underground service wiring conundrum

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RustyShackleford

Minister of Fire
Hearth Supporter
Jan 6, 2009
1,350
NC
I'm trying to run 30-amp/240v electrical service to my shed, maybe 60ft. I bought a 100ft roll of 10-3 UF-B awhile back. But today I tried to dig the trench and it was all I could do to get 18" deep (instead of 24" required). So I've gotta do PVC conduit. I'm wondering if I should return the UF-B and buy THHW; it'll be about a wash in cost but a bit of a hassle. I'm worried about trying to pull the UF-B, especially since there's a 90-degree bend midway (maybe I can smooth it out to two 45s). Speaking of pulling, what size conduit should I run ? Also, reading the NEC, it looks like I can go to 35 amp service if I use THHW (instead of 30 amps for the UF-B)
 
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Pulling UF-B thru conduit can be a bear, if favor THHN. In overhead conduit, I use what’s called a “pulling elbow” or an outlet body (eg. Type LB), when I need to install a 90 elbow, but you’d have to check to see if they’re rated for burial.

I can look in my book tonight for minimum conduit size, but have you de-rated your conductors for distance, if over 100 feet total from breaker to load? There is margin in the 10/30 combo, so the derating is likely nil for up to 150 feet, but I’d have to look that up to be sure.
 
For that distance I'd go 8/3 if power equipment will be a frequent load. If just lighting and an occasional hand tool then 10/3 might suffice. If this is at the shed soften the radius with a wide sweep elbow. Or if this is in ground, eliminate it completely with a gentle curve over a greater length of the conduit.
 
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Pull the wire through as you bury the conduit. Or run a rope through it while burring it and pull it through.. Put a extra pull rope/string in the conduit as well incase in the future you want to add something else.
 

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All excellent suggestions. As Bgreen said, whats the service for. If your running motors that have starting current, oversize the wire based on the distance. See the online calculators for this. Whatever you use avoid any 90's. I like the Idea of sliding the pipe over the wire.
 
Pulling UF-B thru conduit can be a bear, if favor THHN. In overhead conduit, I use what’s called a “pulling elbow” or an outlet body (eg. Type LB), when I need to install a 90 elbow, but you’d have to check to see if they’re rated for burial.
Since even the inside of a conduit is a "wet" location, I don't see why it'd be a problem. But since underground, it doesn't really help unless I pull the wire as I bury, in which case I don't really need it.

I can look in my book tonight for minimum conduit size, but have you de-rated your conductors for distance, if over 100 feet total from breaker to load?
It's definitely under 100ft. Wire can crry the same amperage regardless of distance, except, of course, for voltage drop. The resistance of 10awg is exactly a milli-ohm per foot, so my <200ft round-trip would drop <6 volts when pulling the full 30 amps. I think that's ok.

For that distance I'd go 8/3 if power equipment will be a frequent load. If just lighting and an occasional hand tool then 10/3 might suffice.
I've been using a heavy-grade extension cord (probably 12awg, certainly not over 10awg), on a house circuit protected by 20 amp breaker, and it's fine. I can start up compound-miter saw while small pancake compressor is running, and no issue - that's good enough for me.

If this is at the shed soften the radius with a wide sweep elbow. Or if this is in ground, eliminate it completely with a gentle curve over a greater length of the conduit.
Trench is dug, machine returned. No long gentle curve is gonna happen, but wide-sweep elbow will fit.

Pull the wire through as you bury the conduit. Or run a rope through it while burring it and pull it through.. Put a extra pull rope/string in the conduit as well incase in the future you want to add something else.
Good ideas. One thing i wonder: it means you gluing the next piece of conduit with the wire already pulled past the joint; is it ok to get the PVC cement on the wire ?

Hello
I did the direct burial, I do not like the idea of water collecting in conduit.
Ship has sailed: trench dug, machine returned.

So back to my original question: assuming I'll pull the wire AS I'm burying the conduit, should I go to the trouble of replacing the UF-B with THHW ? And, what size conduit should I use ?
 
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A hard 90 may be required above ground at the building. If so, use a type LB for pulling access.
 
It's definitely under 100ft. Wire can crry the same amperage regardless of distance, except, of course, for voltage drop. The resistance of 10awg is exactly a milli-ohm per foot, so my 200ft round-trip would drop 6 volts when pulling the full 30 amps. I think that's ok.
It depends on the incoming line voltage and what's normally running in the shed. If at a later date a freezer or beer fridge is added, this can matter.
 
