Outdoor electrical circuit install check

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.

Pipe threads are tapered also.

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

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

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

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

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.

Anyway, I'll use THWN.

Thanks, Wayne

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.