Running 220 via underground flex NM conduit


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Old 03-18-18, 08:34 AM
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Question Running 220 via underground flex NM conduit

I have a sub panel that's wired for 20A 220. It's at an outdoor shed, with the cable run underground (not in conduit) from a junction box at the corner of the house. The underground cable developed an open in one of the hot legs. I want to replace this, but this time, using conduit. I thought that it HAD conduit, as there was a NM flex conduit where the cable entered the ground at the house, and exited the ground at the sub panel. Anyway, I need to get this fixed fairly quickly, so I'd like to wire it up, in conduit, then later bury the conduit. I would like to use liquid tight NM conduit, which is already used in a few places around the sub panel. The run would be a little less than 100' from the corner of the house to the sub panel.

My background - I have done most of my ac wiring repairs in the past, including adding a breaker to a main panel for a GFCI outlet in a garage right near the panel (for running an air compressor that kept tripping the "garage" breaker). For this problem, I measured voltages, but I kept getting some weird results, so I called an electrician. Two showed up, and diagnosed the problem. The quote to repair it is over $1300, so I'd like to do it myself.

Questions:

- Does the NM conduit meet code for this application?

- Can I use this conduit for runs this long? I'm thinking that the NM conduit has a better chance of staying dry than rigid PVC with 10 or so joints. But also... if this ever happens again, I want to be able to just pull new cable or wires through the conduit.

- If so, what size conduit should I use?

- What wire or cable do I need to use? The current underground cable is 12/3 with a ground. I'm guessing UF, but I can't read anything on the insulation. I've read that in some places, you must run individual wires through conduit like this instead of cable. Do I need to run wires or cable?

- Do I need to run a ground? One of the electricians told me that I needed to ground the sub panel to a nearby ground rod. If so, why would I need to run a ground from the house?

- Will I need to use lube on the cable or wire to get it through 100' of conduit?- Anything special (say, tools) needed to pull cable (or wires) through 100' of NM conduit?
- How deep does it have to be buried?
- Any other "gotcha's" in my plan?

Thanks.
 
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Old 03-18-18, 10:31 AM
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What is NM conduit????. use 3/4” PVC and pull THWN individual conductors, the pipe run must be completed before conductors are pulled in. No need for a ground rod if you pull the ground wire out from the house and you only have a single circuit .
Geo
 
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Old 03-18-18, 11:15 AM
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By "NM conduit" you are referring to Seal-tite or flex conduit. This question comes up routinely. Can it be used for your run.... yes.... I don't believe the code states a maximum length.... just that it needs to be properly supported. It also must be designated for direct burial in its specs.

Is it a good idea.... I wouldn't use it. Expensive and hard to keep in the trench to cover over. I'd just use standard PVC. 3/4" would be a good choice. It needs to be buried 18" deep. As mentioned... the conduit system needs to be in place before the wires are pulled thru.

You currently have 12-3. That's four wires.... so yes.... you will need to pull four #12 THWN wires. That pull in 3/4" pipe is easy. You could use a bit of pulling soap but isn't really required.
 
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Old 03-18-18, 11:39 AM
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No conduit will stay completely dry under ground. Condensation over time makes the interior of the conduit to become damp and even form puddles in low spots.

With having a subpanel you should have a ground rod at the shed. The ground that runs with the feeder from house to shed is an equipment ground and is for clearing fault current (trip breaker). The ground from the subpanel to earth (ground rod) is for lighting strikes. You don't really want lighting strikes at the shed possibly traveling back to the house via the equipment ground.
 
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Old 03-18-18, 02:03 PM
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"NM" is non-metallic liquidtight flexible conduit.

"No need for a ground rod if you pull the ground wire out from the house and you only have a single circuit ."

Single circuit... from the house? Or from the sub panel? There is a single cable from the house to the sub panel, but there are two breakers in the sub panel. One for a pool pump, and one for the pool light. So going into the sub panel is a single circuit, but out of the sub panel are multiple.

"Is it a good idea.... I wouldn't use it. Expensive and hard to keep in the trench to cover over."

