a 240V circuit don't count as a
conductor for temp derating and neither do grounds. But a white neutral in a
120V circuit counts unless its a "multiwire branch circuit".
If you keep the conduit or nipple to the J box to 24" or less, none of this
temperature derating applies. But fill still does, but I believe a higher
percentage then normal is allowed.
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