Ek's Home   |   Forum   |   Chat   |   Electrical Links   |
	Subject - Underground Conduit vs Direct Burial Cable Scott Vickrey In an effort to document the pros and cons of each wiring method. I invite you to share your thoughts and experience concerning underground conduit verses direct burial cable. frenchelectrician hello guys and ladies: thanks scott for pointing it out here from other page. what we been talking about the subfeed box . the reason the moot point is the cost wise . sure the uf wire are little cheaper than pvc underground conduct but really in my option here i feel better to use the conduct [pvc] because it is eaiser to repair the wire[s] and for future expandation like increaseing the wire size up if the conduct have plenty room there and my standard practice i useally bump up one size larger if i can espcally true with very long runs. this is very common curpit on mobile home parks i did see alot of wire were buried with uf or underground service cables and they dont last very long with it and end up with damaged wires have to replace it. [ every time i replace the wires i run new pvc conduct 2inch size due many new mobile home now are comming out with 200 amp breaker box]and put a 12 by 12 pull box for spliceing in there and it look neater too .. scott i know what you mean about prefab elbow sweeps they are kinda pain in butt too one trick i did it work i took a pvc remer kinda like cone shape and what it do is take sharp edges off from the pvc pipes the same way with emt conducts. one more thing scott: i not really catch the picture what you called yellow wrap can you explain little more clear for me ?? thanks. merci, marc JimmyDee I like oversizing the conduit on an underground run rather than using UF cable or proper size conduit for future increase in wire size. I've never had a problem with a conduit underground but have had several service calls on UF cable and underground feeders (non cable) that have gone bad. Some have been under a drive way for lighting. The biggest problem I saw was a family of shrews decided to make the coating of the UF cable their lunch. Several dead, several alive, but the wire didn't make it. Jim iwire I have never understood the economics of using direct burial cables. Around here we pay a lot for trenches to be dug, add asphalt patching and it gets real expensive. If we have a trench dug we put in PVC raceway, most times larger than needed. To my own garage only 20' x 12' I have one 2" and two 3/4" PVCs. cs409 i have repaired more direct burial runs and very very very few conduit runs ....as stated, always best to go to next size conduit for ease of work and future size increase.... Scott Vickrey I only use PVC for underground runs. I also oversize every run. I like the fact that I can pull new wire in if I need to and knowing the person that has to work on it after I'm gone isn't cursing me. I don't like the PVC prefab elbows because in a tough pull the pull string or rope will cut clean through them and it's also really tough to push a fish tape through them. At both ends I always take a 10' stick of RMC and bend it to correct length so it reaches the bottom of the ditch. I use the yellow field wrap on the metal from the end to a few inches above grade. I change over with a female adapter and if there are any turns that need to be made I do it in a large sweep. If I have to turn a 90 I use a RMC elbow again wrapped in field wrap and connected with female adapters. iwire Scott I am with you on the prefab PVC elbows, most of our work we have the space not to use any bends at all just long slow turns. However coming up out of the ditch we use metal rigid prefab 90s if we are coming up in the open we use metal all the way up or if we are coming up into switchgear we use metal at the 90. When we run primary ducts for the local power company the rules are a little different, 5" schedule 80 PVC until the last 10' before 90 turns, at any 90s they have us use 10' of 5" GRC than a large radius 90 sweep (about 5' tall) than 10' of GRC again. This will weight about 400 lbs (about 28' of 5" GRC) When you watch them pull the cable in with a steel cable from a truck bed winch you can see why they require it. cal_sparky Scott, Bob, If you bring the GRC to and above grade, how do you bond it? I use GRC 90 sweeps on Schedule 40 runs, but the top of the GRC elbow is 18" below grade, so does not have to be bonded. Cliff David Hyatt Well, at my old house I ran PVC to my building and pulled THHN. I had a 3-way and hot circuit so it made sense to run conduit. UF cable deep enough not to be nicked by landscapers will last "forever" I know nothing last forever. On large cable it is hard to pull wire through and the cost is higher, so I prefer direct burial. This will last if no one cuts it. I do however alot of service calls from broken underground lines. It is always either an installer error, or someone digging. Never have I had a wire that wasn't damaged at the install that needed repair. The time I save as well as the money its worth burring the wire to me. Under drives or sidewalks I will use a conduit sleeve for future needs. Thats just my opinion. I do understand and agree with your post too. Scott Vickrey Marc, Sorry, I didn't see your question about the tape. I'm not sure what the proper name for the stuff is. We call it field wrap. It's a yellow heavy plastic 2" tape that has a sticky polymer on the underside that resembles tar. I wrap it around rigid metal conduit (even though it's galvanized) where it will be in contact with earth. This keeps it from corroding. Cliff, As far as bonding goes, I should just tell you I use a bonding locknut but, this wouldn't be truthful. I prefer to come up with rigid metal into a threaded collar and attach it with a chase nipple. This makes for really good pulls, protecting the insulation from damage. I believe it beats the heck out of those flimsy plastic or fiber bushings. They are about half worthless, not wanting to stay on the well reamed but still sharp conduit. Since I can make the chase nipple bite into the metal, I feel the superior protection it gives the wire insulation far outweighs the slightly better bond of a bonding locknut. I wish they would make bonding chase nipples so we can have the best of both worlds. I have used some that I suspect may be listed for this but, I don't know. Another thing about those bushings how many times have you terminated a wire only to have to take it apart to put that darn bushing on? David Hyatt Scott, speaking of tape, it is good practice to install 2" caution tape the length of the trench to help prevent someone from digging up your wires. Wire, dirt, tape, dirt. Hopefully they will see the tape before they hit the wire. cal_sparky Scott, I like your approach--"come up with rigid metal into a threaded collar and attach it with a chase nipple." This will bond the conduit at the one end. And I think you don't need a bonding locknut unless you're either coming into a non-metallic enclosure, or the conductors are at 250V or more to ground and the MA enters through a concentric or eccentric KO. I though I recalled a requirement for any metallic raceway to be bonded at both ends, but I looked and can't find it--except for a requirement for adequate bonding where the raceway is used a the EGC. And this ain't the case here. Thanks for the idea Scott. Cliff renosteinke As other have pointed out, you can fix wire in pipe. More pragmatically, UF usually has to be buried deeper than pipe. A 24" ditch is twice as hard to dig as an 18" ditch. If anybody want UF, give them the shovel!