Buried conduit, wire size, type??

[BFBOB]

I've got to replace the cable in my conduit from the meter to my garage. I guess I made a bad decision and used some old aerial wire I had lying around. Aluminum 1 0r 2 ga, with the steel strand removed from the bare neutral - otherwise I'd never have been able to get it bent around the corners! It worked fine at first, but 3 years later I have one phase open completely and one with so much resistance my voltage drops to 90 when I turn 200W worth of lights on. Power tools won't run at all. I suspect water problems though I carefully inspected every inch of the stuff and found no damage to the insulation. And I did use de-ox on the connections. You'd think something designed to be hanging out in the weather would be waterproof, but there it is.
So here are the questions: Assuming no conduit is perfectly dry, is THWN better than THHN? The W stands for water resistant, y'know - or are there other tradeoffs?
I think I'll go with an insulated neutral this time, again for the dampness.
Size - according to which chart you choose to believe, 3 or 4ga should be sufficient. It's only 55 wire feet - 100A panel, coming right off the meter lugs - it's a fancy new meter with a main disconnect and three sets of lugs.
I'm inclined to skimp a bit and use the 4 ga, which is rated 95A. Close enough? The 3ga is rated 110A, but costs quite a bit more - 1.04/ft vs. .81/ft. (at wireandcabletogo.com. haven't checked my local supply houses yet)
That's about $40 difference. Hmm, maybe cheap insurance.

Is this short enough that some of you sparkies might have some cutoffs to get rid of?

 

[RPH]

Thwn only, conduit is considered a wet location.

 

[Norcal]

I've got to replace the cable in my conduit from the meter to my garage. I guess I made a bad decision and used some old aerial wire I had lying around. Aluminum 1 0r 2 ga, with the steel strand removed from the bare neutral - otherwise I'd never have been able to get it bent around the corners! It worked fine at first, but 3 years later I have one phase open completely and one with so much resistance my voltage drops to 90 when I turn 200W worth of lights on. Power tools won't run at all. I suspect water problems though I carefully inspected every inch of the stuff and found no damage to the insulation. And I did use de-ox on the connections. You'd think something designed to be hanging out in the weather would be waterproof, but there it is.
So here are the questions: Assuming no conduit is perfectly dry, is THWN better than THHN? The W stands for water resistant, y'know - or are there other tradeoffs?
I think I'll go with an insulated neutral this time, again for the dampness.
Size - according to which chart you choose to believe, 3 or 4ga should be sufficient. It's only 55 wire feet - 100A panel, coming right off the meter lugs - it's a fancy new meter with a main disconnect and three sets of lugs.
I'm inclined to skimp a bit and use the 4 ga, which is rated 95A. Close enough? The 3ga is rated 110A, but costs quite a bit more - 1.04/ft vs. .81/ft. (at wireandcabletogo.com. haven't checked my local supply houses yet)
That's about $40 difference. Hmm, maybe cheap insurance.

Is this short enough that some of you sparkies might have some cutoffs to get rid of?

#3 copper is only good for 100A, you cannot use the 90 degree C column to size wire.

 

[DesertSparky57]

RPH is this wet location in a conduit just a Michigan thing???

County, city and 1000's of commercial locations here have miles and miles of THHN underground in conduit. I know I pulled my fair share of street lights that's for sure...

To the OP, you say you have three sets of lugs? Describe this part more accurately please because if you are pulling more than 3 current carrying conductors in this conduit of yours.... You must de-rate the current carrying capacity of your chosen wire to 80%. I would be surprised to learn you have 3 phase power but you never know.

Oh and seriously turn off the power supply feeding your trashed set of feeders man!! Jeez!! Ka-Blooey probably already happened but come on man! Are you trying to weld them all up in the conduit and lose the pipe or what?!?!

Money says they've blown up in the conduit already.

 

[sands35]

The stuff that HD and Lowes carries is dual rated for THHN and THWN. Not sure what the 3g and bigger is though.

 

[LXCam]

You should use 1-1/4" min conduit and #2 wire. And you're damn lucky no one got hurt off your old service.

 

[wyliesdiesels]

RPH is this wet location in a conduit just a Michigan thing???

County, city and 1000's of commercial locations here have miles and miles of THHN underground in conduit. I know I pulled my fair share of street lights that's for sure...

To the OP, you say you have three sets of lugs?Describe this part more accurately please because if you are pulling more than 3 current carrying conductors in this conduit of yours.... You must de-rate the current carrying capacity of your chosen wire to 80%. I would be surprised to learn you have 3 phase power but you never know.

