Outdoor electrical circuit install check

Hello,
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?
Thanks, Wayne
P.S. Is RMC identical to iron pipe for water?
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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.
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Pipe threads are tapered also.

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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?
Cheers, Wayne
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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

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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 junction box.
Cheers, Wayne
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The box on the house is fine. In my opinion, the whip is fine too. I've just had some real particular inspectors give me grief about conductor types in outdoor conduits
wrote:

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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.
Thanks, Wayne
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On Sat, 28 Jan 2006 18:01:10 -0600, Wayne Whitney

You can use U/V protected type UF. It looks like Romex but is a wet location wire. It can also be substituted for romex in the house.

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