Running Partial Conduit with MHF

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Old 03-20-17, 10:53 PM
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Running Partial Conduit with MHF

So I have some 2-2-4-6 MHF on its way. Dug a 24" trench from the house to the shop, about 120'. I had intended to direct bury but my soil is far too rocky to do without bedding in sand. Given the volume of sand that would be required this is not an acceptable solution. So, 2" conduit the whole way it is...

Except, it looks like I'll be over the 4-turn/360 degree limit, plus 4 turns of that MHF would be a major PITA. I had an inspector out for other work and he made an interesting suggestion. I can run partial conduit to the second bend, then just leave it open and run into another conduit the rest of the way to the shop. He said I can just bed that open section with sand. Is this a common thing to do?

Also I have some concerns about leaving an open conduit as the service entrance in my shop will be about 2 feet below where that open section would start, so if that conduit fills with water it could become a hose into my shop... no bueno.

Perhaps, I have two open sections, one at the bend and another just before the shop, then I can also use the shop break to run the wire to the ground rods? Thoughts?
 
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Old 03-21-17, 06:55 AM
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It is not common, but yes you are allowed to do it.

The best way to deal with wet conduits is to bring the pipe up above grade on the exterior of the building, then use an LB fitting to go through the wall. Optionally you can drill a small weep hole through the pipe on the riser section. That way it doesn't matter if the pipe gets wet, the water can't rise high enough to enter the building. All exterior conduits should be sealed with duct seal putty at the panel to prevent humid vapors from coming in.

Yes you can sink the ground rods into the bottom of the trench. The GEC can run through the plastic conduit or be zip tied to the outside of the pipe.
 
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Old 03-21-17, 08:35 AM
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Pull boxes can be buried to take care of the 360 degree limit. You need to have the box in a place that can be excavated without having to tear up sidewalks, paving, etc., and the location needs to be identified.
 
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Old 03-21-17, 08:59 AM
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Excellent, thanks for the advice!
 
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Old 03-21-17, 06:37 PM
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I had an inspector out for other work and he made an interesting suggestion. I can run partial conduit to the second bend, then just leave it open and run into another conduit the rest of the way to the shop. He said I can just bed that open section with sand. Is this a common thing to do?
Not common and I wouldn't do it.

I don't like the idea of buried pull boxes either, but I do like the idea of using either LB or C fittings on the riser at each end of the trench.
 
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Old 03-21-17, 07:25 PM
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I wouldn't do it.
May I ask why? How is it any different than direct burying the MHF and bedding in sand? The goal here is to keep the MHF from being damaged by rocks and this would appear to accomplish that.
 
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Old 03-21-17, 08:04 PM
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That's CasualJoes preference. Mine is to use conduit the whole way using handholds or buried pull boxes. Do whatever works for you.
 
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Old 03-21-17, 08:28 PM
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I wouldn't do it.
May I ask why? How is it any different than direct burying the MHF and bedding in sand? The goal here is to keep the MHF from being damaged by rocks and this would appear to accomplish that.
I wouldn't do it because leaving a gap in the conduit system is like having a collapsed conduit if you ever have the replace the cable and it would all have to be dug up. How could you pull the cable without a complete conduit system? I prefer a complete conduit system, but you said you would have more than 360 degrees of bends. Installing buried pull boxes would require you remember where the pull boxes are located, what if it were a new owner, he would have to dig up the entire system to find them. I would install a complete conduit system with either LB or C fittings in the riser at each end providing pull points so you wouldn't have the 360 degree pulling problem. In a complete conduit system I wouldn't use the MHF either, the quadplexing of the cable makes for an extremely high resistance pull, I'd use aluminum Type XHHW conductors.
 
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Old 03-21-17, 11:19 PM
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So no different than burying it. As long as it's safe I don't care if the next guy has to dig it up, I did...
 
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Old 03-22-17, 07:43 AM
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I wouldn't do it because leaving a gap in the conduit system is like having a collapsed conduit if you ever have the replace the cable and it would all have to be dug up.
No different than totally direct buried.

Installing buried pull boxes would require you remember where the pull boxes are located, what if it were a new owner, he would have to dig up the entire system to find them.
Not if location is properly identified as required by NEC.

In a complete conduit system I wouldn't use the MHF either, the quadplexing of the cable makes for an extremely high resistance pull, I'd use aluminum Type XHHW conductors.
I've pulled plenty of both and I don't recall any high resistance pulling issues with MHF or Quadruplex cable through conduit. Just don't use the minimum required conduit size. XHHW or THHN/THWN makes more sense in a complete conduit system because a direct bury rated wire is not needed.
 
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Old 03-23-17, 11:54 AM
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XHHW or THHN/THWN makes more sense in a complete conduit system because a direct bury rated wire is not needed.
Agree! I have never been a fan of direct burial cables. I have seen many more failures of direct burial cables than failures of wire pulled into complete conduit systems.
 
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Old 03-31-17, 10:06 AM
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Inspection

Just an update, I got my approval sticker today. This was actually my 2nd inspection on this project. 1st time he came out the GFI outlet I had installed was faulty, double checked the wiring while he observed and it was correct so I just had a faulty unit.

Unfortunately this led to "while since I'm coming back, do these other things...". I had assumed since the existing wiring and outlets were already in the shop they'd be grandfathered... nope. So I had to replace two metal boxes, add some staples, and fill a knock-out in my main panel (that he presumably overlooked when he was inspecting my other project). So be sure to test your GFIs before you get the inspector out otherwise you'll have to schedule a re-inspection if they don't work. I'm positive if the GFI had worked the first time he wouldn't have asked for the other things. The more you know...
 
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