UF cable question.


  #1  
Old 09-15-03, 01:33 PM
batmobile78m
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UF cable question.

I know UF cable is OK for direct burial but can it be run through conduit and are there any disadvantages to doing this?
#6copper UF, I'm speaking of, if that makes any difference.

I see UF has to be buried 24" and conduit only !8". I'm trying to avoid the extra 6" of digging and also I can't seem to find anyone with THWN single conductor wire. Although some guy at Home Depot told me I could run THHN underground outside in conduit. It didn't sound right though????

thanks for any replies
 
  #2  
Old 09-15-03, 02:44 PM
J
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UF may be run through conduit for physical protection. You'll need pretty large conduit to make it possible to get a large cable through it.

Almost all (and all that Home Depot sells) of THHN wire is dual rated THWN, so yes, you can use it in underground conduit. And it will be a lot easier to get it through. Check the THHN specs closely and you will see that it is also rated THWN.

If you use rigid metal conduit (not EMT), you only need to bury it six inches.
 
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Old 07-24-07, 12:29 PM
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UF cable

I have a couple of follow up question on this and need your advice.

I am trying to install a light on the tree. I was told by someone at Home Depot that putting a direct burial UF cable (the one with rubber jacket with 3 wires- black, white and neutral) in a gray pvc conduit would generate too much heat (??) and therefore not recommended (or against code?). Is it true? I would think more protection the better. This person recommened instead use a single conduct (strand or solid) (i.e., coler coded ) cables with the conduit.

My second question is, if I can use a direct burial UF cable in a conduit, then can I combine the 12 guage 3 wire UF cable and a single strand (coler coded) cable to create a 12 guage 4 wire (to add an outdoor receptacle). I will be using a 1/2" gray PVC conduit and it is not big enough for two 12 guage cable. Thank you for your advices. By the way, the conduit will be insalled near the fence behind the tree and no one will be near it.
 
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Old 07-24-07, 12:38 PM
R
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You cannot combine a cable assembly containing three conductors and a single wire to make a multi-wire circuit.

Either use four conductor cable, or use four individual wires.
You can run UF in conduit, but it makes no sense to do so, except for short runs where protection is needed.
 
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Old 07-24-07, 12:58 PM
I
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To add further, if this circuit is 20A or less and GFCI protected before it enters the ground the depth only needs to be 12" regardless of whether you use UF-B or THWN in conduit.

If you use conduit, use THWN conductors; UF-B is nearly impossible to pull through. Look closely at the THHN at Home Depot: it will have "THHN / THWN-2" printed on the insulation indicating it meets both standards.
 
  #6  
Old 07-24-07, 01:43 PM
B
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Thank you for your help.

Just to make sure that I understand, I can just bury the UF cable about 12" underground without a conduit (What if it is not 12"?). It is about 30 feet length and I just wanted to provide addtional protection from animals, etc. Do I need to worry about this? As I said, this is an area that no one will venture out. As I said, I am trying to put a flood (security) lights on the tree, about 10ft above ground. I suppose I need to install a protective conduit with waterproof wires for this section. Am I right?

"You cannot combine a cable assembly containing three conductors and a single wire to make a multi-wire circuit."

Would you please explain to me why I cannnot combine a UF cable with an single wire. What would be the consequence? What is the difference between 4 wire cable and this? I think I can pull these wires with no problem through the counduit.

"You can run UF in conduit, but it makes no sense to do so, except for short runs where protection is needed."
Is 30ft segment considered a short distance?

What does THHN or THWN-2 mean?

Thank you again for your help.
 
  #7  
Old 07-24-07, 01:59 PM
R
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The NEC has rules that I am not even going to pretend to understand. Not everything makes sense all the time.

Cable assemblies are designed to be used as a unit. That's why (for branch circuit sizes anyway) they include a ground wire and at least two current carrying conductors, where one is white and the others are not white.

In a cable assembly or in a conduit the current going one direction must equal the current in the other direction. If you made a multi-wire circuit out of a section of 12-2 UF and a section of 12 gage THWN, the currents through the UF cable would not be equal when the section of 12 gage wire is carrying current.

This would violate the rule. Would it be unsafe? Probably not. Would it confuse people? Probably yes.


I doubt very much that you can pull UF through a conduit of any significant length that is not straight. It just isn't as easy as it sounds. I KNOW you cannot pull it through half inch conduit.


THHN and THWN are ratings for the wire. They identify type. The W in THWN means water proof, so it can be used outside in conduit.


Make your entire run 12-3 UF-B, and use conduit up the tree to the receptacle. Then use conduit up to the light with 12-2 UF-B or individual wires in the conduit.

Or use conduit the entire length and use individual wires.

If using individual wires use three or four of them (depending on what section of the circuit), one bare or green for the ground, one white for the neutral, and one or two black or some other color for the hots.
 
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Old 07-24-07, 02:12 PM
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Thank you. I will follow your advice and honor the NEC code.
 
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Old 07-25-07, 07:55 AM
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> I can just bury the UF cable about 12" underground

You can bury it 12" deep if it has either GFCI breaker or a GFCI receptacle protecting the circuit before it goes underground. This does not count if the GFCI receptacle is at the end of the run. I recommend you follow racraft's advice and use 12/3 UF-B cable; sleeve the cable in 3/4" PVC conduit (or larger) when it exits and enters the ground and up the tree for protection.
 
 

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