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I am in Washington state. I want to install two 4000W infratech infrared patio heaters.

My main panel is in the garage and the run is about 100ft to where I want to place a sub-panel. Part of the run from the main panel is in a crawl space and then transitions outside.

Does all the wiring need to be in conduit or can the inside portion(35ft) be 6/3 NM cable (or other recommendations) and then junction to 6 AWG THHN wire in conduit for the outside run(65ft), Or can the entire run be 6/3 NM cable?

I plan to use a 50 amp breaker at the main panel and 6AWG(per above question) to wire to a sub-panel with two 30amp GFCI breakers in the sub-panel and 10AWG THHN in conduit to wire to each of the 4000 Watt heaters. Then back from the heaters with 10AWG THHN in conduit to two control switches.

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    Hmmm. To answer your question, yes cable can be run inside conduit, just be sure to up-size it enough so it can be pulled more easily. But why are you running a sub-panel for just 2 heaters? Are you planning on running something else off of that at a later date? Do the heaters require a neutral? If not I'd be inclined to run TWO 30 amp circuits via conduit the entire way.So you'd be looking at 4 hots and a ground and maybe a neutral. 2 30 amp breakers in your main panel. 6/3 is EXPENSIVE! BTW, I'm a fellow Washingtonian! – George Anderson Nov 28 '20 at 22:55
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Firstoff, NM cable cannot go outside. Fullstop. Doesn't matter if it's in conduit: outdoor conduit is not "indoors". It actually tends to fill with water due to condensation.

If you really, really, really need to use cable outside, UF-B or MH feeder are the right stuff. However UF-B is very flat and wide, and does not work inside conduit. (the conduit must be outlandishly large: 2" conduit for 6/3 UF-B).

Given the distance, size of the wires and the fact that you're going to a subpanel, you might consider running aluminum wire to cut down on the cost. There is nothing wrong with aluminum wire at these large sizes. Further, at #4 and larger, you can use the same THHN wire, no need for white neutral or green ground.

You can decide for yourself whether to run conduit all the way across the crawlspace. It's not that hard once you get the hang of it. For novices, I strongly recommend EMT conduit because it has an "Undo" and a "Save game" button: you can do a few segments, take a break, snap photos, get feedback, undo mistakes, and resume as needed. Then the hard part is behind you; actually pulling the wire is just a "victory lap".

(in conduit you always build out the entire conduit run before you pull ANY wire. Never assemble conduit over wires: that's simply wrong.)

Some people say "oh, PVC is better"... I suppose that's true if you don't glue it, but then you better really anchor it down well, or come back when it's finished and glue it. Also EMT provides the ground, so one less wire.

By the way, if your garage is an outbuilding, you can't run separate circuits, you must do a subpanel. On this forum we strongly recommend people "Think BIG" when choosing a subpanel. I would choose a 24-30 space even if it is laughably bigger than your main panel. Spaces are dirt cheap, and when you add something to the house, it's nice to find the spaces you need waiting for you, instead of having to do a bunch of expensive rework. While the heaters will largely "max out" the panel, if you aren't running the heaters you could run a machine shop, compressor, welder, even an electric vehicle charger. These days an EV charger is something homebuyers will pay extra for, and you'll be 90% there.

One other reason to go THHN over cable: With #6 copper or #4 aluminum,

  • NM and UF cable are limited to 55A (breaker at 60A).
  • THHN can go to 65A (breaker at 70A). So in that case you could run a 120V saw + dust collector even with both heaters going full-bore.

Your 4000W heaters need to have 125% of that provisioned for them, so 5000W each or 10,000 watts the both. That's 41.7 amps. That leaves you 13.3 or 23.3 amps to play with depending.

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  • Thanks for both consults. Regarding UF-B 6/3, could this be run without conduit since it can be buried underground without conduit? In my application it would be fixed to shake siding under an overhanging 2nd floor deck. – David Brook Nov 30 '20 at 16:39
  • also, regarding 10,000W total requirement and 41.7A , if I don't plan to add any other devices, could I use a 50A breaker at the main panel? – David Brook Nov 30 '20 at 16:48
  • You say that now... lol... Yes, UF-B can run without conduit, but it'll take about a 30" trench: to pack above and below the cable with sand so a rock won't penetrate it, and achieve the mandatory 24" of cover. With Rigid conduit you only need 6" of cover and you don't need to care about rocks. You will need conduit to protect the "stup-ups" where the wire transitions from burial depth to the side of the building. By the way, consider #4 aluminum MH feeder instead of #6 UF. On feeder of this size, copper buys you nothing. – Harper - Reinstate Monica Dec 1 '20 at 18:03
  • Thanks, again. I mislead your answer. I know UF-B can run underground without conduit but can it be fixed, exposed to the side of a house more than 8 ft above grade without conduit? I will look at #4 aluminum MH feeder, the same question can it be fixed, exposed to the side of a house more than 8ft above grade without conduit? – David Brook Dec 3 '20 at 16:11
  • @DavidBrook Yeah, you'll need conduit for physical protection up to about 8' (note Sched 40 PVC is not good enough). The problem is how do you treat the top of the pipe? It makes more sense to me to have an LB conduit body at the point it turns into the house, use conduit for the thru-wall bit, and caulk that. I'm not entirely sure MH feeder is legal for indoor wiring without protection, shoulda mentioned that... – Harper - Reinstate Monica Dec 3 '20 at 19:44

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