Proper way to wire a sub panel to an attached garage?

Question

I am planning to install a new sub panel in my garage. My concern is following proper procedure/code. I contacted the local building code office and confirmed that they follow the 2014 NEC.

My main panel is located in an interior room approximately 40 feet from where I would like to install a 70 amp sub panel. For the 70 amp sub panel, I would run 3 4 gauge conductors (2 hot, 1 neutral) and 1 6 gauge ground (all wires THHN/THWN). Since they are individual wires, I would have to use conduit, so I was planning to use between 1" and 1.5" sch. 40 PVC conduit from the main panel all the way to the sub panel.

Since the main panel is in a finished area, my only thought is to cut the drywall beneath it and feed the wires from the panel through the floor and into the crawl space.

I am planning to use between 1" and 1.5" sch. 40 PVC for the 30 ft run in the crawl space where the crawl space ends at a wall which has a utility closet on the other side located in my garage.

Drilling through this wall would put me into the utility closet in my garage between my gas water heater and a utility sink. My thought was to continue with the conduit 7 ft vertically, then turn and run approximately 4 ft horizontally (and over the utility sink), turn a corner and proceed another 6 ft horizontally until I arrive where I would like the panel to be which would require 1 last turn down and into the panel.

Does my plan seem ok, or have I missed a part somewhere?

Answer 1

When wires get this large, using copper is just throwing money away. 2AWG aluminum is a very appropriate choice for this application. Whatever ooga-booga you may have heard about aluminum, never applied to the large conductors like this, and doesn't apply to the new AA-8000 alloys.

How about EMT steel conduit? Use that, and you don't need to pull a ground wire. The EMT is the ground. (Not only that, it's ground for any other circuit that needs to be grounded back to that panel, in case you are retrofitting grounds in an older home).

If possible, I would use 2" conduit. I know the tables say 1.5" is adequate, but this assumes you have an electrician's truck chock full of pulling tools and I'm guessing you don't. The more oversize you go, the easier the pull, and the less likely you'll need to hire the guy with the truck. And guess what, he won't work with you unless you hire him for the whole job. I managed to hire one just for the pull, but he was a friend of my family, and even he needed to delay the job a day so he could buy another tool he didn't have. Anything you can do to ease the pull is a good idea.

Watch the bends. Every bend makes the pull much harder. When I lay conduit, I allow myself only one 90 degree bend between accessible points (Code allows 4, but you'd need a power winch and a bunch of lube to pull 3x#2AL around 4 of them! Yikes!) Whenever you use a junction box or conduit body at a 90 degree bend, that replaces the bend with an accessible point! Quite a bargain! It also makes the turn sharper and neater. If it were me, on a big one like this, I'd put an access point at every single 90... which would make the pull easy, and could make it workable with smaller pipe. I still wouldn't go smaller than 1-1/2".

Food for thought (i.e. where to use a junction box instead of a conduit body), you can have up to 4 circuits in a single conduit. (literally, 9 conductors, but grounds don't count, and neither do neutrals on split-phase 120/240 circuits and MWBCs, so you've only used 2 of 9.)

Answer 2

First off, I'd use 1.5" conduit for this -- 1" is an extremely tight (214mm^2 available vs ~214mm^2 used) fit with compact stranded conductors and doesn't fit (~236mm^2 used) if you have regular stranded wire instead.

Also, using conduit bodies (LBs, LLs, LRs) instead of elbow fittings or bent sweeps for the turns is likely a good idea for the sake of ease of pulling. This also means you don't have to worry about the limitation on bent conduit of 360° between pull points, as each conduit body counts as a pull point.