Wiring solution for 240V EV charging outlet in garage with long run to breaker box

Question

I am trying to come up with a wiring solution for a NEMA 14-50 240V outlet in the garage for an EV. Specifics of my situation:

• Max charging current will be 32A, so I need #8 wire.

• Breaker box is on an exterior wall, at a wire length of 90-100 ft from the outlet.

I'm proposing to run the first 70' through the attic; then pop out for the last 20' or so. I had originally thought I'd just run 8/3 NM Romex all the way, but found out to my dismay that Romex cannot be used in outdoor conduit. Then I figured I might strip away the sheath and treat it as THHN outside, but one of the answers to the linked question made short shrift of that idea.

So now I am faced with the following restrictions:

1. Cannot use Romex in the outdoor conduit.
2. Cannot remove the Romex sheath and treat component wires as THHN.
3. I would rather not run conduit over the entire 90 ft run. It would involve many 90 deg turns and be a hassle to set up. Romex still looks appealing for the 70' linear length in the attic.
4. UF cable over the entire run would work but be very expensive. Also, overengineered, since only the last 20' of the run is outside.

So my current thinking is to run Romex to near the exit point, and hop over to THHN/UF in a junction box before exiting the wall to the exterior of the house. The remainder to the breaker box will run in conduit outdoor. However, it will need connecting the two different conductor types with wire nuts. I am wary about doing this on thick wires, for a circuit that will have a constant 32A draw.

My questions are:

1. Does this plan sound reasonable? Can someone suggest better options?
2. Are my misgivings about wire nuts warranted? I looked for alternatives; it looks like bus bars could be used, or I could butt-splice wires together. I haven't investigated either of these in depth.
3. If I do follow this scheme and switch to a different wet-rated solution for the outdoor run, would THHN be markedly better than UF for pulling through conduit? What if I use UF but not pull, rather build sections of conduit as-I-go? (so I'm threading it through say 5' of PVC conduit at a time?)

Also a comment/question about pricing: #8 THHN is $0.64/ft at the local home improvement store. So $64 per 100ft for one wire! $256 for 100 ft for 4 wires. I thought THHN was supposed to be cheap; how/why is it so much more expensive than Romex, which one would think has the same amount of copper? For comparison, a 125’ 8/3 Romex reel is $185.

EDIT: Adding a further restriction on my setup to address @Harper's suggestion of using thicker wire. If I am to use the only side (bottom-left) knockout available on the breaker box, I need to do a tight 3" radius turn into the breaker owing to obstruction from a fiber box mounted infuriatingly close. I have seen that only 1/2" LiquidTight metallic conduit is able to do that, and four #8 wires already is technically past the fill for that size (but thinking pushing the rules a little might be ok for the last 6" of the run?). The only alternative is drilling a hole on the top-right side of the breaker box, but I fear I might be biting off more than I can chew at that point. Space inside the box is also quite constrained, so stuffing thick wire inside and running it to my circuit breaker will be a minor challenge as it is. These breaker box constraints are why I dropped down from #6 to #8 in the first place. :-/ (Of course, found luckily that #8 is electrically sufficient for my projected need).

Answer 1

That sounds like a reasonable plan. The only thing making me itchy is the number of 50-60A (read: 40-48A continuous) EVSE's on the market. Given the upfront expense and labor, I'd really rather see you wire this with #6Cu or better, #4Al (Al since it becomes very appropriate at these large sizes and you'll like the price.)

The only downside of this is you will need a longer junction box to make the splices. A simple 4-11/16" box won't cut it anymore.

Technically, you need THWN or preferably THWN-2 rated wire. However electrical supply houses don't like stocking 2 kinds of wire merely to save a nickel a reel on the price difference between THHN and THWN-2, so almost any wire you buy these days is dual-rated THHN/THWN-2.

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As far as splicing, you're right. Wire-nuts are going to be a pain at #8. So I recommend Polaris connectors (lug connectors with a rubber wrapping), and since Polaris is very capable of handling sizes larger than #8, and handles aluminum very effectively, again this reduces the barrier cost for using #4 aluminum in the THWN-2 section.

