burial requirements for RMC or IMC

[RustyShackleford]

I want to re-locate our electrical vehicle charger (EVSE) which is now on the side of the house and located pretty inconveniently. I want to move it to the edge of the gravel parking area. To do that, I will need to bury about 20ft of conduit across an area of lawn and a gravel path. Unfortunately I'll be crossing the main power feed to the house, theoretically buried 24" deep, but I don't completely trust that and the penalty for failure is high; so I don't want to attempt the 18" burial depth for PVC conduit (never mind the grunt work).

So I'm looking to use intermediate metal conduit (IMC) or possibly rigid metal conduit (RMC). I am thinking I can get away with only burying it 6", although I'll probably do more like 12". I am basing this on NEC Table 300.5 (Screenshot here).



Using column 2, it looks like the top row ("All locations not specified below") would apply, allowing 6" burial. It seems like the only thing that could get me would be the next to bottom row ("One- and two-family dwelling driveways and outdoor parking areas, and used only for dwelling-related purposes"). But as long as the IMC/RMC only comes up the edge of my parking area (and never crosses the driveway itself), I should be ok with 6" per the top column. Make sense ?

I'm also thinking the vertical runs coming up at each end, to the house and to the EVSE, can be done in PVC. Simply to save having to buy all those expensive IMC fiittings. So hopefully two 10-ft sections of IMC is all I need, the remainder can be PVC (assuming the transition can be gracefully done, with the threaded PVC fittings mating up with the threaded fittings at the ends of the IMC).

 

[Rusty18]

Does the potential exist to drive across it? If so I’d say your are still at 18”. As far as digging across your feed coming in…dig as much as you dare with a shovel then”hydro excavate” with a water hose. Are you pulling solid or stranded wire? Solid wire doesn’t pull well through fittings like elbows…I was told…by a friend . For that matter stranded doesn’t either but it is a little more forgiving. Put in the sweeps or bend the pipe…you might be the next guy that has to work on it.

 

[NHWS]

As far as digging across your feed coming in…dig as much as you dare with a shovel then”hydro excavate” with a water hose.

Have the utility companies come out and mark the positions of the various feeds.

Hydro excavation is how most of the pros do it; some use compressed air. Just be sure to have a path for the water/mud to drain away, or a pump. A pressure washer on a low pressure setting works well. Set the pressure just high enough to dig with - that will be low enough to avoid damaging direct burial cable or any kind of conduit, as long as it is in good condition. If a using pressure washer for digging makes you nervous, drop back to a garden hose close to the location of the marked feed locations.

 

[RustyShackleford]

Hydro-excavating, I love it. Good idea.

 

[sam45]

Also interested in.

 

[RustyShackleford]

Coming back to this ... I've definitely decided to go with RMC (rigid metal conduit), due to the 6" burial requirement (if I never cross the parking area with it). The stuff is expensive, BUT ... I found a bunch at the scrap-metal yard (it's like a vintage clothing store, for guys ) , however, it is aluminum.

Which is fine, aluminum evidently has the same burial requirements (per NEC Table 300.5). But as per NEC Section 344.10, if aluminum RMC is used for direct earth burial, it must be with " additional corrosion protection". What this means is never stated, and apparently it's simply up to the AHJ. I suspected this meant that soil conditions is various areas could affect the degree of " additional corrosion protection" required. But my AHJ had no opinion on this (per email), and I'm not planning to get the work inspected. So I simply want to do what's right. So the first of my many questions is ...

1. What is good " additional corrosion protection" for direct burial aluminum conduit ? I'm thinking maybe a good primer meant for aluminum, such as (broken link removed)

2. Apparently it's allowable to mix the aluminum with galvanized steel (if no severe galvanic corrosion conditions are present, which I believe is true), which is good since the coupling, elbows, etc are unavailable in aluminum and will need to be galvanized parts. So, what's a good product to use on the threads where I'm joining these dissimilar metals ?

3. When I bring the RMC out of the ground at the EVSE end, I'm going to want to go into a weather-proof box, where I'll connect it to the "whip" from the EVSE. What's a good way to terminate the RMC into such a box ? Do I need to use the type with threaded knockouts ? Or is there a way to do it with a blank PVC box ? Or should I just transition to PVC conduit before I come out of the ground, and then I can just use a PVC male adapter ?

4. How can I ground the conduit ? There are "grounding bushings", which screw onto the end of the conduit threads and have a lug to attach the ground. wire. But it seems like if the conduit threads screw into a threaded knockout at the EVSE end and a LB conduit body at the house end, the end of the threads won't be exposed to be able to install the bushing. Do RMC conduit bodies have a grounding screw ?

 

[RustyShackleford]

Looks like this will handle question #3 ...