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My garage is not connected to my house and it's on a circuit that contains a few outlets inside the house as well. I found out the first time I tried to charge my PHEV that the entire circuit isn't properly grounded. I had an electrician come look at it, and found some old wiring that he replaced and it worked! Until it didn't... All of a sudden it wasn't grounded anymore.

The electrician came back over and when he went to test it, it was grounded properly?! Turns out this was just after a rainstorm. Now I've found this to be true that every time it rains, the garage circuit is grounded properly and when it's dry, it is un-grounded again.

Now the electrician want to tear up the underground conduit connecting the garage to the house, but I'm not sure why that would fix it? If anything I would think that it would be un-grounded when it rains, or trip the breaker every time it rains?

Also, there are no outside outlets and the underground wiring is the old metal sheathing style that runs through a 1.5" dia pipe into the house. He has already tried pulling the wire through one end with a stringer but apparently it didn't work.

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    Has the actual ground rod been checked? Yours would not be the first installation where corners were cut and the rod is not installed properly. I've seen a 10' rod cut so that only about 6" was actually driven into the ground.
    – jwh20
    Commented Aug 25, 2020 at 17:06
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    Hahaha - appreciate the rod puns. But, if the rest of the house is grounded properly from the same breaker box, it couldn't be the grounding rod right?
    – Andy Barnes
    Commented Aug 25, 2020 at 17:14
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    How do you know the rest of the house is properly grounded? Most stuff doesn't care. It seems the EV charger is attempting to be extra safe here. I guess my point is to check the obvious problem first. The thing says the ground is bad, perhaps it really is.
    – jwh20
    Commented Aug 25, 2020 at 17:27
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    @AndyBarnes Because ungrounded appliances with 2-prong polarized plugs rely on neutral not being hot to keep you safe. Flip the outlet and the inner pin of your lamp socket becomes neutral and the metal shell outside becomes hot.
    – J...
    Commented Aug 26, 2020 at 19:34
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    @J...: Do appliances like that (which "rely on neutral not being hot to keep you safe") actually exist in North America? I've never seen or heard of any here in Europe, which makes sense since our plugs are mostly unpolarized. We do use the same kind of Edison screw sockets for light bulbs as you do, and the (supposed) neutral in them is indeed more exposed than the hot, but not enough to create an actual safety hazard when the bulb is in place. Indeed, lamps with symmetric 2-prong Europlugs and Edison sockets are still commonly and legally sold here.
    – Ilmari Karonen
    Commented Aug 27, 2020 at 9:03

2 Answers 2

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To me it sounds like the conduit was used as the grounding conductor, as allowed by code, and it rusted through, therefore it works when the ground is wet and you have a ground connection through the soil and water. If this is the case, then an additional electrode at the garage probably won’t help as the pipe is in the ground.

Your electrician wanting to dig it up makes sense to me because when debris get in the pipe, it becomes impossible to pull new wire. With current code, the ground can be run separately, but that would be pushing the code in my opinion but legal. You can run a ground as long as they originate from the same panel.

Earth is a horrible conductor. I see high resistance quite often; this is why I don’t think another rod will help.

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    That is a really good answer. I would add that this will eventually fail during rain too.
    – DMoore
    Commented Aug 25, 2020 at 17:21
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    Wait... This just made sense to me... You're saying the conduit has rusted to the point where it's essentially two disconnected pieces that used to fully enclose the wire, but now since they are disconnected, wet ground will act as a conductor when it rains. Is this right?
    – Andy Barnes
    Commented Aug 25, 2020 at 17:24
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    @AndyBarnes You got it. "he tried pulling a wire through..." this is a good clue that there's a collapsed , rusted conduit. +1
    – JACK
    Commented Aug 25, 2020 at 17:31
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    Andy the depth is not as important as the length code allows for a pipe electrode in contact with earth for 10’ to be used. If your charger requires the ground in many cases a solid electrical connection is needed. I have measured several hundred ohms on grounding electrodes even with multiple rods some electronic devices that monitor the system have not been low enough (it sounds like that is your problem). You can test this theory by running a ground wire from the panel or electrode at the house to the garage
    – Ed Beal
    Commented Aug 25, 2020 at 17:32
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    @JimStewart Heck no, don't bury bare copper! Has to be either in conduit or protected cable suitable for direct burial.
    – J...
    Commented Aug 26, 2020 at 10:57
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An outbuilding needs two kinds of grounds

Electricity wants to get back to source, not ground. Source is the supply transformer, or in the case of natural electricity (lightning, ESD), then yeah, it's earth.

The grounding rod is designed to handle natural electricity. Note that natural electricity has VERY high voltage compared to the amperage.

The ground wire handles artificial electricity. It returns wayward fault current to the main panel, where it gets back to the supply transformer via the neutral-ground equipotential bond at that location, and the neutral supply wire.

Your site's problem is lack of a ground wire. That means if there are any ground faults, it will tend to energize all the grounds in the building, and the earth in the vicinity of the building. There will be a "voltage gradient" across the earth along the gap in the grounding. At extremes, an animal with two feet on one side and two feet on the other side could get shocked. Or even a human taking strides. Metal fence lines are particularly dangerous; they could be grounded at one end and present near 120V to ground at the other end.

Metal conduit as the ground "wire" ... underground

Your installation goes back to the day when all conduit was metal. Yours was installed one of two ways: either with no thought to grounding at all since it's that old; or intentionally using the metal pipe as the ground "wire". This is a technique I'm normally all in favor of, but underground, it is prone to rusting out. EMT (thin wall) conduit doesn't perform well underground because of this issue.

The electrician's inability to pull or fish through the conduit would support this theory.

So your ground "wire" has broken, and you have all the normal problems from that.

Regardless of what type the conduit was, if it is corroded through in one location, it is likely in very poor condition for the rest of the run.

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  • Actually emt is allowed now I remember being surprised by that I will find the code reff I don’t think it was a local thing mug it may have been.
    – Ed Beal
    Commented Aug 25, 2020 at 18:13
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    I found the reference it was new in 2017NEC. 358.10.A.1 uses permitted: 1) in concrete, in direct contact with earth. The water tight fittings have to be used the same as outdoor but it is allowed in the ground.
    – Ed Beal
    Commented Aug 25, 2020 at 18:20
  • @EdBeal edited but whoa... that's crazy. I would not do that. Maybe it works in Arizona or something... not in any soil around here.
    – Harper - Reinstate Monica
    Commented Aug 25, 2020 at 18:33
  • I would almost agree where I see a difference is rigid once threaded the groove thickness is not much different than EMT (this may be why the change was allowed). The difference is the EMT has a uniform galvanized coating. By code threads are supposed to be protected NEC 300.6 in the past I have done this but at my current location the owner did not want the extra expense so I only do it on underground fittings now where I used to use it everywhere. So far I haven’t got hit on an inspection. I have seen other electricians not using corrosion protection similar to switch leg marking missing.
    – Ed Beal
    Commented Aug 26, 2020 at 14:34

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