Another reason I am trying to sneak up to #4 is that at this size or above, wire marking is allowed. That means you can simply designate hots, neutral and ground by marking wire ends with tape, which means you can simply buy 1 spool instead of having to buy 3-4 cut sections. Speaking of that, there is never any need to distinguish hot phases in a split-phase circuit, so don't swerve out of your way to make one hot red.

The downside of #4 and up is it changes the splice rules from cubic inches to bending-radius. So get a long narrow junction box. Don't shop for big boxes at big-box stores, they won't have decent selection or price. They also tend to obscenely rip you off on anything but certain commodities they think you'll price-check, that is why they're out of line on by-the-foot THWN-2 (not a commodity, unlike Romex). Visit a few electrical supply houses and see if you can get trade pricing out of them. Most will give it to you if you reveal yourself to be not an idiot.

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I just saw a Deep Space Nine episode where a customer runs a sexy holo-suite (holodeck) program. The camera reveals the digital companion's legs, waist, and finally face -- the ugly mug of the Ferengi bartender. I thought, "Finally! A way to express what pulling cable through conduit is like!"

Cable in conduit isn't illegal - it doesn't need to be. It's a miserable hateful pull which risks damaging the wire. The irony is, if that's your first conduit pull, you'll never know the difference. You'll just think conduit is a miserable experience. I use stranded THWN-2 wire almost exclusively, and my experience with conduit is so good that I fit conduit even where I don't need to.

It really helps a cable-in-conduit pull to make the conduit quite large. In the case of oblong cables like UF, that's mandatory. For very sensible reasons, a wide flat cable like UF (or any non-round cable) is treated as a round wire of the largest dimension. This requires larger conduit even for 1 circuit. For 2 circuits, the sizes get truly wasteful, and that's a shame because conduit can support up to 4 circuits, after all.

I once did a calc on one 6-3UF and one 12-2UF. The conduit size was frightful.

As far as routing conduit, remember there is such a thing as a conduit body - typically found in the form of a corner. This allows conduit to make sharp 90s and provide a pulling point, at the cost that the cover must remain accessible. These come in LL, LB, LR or inline. Other than that, aim for no more than 1 or 2 90 degrees between access points. Up to 4 is legal, but starts to require serious pulling tools, and for a novice, that often means project failure.

Assembling conduit sections 1 stick at a time over the wire/cable is specifically forbidden in Code. That tells you somebody managed to cause a warehouse fire by doing this. Generally if a pull is that hard, that is God and NFPA's way of saying "don't do that pull" or "more conduit bodies".

Answer 2

That should be OK. Your junction box might need to be outside, as enclosing it in the wall may not be code (some places dislike hidden junction boxes). As long as the NM terminates there it should be fine. I'd put some wet location nuts on it just to be safe (Overkill but you'll need to buy some larger wire nuts anyways so spend the extra money and err on the safe side).

THHN is pricier than NM because it's better coated than NM. Standard NM is not wet rated. It's more comparable to UF in that regard.

Answer 3

Conduit can be bendy

One little-known secret is that ENT (Electrical Nonmetallic Tubing aka "smurf tube") is legal to use basically anywhere NM can be used; as a result, you can eliminate a junction box from this setup by using 3/4" ENT (with 2 6AWG copper or 4AWG aluminum THHN/THWN hots and a 10AWG ground) for the attic run instead of the 6/2 NM, especially if you run the conduit in the attic via the most direct path instead of purely at right angles.

Use your body!

Tight bends in conduit are made using conduit bodies; while most people think of the LB used to go into or out of a wall as what exists in this department, there are also LL and LR bodies used to make 90 degree turns along walls, as well as C bodies for straight-through connections and T, TB, and X bodies to allow for branches.

In your case, you can use a 3/4" PVC LR body with a bell adapter fitting (instead of the normal male adapter + nipple) to exit the breaker box with a bit of room to spare, followed by some 3/4" Schedule 40 PVC to go up the wall into a LB at the entrance point. ENT uses the same conduit bodies as PVC, as well, so if you are using it, you can simply run the ENT into the back of the LB without the need for an adapter fitting. If you stick with the NM route for the indoor run, you'll instead need to use a short length of PVC to transition into a 4 11/16" square junction box (there's enough room in one to deal with something that fits in 3/4" conduit by the pull box fill rules) located indoors where you can transition into the NM.