Oh and seriously turn off the power supply feeding your trashed set of feeders man!! Jeez!! Ka-Blooey probably already happened but come on man! Are you trying to weld them all up in the conduit and lose the pipe or what?!?!

Money says they've blown up in the conduit already.

I took that to mean 3 pairs not 3 hot lugs! He did say sets...

 

[Charles (in GA)]

RPH is this wet location in a conduit just a Michigan thing???

County, city and 1000's of commercial locations here have miles and miles of THHN underground in conduit. I know I pulled my fair share of street lights that's for sure...

Virtually all THHN is dual marked with THWN. All conduit in ground and in concrete slabs are considered to be wet locations per the NEC

300.5 Underground Installations.
(A) Minimum Cover Requirements. Direct-buried cable
or conduit or other raceways shall be installed to meet the
minimum cover requirements of Table 300.5.
(B) Wet Locations. The interior of enclosures or raceways
installed underground shall be considered to be a wet location.
Insulated conductors and cables installed in these enclosures
or raceways in underground installations shall be listed for use
in wet locations and shall comply with 310.10(C). Any connections
or splices in an underground installation shall be approved
for wet locations.

300.50 Underground Installations.
(B) Wet Locations. The interior of enclosures or raceways
installed underground shall be considered to be a wet location.
Insulated conductors and cables installed in these enclosures
or raceways in underground installations shall be
listed for use in wet locations and shall comply with
310.10(C). Any connections or splices in an underground
installation shall be approved for wet locations.

 

[BFBOB]

Thanks for all the info. I was unaware of the dual THHN/THWN rating - was having a hard time finding THWN!
By three sets of lugs I meant three hot1, three hot2 and three neutral/ground. I've only got three conductors going through the conduit to the garage - the house is strung overhead. I've had to leave the power on because there's only one disconnect, and it would cut power to the house too.
It's 1 1/2" conduit, and plastic, so plenty of room. I suspect corrosion rather than a short, but that can lock up a conduit just as surely as welding a phase conductor to it! I'm hoping it's eletrolytic action due to water in the pipe and tiny flaws in the insulation. That could account for it all without locking the conductors in place. It can't be overloading the wire since the heaviest load I've put on it is a 1/2 hp bench grinder. plus a few lights.
Worst case, if I have to dig it up, it's only 40'. I did it by hand originally and I can do it again ... if I have to ... I guess.
It looks like I can use 3ga copper or 1ga aluminum. The only aluminum I've found is 3-con cable. (they call it cable, but actually it's wire. No overall jacket, just 3 conductors bonded like zip cord). Could this be a problem pulling around 2 90 degree bends, vs 3 separate conductors? To get the original wires through took a pulley and a rope hooked to my trailer hitch!

 

[BFBOB]

You should use 1-1/4" min conduit and #2 wire. And you're damn lucky no one got hurt off your old service.

Since it's a service entrance panel, not a sub-panel, it's grounded properly and bonded to the neutral. How would anyone get hurt? If a sub panel, I can see that it's conceivable the neutral could blow and fuse to the second phase inside the conduit, resulting in the neutral becoming hot. But not in this case. It would just go to ground and trip the main breaker on the meter.

I thought about disconnecting the garage wires at the meter, but it's pretty close quarters, and I couldn't see a good way of making sure there was no way to end up with a short across the phases. Can't just flip the main breaker because the house needs power.

 

[wyliesdiesels]

If u need to disconnect the garage, u could do it hot with an insulated screwdriver. Remove one leg at a time and wrap the bar end with tape so it doesnt touch one of the busses. If u wanted a disconnect for the garage, u could mount one next to your main service panel still using the lugs to feed it.

Have u tried looking for MHF/mobile home feeder? U could use #2-2-2-4 AL MHF @ a max of 90a.

 

[RPH]

All I can tell you here where I live is it better be thwn rated or you will do it over.
I may just be the inspector but is it worth the trouble when you got to buy it to begin with. I make sure I get what he wants and life is much simpler.

