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Old 01-04-2009, 07:53 PM   #1
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Direct burial vs. conduit?


I have a run (140' panel to panel) from the house to an outbuilding to install. Will be #8 or maybe #6 - 3 conductors plus a ground. What are the pros and cons of direct burial cable vs wires in conduit?
  • Materials costs?
  • Labor? (my own, plus a day laborer or two to do the digging)
  • Other?
I understand 18" depth for conduit, 24" for direct burial. I have not done conduit before - how does one fish wires through this long a conduit?

thx!
Vince

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Old 01-04-2009, 08:13 PM   #2
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Direct burial vs. conduit?


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Originally Posted by vsheetz View Post
I have a run (140' panel to panel) from the house to an outbuilding to install. Will be #8 or maybe #6 - 3 conductors plus a ground. What are the pros and cons of direct burial cable vs wires in conduit?
  • Materials costs?
  • Labor? (my own, plus a day laborer or two to do the digging)
  • Other?
I understand 18" depth for conduit, 24" for direct burial. I have not done conduit before - how does one fish wires through this long a conduit?

thx!
Vince
Where conductors enter and leave the ground they must be protected by conduit. One hundred plus feet long and 18-24 inches deep - I'd rent one of those trenching machines. Last time I checked it was about $115 for 4 hours at Home Depot.

One would use a fish tape or pulling rope to snake thru the conduit then attach the wires and pull them thru.

Not sure about the difference in cost between direct buried cable and wires in conduit.

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Old 01-04-2009, 08:33 PM   #3
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Direct burial vs. conduit?


I've always elected to run high voltage in conduit between buildings to ensure it is not accidentally found. I believe THHN is cheaper than URD, but I don't know for sure.

If you go conduit, make sure you go the correct size for the gauge of wire you will be running and make sure you use wire that is rated for underground conduit such as THHN or THWN-2. All conduit will get water in it, so it is important to use wire rated to be submerged.
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Old 01-04-2009, 08:34 PM   #4
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Direct burial vs. conduit?


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One hundred plus feet long and 18-24 inches deep - I'd rent one of those trenching machines. Last time I checked it was about $115 for 4 hours at Home Depot.
About 80' of the run is in a place it's difficult to get to with a trenching machine - therefore a couple guys with shovels will be best.

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One would use a fish tape or pulling rope to snake thru the conduit then attach the wires and pull them thru.
Thus my question about how to use a snake for a run this long? If using a pull string - how is this best installed, as the conduit was pieced together?

thx!
Vince
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Old 01-04-2009, 08:47 PM   #5
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Direct burial vs. conduit?


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Thus my question about how to use a snake for a run this long? If using a pull string - how is this best installed, as the conduit was pieced together?

thx!
Vince
Dig the trench, glue up the conduit and place in the trench. Cover the trench. Use a shop vac to pull a plastic bag with string attached thru the conduit. Use string to pull in heavier rope. Use rope to pull wire. done.
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Old 01-04-2009, 08:51 PM   #6
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Direct burial vs. conduit?


I usually use a 200' fish tape that I push thru the conduit after it is installed. I've only heard about the pull string, but never actually used one. To my knowledge the pull string is sucked thru the conduit with a vacuum. I have seen this equipment in electrical tool/equipment catalogs and I have also heard of some homemade types. I believe after the pull string is thru you can then pull a larger rope thru (I can't remember if you can actually use the string to pull conducts thru - it may not be rated for that) and then attach that rope to the conductors.
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Old 01-04-2009, 09:13 PM   #7
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Direct burial vs. conduit?


Quote:
Originally Posted by AndrewF View Post
I've always elected to run high voltage in conduit between buildings to ensure it is not accidentally found. I believe THHN is cheaper than URD, but I don't know for sure.

If you go conduit, make sure you go the correct size for the gauge of wire you will be running and make sure you use wire that is rated for underground conduit such as THHN or THWN-2. All conduit will get water in it, so it is important to use wire rated to be submerged.
THHN is not rated for wet location use. You must use THWN, the W signifies the wet location rated wire.
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Old 01-04-2009, 10:44 PM   #8
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Direct burial vs. conduit?


Use this as your string. It can be found at lowes or HD.
For that distance you will need a strong shop vac.

http://www.mygreenlee.com/Products/m...c_number=12598

Here you go Steelhead. A vac system
http://www.mygreenlee.com/Products/m...c_number=31339
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Old 01-05-2009, 03:03 AM   #9
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Direct burial vs. conduit?


I would be coming out of the earth right under the panels at each end, entering the wall via a LB. If running THWN wires does the conduit need to extend inside the wall the couple feet to the panel - or can the wires be left uncovered there (the wall is enclosed - not open studs)? Or do should I transision in a box to cable somewhere?

What size ground:
with #6 - #10?
with #8 - #12?

What size breaker at the house panel:
if #6, 60 amp?
if #8, 50 amp?

What size conduit for:
6-6-6-x - 1" or 1.25" or other?
How about if 8-8-8-x?
I'll have three 90 degree bends plus two 45 bends LB to LB.

a huge thx!!
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Old 01-05-2009, 10:54 AM   #10
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Direct burial vs. conduit?


You cannot have three 90's and two 45's in one run. 360 degrees maximum. But even at 360 degrees will make for a very hard pull. Forget the fishtape, you will need a shop vac. You don't need a big one. Just one with a clean filter and good suction. You will need to install a box or find a way to limit your conduit turns.

Ground either size = #8
House panel breaker = You pick
Conduit size = 1" will work for both. However, since you have some turns the smallest I would use is 1.25.
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Old 01-05-2009, 12:00 PM   #11
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Direct burial vs. conduit?


Thanks for catching my typo. Yes, THWN or THWN-2 is suitable for this project.

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THHN is not rated for wet location use. You must use THWN, the W signifies the wet location rated wire.
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Old 01-05-2009, 03:13 PM   #12
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Direct burial vs. conduit?


Yes, I am aware of the 360 limit - three 90's and two 45's add up to that. Two of the 90's are the turn-up coming out of the ground, so I guess I can put them on after the cable pull? And maybe I can reduce one of the 90's. I don't have a straight shot building to building due to having to go around a swimming pool.

How about the conduit need once I enter the building via the LB fitting - does it have to be conduit for the couple feet inside the wall to the panels?

thx!
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Old 01-05-2009, 03:40 PM   #13
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Direct burial vs. conduit?


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Originally Posted by vsheetz View Post

How about the conduit need once I enter the building via the LB fitting - does it have to be conduit for the couple feet inside the wall to the panels?

thx!
Yes!!
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Old 01-05-2009, 06:28 PM   #14
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Direct burial vs. conduit?


Direct burial is stupid.

Use conduit and protect your wiring and leave the possibility of pulling new ones if something should go wrong.

Conduit is cheap and lasts a very long time. I'd run minimum 1 1/2". Ease of pulling and room for upgrades.
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Old 01-05-2009, 06:33 PM   #15
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Direct burial vs. conduit?


Forgive my ignorance... but is the 360 degree rule for every single run, inside and outside? I plan on upgrading the wiring in this house as it is the old cloth style wiring... I'll be running most of the lighting through ceiling joists not really at any angles, but there are some diagonal runs I'll need to make from one corner of a room to the other... I guess that's only two 30 degree turns maybe, and the 90 degree coming from the panel to the actual run...

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