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?

  • Are you going to be using an insulated (green) or bare wire for the ground? Also, are you using copper or aluminum conductors? – ThreePhaseEel Dec 28 '16 at 23:10
  • Copper for everything and insulated green for the ground, insulated white for the neutral and insulated black for the two hits. – Ryan Marino Dec 28 '16 at 23:16

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.)

  • PLease cite code to verify Al is acceptable for this purpose. In addition, you should identify proper connection of Al to Cu sources. – Carl Witthoft Dec 29 '16 at 14:26
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    @CarlWitthoft Why wouldn't AL be acceptable? Also, if the AL if going from a breaker terminal to a panel terminal, there's no CU involved. As long as the terminals are rated for AL, there's no problem. Just make sure the lugs are torqued to spec, and use some antioxidant goop. – Tester101 Dec 29 '16 at 15:19
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    @CarlWitthoft -- see 2014 NEC 310.106(B) -- it calls out clearly that aluminum wire can be used in all chapter 3 wiring methods, and sets out the requirements for alloys used (i.e. AA-8000 instead of cruddy old AA-1350, at least for wire types used in branch and feeder service). – ThreePhaseEel Dec 30 '16 at 4:22
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    @CarlWitthoft Table 310.15B16. You can handle 75A with 3 AWG AL (if you can source it) or with 4 AWG AL with 90C terminations (if you can source them). He's just on the wrong side of the cusp, 4AWG AL is good for 65A@75C. So lots of headroom. Big Orange stocks several kinds of 2/2/2/4AL and very little large Cu. – Harper Dec 30 '16 at 5:38
  • Thanks to all who responded -- upon re-reading my comment I can see that it appears skeptical. I didn't mean that; just wanted to see the code refs included for future reference. – Carl Witthoft Dec 30 '16 at 12:58

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.

  • Thanks for the suggestion. I had used a fill calculator for 3- 4 gauge wires and 1- 6gauge, but I know those aren't always accurate. I am an engineer by trade and tend to "over-engineer" projects so I was already leaning toward 1.5" even though it seemed like 1" was suitable. Thank you for the conduit body suggestion also. I was concerned about the 360 degree rule and I think conduit bodies would eliminate that, right? – Ryan Marino Dec 29 '16 at 1:13

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