 

[BFBOB]

UPDATE: The power is back on! I decided on 1ga aluminum rated THHN/THWN. It was so much cheaper than copper I couldn't resist. My local supply house had it for 38c per foot in bulk, so $75 worth did the job. The old wire pulled out very easily, and it all came in one pull. I thought I might have to fish for fragments from both ends, but not so. It was soaking wet, and in a condition that -- well, take a look at the pictures. The reason for the dropping voltage on light loads is plain to see. The only "neutral" was through the earth ground.
One reason for the water was that the conduit had settled in its trench far enough to pull the fitting out of the meter base, along with the next-size knockout. I had to dig up about two feet and tamp earth firmly under it. I used the blunt end of a 25 lb pinch bar to tamp it under and alongside, forcing it up enough to actually put some upward force on the meter base. Oversize washers top and bottom secure it now securely. At the garage end, it hadn't moved a bit.
With that much water, I wanted to dry out the conduit as best I could. Compressed air and vacuum didn't get any out, which I guess means it wasn't full. I cut a close-fitting plug of closed-cell foam and pulled it through, ejecting about half a gallon on the first pass, and just a few drops on the second. Good enough.
Pulled the new wire though with no sweat, taped the ends of the neutral gray, and fired it up. Let there be light!
Took a few hours. Could have been sooooo much worse.

 

[Norcal]

That cable is not permitted in any conduit, indoor,outdoor,underground, whatever, know of a house in Arkansas where they used that cable in metal studs w/o a conduit for the main panel, just a FYI, there are no inspections except septic in that County. ****** scary.

 

[BFBOB]

That cable is not permitted in any conduit, indoor,outdoor,underground, whatever, know of a house in Arkansas where they used that cable in metal studs w/o a conduit for the main panel, just a FYI, there are no inspections except septic in that County. ****** scary.

Not disputing your word, just seeking information -- but (first, I sure hope "that cable" in your message refers to the OLD stuff!) logic says wire intended to be installed outdoors is suitable for wet locations. Is there a spec for aerial on how long it can be wet? Rated for wet but not immersion? A code requirement without a 'why'? Obviously something went wrong, and judging from the location of the damage I think it looks like electrolysis. The damage to the bare neutral is only right beside the damage to the hot, not in the several feet in both directions that were immersed too. Damage to the insulation my inspection didn't catch, or incorrectly rated insulation that soaked through?
Just to enhance my reputation as a smartypants, it isn't cable; it's wire. Per NEC, a cable is a factory made assembly of conductors having an overall jacket or sheath. A wire is a factory made assembly of conductors without an overall jacket. So, zip cord is wire, Romex is cable.
In my last certification class the question was asked whether it was permissible to run low and high voltage cables through the same hole in a wooden stud. My answer was Yes, for which I was immediately shouted down by the entire classroom. But I quoted as nearly as I could from memory the relevant section, and during the next mini-quiz the instructors looked it up and found I was right. There is a clearance requirement between low and high voltage conductors and wires, but not cables.
However, out in the real world, most inspectors won't allow it.
And, after all, the last line of the NEC reads: "Or whatever the inspector says."
So, lesson learned. I won't be putting aerial wire in conduit again!

 

[theoldwizard1]

I've got to replace the cable in my conduit from the meter to my garage.

Are you coming right off the meter with no disconnect ? If so install a fused disconnect before the line goes underground.

Consider downsizing the fuses to 90A or even 75A. You can save a lot on wire.

 

[Speedy Petey]

Are you coming right off the meter with no disconnect ? If so install a fused disconnect before the line goes underground.

WHY???
What is wrong with coming from the meter underground to the building?

 

[Speedy Petey]

All I can tell you here where I live is it better be thwn rated or you will do it over.
I may just be the inspector but is it worth the trouble when you got to buy it to begin with. I make sure I get what he wants and life is much simpler.

So when was the last time you saw THHN that was not also rated THWN??

 

[Speedy Petey]

Virtually all THHN is dual marked with THWN. All conduit in ground and in concrete slabs are considered to be wet locations per the NEC

 

[Ryf]

WHY???
What is wrong with coming from the meter underground to the building?

nada, but if he had it fused/disconnect before he could have cut power and not lost it at the house. nothing wrong either way but there are good reasons for both. if the meter was on the house, as alot are, having a disconnect would be easy, even if he had to run it inside and pop back out for aesthetics.

 

[BFBOB]

Yes, a separate disco for the garage is a good idea on general principles. There is a 200A breaker between the meter and the lugs, so it's easy to shut down and work on, but it does cut power to the house too. Problem is the meter base/disco combination is so tall that the bottom of the box is less than 3' off the ground. Add another disco below and that makes it low enough to make me nervous. On occasion the snow could even be that deep. Hopefully future garage wiring (and there will be some!) can be done just flipping the garage's main.