DIYbanter - View Single Post - Extending multiple BX cables: with multiple bx cables or multiple wires in greenfield?

Nate Nagel

wrote:
On Sat, 14 Nov 2009 09:07:07 -0600, bud--
wrote:

But I managed to read the wrong column for ampacity. Particularly
annoying because the value I read was not what I remembered. What I
wrote is revised below.

310.16 gives the allowable "ampacity" for the wire. Assuming it is #14
with *THHN* insulation the ampacity is 25A (but cannot be used at over 15A).

310.15(B)(2)(a) requires the ampacity to be reduced if there are more
than 3 wires in a conduit. If you have 7-9 "current-carrying" wires, the
ampacity is reduced to 70%. For #14 *THHN* the ampacity is then 17.5A
and you can still use it on a 15A circuit. You can not have more than 9
"current-carrying" #14 *THHN* wires in a conduit and connect them to a
15A breaker.

310.15(B)(4) says that the neutral of a 3 wire "Edison" circuit does not
count as one of the "current-carrying" conductors.

310.15(B)(5) says that ground wires do not count as "current-carrying"
conductors.

You can run 4 "Edison" circuits, 12 - #14 *THHN* wires (8
"current-carrying" conductors) in a single conduit. Add a #14 ground wire.

Appendix C.3 says the 13 - #14 conductors can be run in a 1/2"
Greenfield. It is the maximum number and you would have to pull the wire
carefully. With 3/4" Greenfield you shouldn't have a problem

For 2 wire circuits (if you do not combine them to be "Edison"
circuits), you can run 4 - #14 *THHN* circuits in the same conduit - 8
"current-carrying" wires plus ground = 9 wires.

When you are talking about 14, 12 and 10 gauge, don't forget 240.4(D)
that effectively limits that to 15a, 20a and 30a respectively, except
for certain fixed loads. You always have to use 240.4(D) if
receptacles or lamp holder are involved since the installer has no
control over what the user will attach..

right, but he's talking about derating because of all the wires running
in a single pipe, which uses the "real" ampacity

when the ampacity drops below 15A then you just can't put any more wires
in the pipe

nate

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