I'm planning to run an electrical circuit outdoors for the first time
(with permit and inspections), and I was hoping someone with more
experience could tell me if my plans sound OK. For simplicity I'm
going to run NM cable and sheath it in appropriately sized conduit.
Run 6" RMC nipple through sidewall of crawl space into the back of a
wet locations outlet box. Cap ends with insulating bushings. Makeup
connections, ground the box, and install switch plus wet locations
switch cover with gasket. Run about 2' of NM liquid tight conduit
from the outlet box to the device (exterior tankless water heater),
using liquid tight connectors to make the connections. Makeup the
connections and ground the device. Seal house penetrations and the
outlet plugs for unused ports with 100% silicone.
Sounds OK? Also, a few more assumptions: no locknut is required when
attaching a threaded connector to the threaded ports on the outlet
box. The RMC nipple to outlet box connection should get silicone, as
there is no gasket, and the silicone will not interfere with the
ground bond. Are these assumptions correct?
P.S. Is RMC identical to iron pipe for water?
NM Flex does not last but a few years in the sun, where I live. (Phoenix)
Check to see if the switch has an interrupting capacity of at least the load
in the heater. (20 amp)
Electrical conduit has a tapered thread GRC for plumbing has a straight
thread. Other than that the two are interchanged frequently where I live
especially for nipples.
I do not under stand your grounding assumptions cause you do not state from
where the grc nipple starts.
Thanks for your comments. What do you suggest as an alternative?
The gas tankless unit is a Noritz N-063S-OD, and its manual says the
maximum power draw is 125W (when freeze protection kicks in, otherwise
it is 65W). So any switch should be rated for 1 amp. And as I
understand, there is no need for a switch to be rated at the circuit
amperage, just the load amperage?
I mentioned I'm running NM cable and the conduit is just to protect it
from the elements. So I'm going to depend on grounding the box to
ground the nipple, the other end of the nipple in the crawl space
isn't attached to anything. My question was whether using silicone to
seal the connection between the nipple and the box will keep the
nipple from being grounded. I can always test the resistance
afterwards with a multimeter, is that sufficient?
Sounds fine, the only possible issue might be: NM cable and the wire inside
it, is not for use in wet locations. The whip from your switch to the heater
may be considered a wet location. You can use TW or THWN in wet locations
Interesting question, I assumed that the inside of a liquidtight
conduit would be considered a dry location. I see that I do have
enough spare THWN lying around, so I'll use it. Thanks.
Is it OK to run the NM cable into the exterior box, or should I switch
to THWN before leaving the house? I'd prefer to avoid another
Well, I just read a long thread about this topic at mikeholt.com. It
comes down to an interpretation of part of NEC Article 225 "Outside
Branch Circuits and Feeders". The second sentence of 225.4 reads:
Conductors in cables or raceways, except Type MI cable, shall be of
the rubber-covered type or thermoplastic type and, in wet locations,
shall comply with 310.8.
310.8 specifies what wire types can be used in wet locations. The
question is whether this sentence means "conductors . . . in wet
locations shall comply with 310.8" or "conductors in cables or
raceways . . . in wet locations shall comply with 310.8". My money is
on the latter interpretation, since the former interpretation just
seems to be a reiteration of 310.8, so there would be no need for the
second half of the sentence.
Anyway, I'll use THWN.
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