Expensive? 100' of 1/2" is $60.100' of 3/4" is $75. Straight sections of PVC are cheap, I admit, but then you add joints, joint compound, turns, cutting various pieces, etc., and it sounds like the cost would be a wash. For the flex conduit, I would just need to cut to length. I'm admittedly no expert, and I'm not trying to defend the flex conduit. But it would seem that the installation would be easier than rigid PVC, which would need multiple types of pieces (straights, turns, etc), and joints (which need to be sealed).

"You currently have 12-3. That's four wires.... so yes.... you will need to pull four #12 THWN wires. That pull in 3/4" pipe is easy. You could use a bit of pulling soap but isn't really required."

OK. Thanks.

"No conduit will stay completely dry under ground."

Thanks, I didn't know that.

"With having a subpanel you should have a ground rod at the shed. The ground that runs with the feeder from house to shed is an equipment ground and is for clearing fault current (trip breaker). The ground from the subpanel to earth (ground rod) is for lighting strikes."

OK. So where do I connect the ground from the house? I assume that the ground from the ground rod gets connected to everything from the sub panel outward (light, timer, pump, etc). If it makes a difference, I'm going to change the breaker for the pump to GFCI.

BTW, can I, as a temporary measure, run UF cable above ground for a few days until I can get the conduit (whichever I go with) buried? I need to get my pool pump running again as soon as I can. Then, after getting the conduit assembled and buried, cut the insulation off the UF, and pull the wires through the conduit?
 
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Old 03-18-18, 02:10 PM
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If I go with rigid PVC, do I use schedule 40?
 
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Old 03-18-18, 02:54 PM
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The equipment ground connects to the ground bar in the house main panel and then hooks to the ground bar in the shed subpanel. The neutrals in the shed panel are to be isolated from the ground. Usually a subpanel requires a ground bar kit so that all the grounds go to the new bar and the existing bar is only used for neutrals and the bonding screw/strap is removed to isolate the existing bar from the case. The wire (usually bare #6 copper) for the ground rod is connected to the ground bar in the shed panel and then run to the ground rod(s). Usually 2 rods are driven at least six feet apart from each other, because NEC calls for 25ohms or less resistance to earth with one rod using 2 rods satisfies that without having to prove the 25ohms.

You can use Sch40 PVC. Any where the PVC conduit is exposed and may be subject to damage you should use Sch80.
 
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Old 03-18-18, 03:05 PM
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With flexible non-metallic conduit you are still limited to no more then 360 degrees worth of bends. Plus pulling wire in non-metallic flex is a HUGE pain in the butt! I would only recommend it for short lengths.
 
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Old 03-18-18, 07:48 PM
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pattenp, thanks for the info on grounds and PVC.

"With flexible non-metallic conduit you are still limited to no more then 360 degrees worth of bends. Plus pulling wire in non-metallic flex is a HUGE pain in the butt! I would only recommend it for short lengths."

That's what I was concerned about with the NF flex conduit. I only have to do 3 90° turns, but I'm convinced that I need to go with rigid PVC.

OK. So one more (hopefully) question. Is it easier to dig the trench first, and fit your conduit to your trench? Or put the conduit together first, and dig the trench to fit the conduit?
 
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Old 03-18-18, 07:54 PM
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20 amp sounds too small. The subpanel also must supply any 120cv lights and recrpeaccles. What amps does the 240 load draw?
 

Last edited by ray2047; 03-19-18 at 06:12 AM.
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Old 03-19-18, 05:48 AM
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The 240V pump is rated for 7.6A max. The pool light is LED. Not sure of it's rating, but it's only a few Watts... way less than one amp. Those are the only two circuits currently on this sub panel. I may add an outlet and overhead light to the shed, but I I know how to not overload a circuit. I'm an electrical engineer. If I can't figure that out, my employer needs to fire me.
 
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Old 03-19-18, 06:01 PM
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For pool equipment you need an insulated ground so UF cannot be used.

Sealtight will be a pain to pull through at that length. PVC will be much easier.
 
 

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