I'm currently rewiring my 100-year-old garage, in the USA. The existing wiring is very old and not up to any code.

My plan is to remove everything up to where the circuit enters the building. It's one circuit, 12 AWG, 3 wire + ground, buried UF cable from the house. That part seems fine.

I'm using 1/2 EMT conduit, 12 AWG solid wire, and all weatherproof, threaded connectors and diecast aluminum weatherproof boxes, with weatherproof covers and weather resistant GFCI outlets. I know this is probably overkill, but I wanted everything safe in case of any kind of leak, and also extra protection from "garage problems" like dust from my power tools.

So here is my question: generally, with all conduit and metal boxes, I believe you don't need to run a ground wire. The metal acts as the ground. I'd just need to attach the incoming ground from the UF to the first box. Does this hold true for the diecast weatherproof boxes too? Will the ground still be continuous with steel EMT connected to aluminum boxes? Or, do I need to run a ground wire through my entire circuit, attaching it to each switch and outlet?

EDIT Since the fittings seem to be a concern, I've added pictures of the hardware. Even reviewing the manufacturer's documentation, it seems ambiguous as to whether they are designed to work together. What I can say, is that the connector threads into the box very smoothly, by hand. If the threads didn't match, I would not expect that to happen.

Threaded together

One thing I did notice is that the box, and the threads in the box, are painted. I don't know if that would make a difference for grounding, but seems like it could impede conductivity.

Connector: https://www.homedepot.com/p/RACO-EMT-1-2-in-Uninsulated-Raintight-Compression-Connector-50-Pack-2902RT/203637931

Box: https://www.homedepot.com/p/BELL-1-Gang-5-Outlets-1-2-in-Threaded-Weatherproof-Box-Gray-5323-0B/204208013

For what it's worth, it also seems like Raco and Bell may be under the same parent company.

EDIT 2 I contacted Hubbell (manufacturer of both box and connector) regarding compatibility. They gave an impressively fast and knowledgeable response. Their product technician confirmed the box will work with both straight and tapered threads, and the two components can be used together. Furthermore, he stated that they would create a grounding path and have passed UL testing for such, but would (personally) run a ground wire anyway as it is more reliable. That's what I'll do.

I guess I should have just asked the manufacturer to start; I wasn't expecting such helpful customer support!

  • As a sidenote, you are correct that Raco and Bell are under the same parent company (Hubbell, a very long-standing name in the North American electricial biz in its own right) – ThreePhaseEel May 31 '20 at 19:03
  • It sounds like the threads are indeed mismatched, since you're not observing any "tightening" of the threads together. See this link for more details – ThreePhaseEel May 31 '20 at 19:12
  • Not trying to be difficult, but the threaded metal plugs that came with the box screw in just as easily as the connector. I can't tell if they are tapered or not, they only have about 3 threads. – SouthShoreAK May 31 '20 at 20:21
  • Huh. That is odd that the threaded plugs wouldn't screw in tightly, have you tried using the screwdriver slot to drive one in fully? – ThreePhaseEel May 31 '20 at 20:22
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    Thanks for the update with info from the manufacturer. I think the site and other users would best be served by making that "edit" an "answer" as that's where people go to look for answers. – FreeMan Jun 2 '20 at 11:32

You'll need to make sure your EMT fittings are rated for being threaded into a hub

The thing with weatherproof cast boxes (Bell boxes) is that they have cast-in threaded hubs in them instead of ordinary knockouts, so that water can be kept out reliably. However, UL historically did not test EMT fittings for use with threaded hubs, only locknuts, and as a result, many of them have a straight thread that won't mate properly with the tapered thread of the hub in all cases.

Fortunately, there are (weatherproof, even) EMT connectors that are rated for threading into hubs; the Bridgeport 290-RTNPT is an example in ½", and it appears your connector was also tested for hub applications based on what the manufacturer is saying. Note that the Bridgeport version is somewhat longer than a typical compression EMT connector due to having a short pipe nipple, essentially, as part of the fitting, as shown below (photo for illustration only):

a Bridgeport Fittings 290-RTNPT

Once that's done, you're good to go

Once the EMT has been made up using the correct fittings, though, you'll be good to go when it comes to using it as a grounding path. Note that I'd probably be using weatherproof push-on couplings (SIMPush™, Mighty-Bite™) or equivalent for joining EMT pieces together; these are the least dependent on workmanship, and couplings are a common source of issues when using EMT as a grounding path.

  • That is useful information. I'll have to test my fittings. They are raintight, so it would be logical for them the fit a Bell box. Doesn't mean they will. Could you weigh in about the grounding? – SouthShoreAK May 31 '20 at 5:30
  • @SouthShoreAK I'd take your fittings back and get the correct fittings, as using the EMT as a grounding path depends mostly on the integrity of the box connections – ThreePhaseEel May 31 '20 at 7:04
  • Can't use a standard EMT to box compression fitting rated for rain tight? (Grainger) – Mazura May 31 '20 at 11:33
  • @Mazura -- the issue with the standard EMT-to-box fittings is that UL only ever tested them with locknuts, by and large. In practice, many electricians get away with threading them into hubs and couplings, but it's one of the sharper edges for the UL listing limits on conduit fittings... – ThreePhaseEel May 31 '20 at 16:03
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    Ended up contacting the manufacturer, but this answer sent me down a route I would never have explored, so I will accept it. – SouthShoreAK Jun 2 '20 at 4:07

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