A hard 90 may be required above ground at the building. If so, use a type LB for pulling access.
Definitely planning to use those. Am I correct that, if I end up using THHW, I could make the splice between NM-B from the breaker box and the THWW, inside the LB elbow ? (Because it can't be ok to run the THHW all the way to the breaker, unless I run it in conduit of some kind, a PITA, since a rather tortuous path).

What about those slip joints, that allow movement between the LB elbow on the side of the two buildings, and the buried conduit ? When are they required ? They are expensive ! Not much frost-heave here in the sunny south - is that the issue ?
 
I have 2 - 20 amp breakers in the small 2 breaker circuit Panel in the shed with all yellow 12 Guage Wiring. I put a compressor on one breaker and saw and vac on the other breaker. No problems. Since I built a new workshop next to the house with 80 amps, 50 for my welder, it is all set.
 
One thing i wonder: it [pulling the wire as you bury the conduit] means you gluing the next piece of conduit with the wire already pulled past the joint; is it ok to get the PVC cement on the wire ?
A quick googling suggests, as I suspected, that it is NOT ok to get PVC cement on wire - since it basically does a solvent weld. So I guess the solution is to run a pull cord in the PVC as I'm burying it, and then use that to pull the wire.

All this is making me think I should definitely use THHW (and return the UF-B). And wireandcableyourway.com sells 8awg by the foot but 10awg only by 100ft rolls, so since I only need 80ft or so, it won't cost much more to go with 8awg. So I might do that after all. Given that: what size conduit ? Considering conduit is cheap.
 
I have 2 - 20 amp breakers in the small 2 breaker circuit Panel in the shed with all yellow 12 Guage Wiring. I put a compressor on one breaker and saw and vac on the other breaker. No problems.
But what size breaker protects the feed to the shed ? Double-pole 20 amp, I presume.
 
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But what size breaker protects the feed to the shed ? Double-pole 20 amp, I presume.
The main panel has a 30 amp 220v breaker because the shed panel is rated for 30 AMP 220vac and the wire is 10-3 UF-B
The conduit goes 3 feet below ground and then the direct burial cable is just in the dirt. After 12” of dirt, the red electric cable warning tape was laid in. Remember Dig Safe! :)
The small conduit is for the phone & cable TV wires, don’t forget those to have with the kegerator!
 

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The main panel has a 30 amp 220v breaker ...
You didn't use the expansion couplings ? I can't decide on that. They say if things will move more than 1/4". The horizontal buried run is not an issue because the soil is so constant temperature. The coefficient of thermal expansion of the PVC is 34ppm per degree-F. So if your riser comes up 50" out of the ground, a 100 degree temperature swing is gonna move it about 1/6". So should be ok. Except for ground settling and/or frost heave. Would be a no-brainer, but those things are expensive. I'm already pissed about having to buy PVC at all (since I couldn't get to 24" deep).
 
I don't believe it is ok to splice in an LB fitting unless there is enough volume.
Those things are huge; even if I go to #8 wire, it's only 3 cu-in per wire, or 21 cu-in (ground counts as 1, I believe).

But, it turns out it's not ok just sufficient for the box to be big enough, its size needs to be marked: https://www.ecmag.com/section/codes-standards/box-fill-calculations-part-xii . (scroll down to 314.16(C)(2))

Weird, since for normal boxes, they aren't required to be marked (I think, at least I rarely see it on metal ones).

I will be using much larger than 3/4" conduit. But that's one thing I'm trying to get an answer to: how big should I go ? (I don't care about the conduit fill calculations, those are ludicrous, at least for a run this long).
 
You didn't use the expansion couplings ? I can't decide on that. They say if things will move more than 1/4". The horizontal buried run is not an issue because the soil is so constant temperature. The coefficient of thermal expansion of the PVC is 34ppm per degree-F. So if your riser comes up 50" out of the ground, a 100 degree temperature swing is gonna move it about 1/6". So should be ok. Except for ground settling and/or frost heave. Would be a no-brainer, but those things are expensive. I'm already pissed about having to buy PVC at all (since I couldn't get to 24" deep).
Hello
All 8 cement blocks under the shed rest on 2 foot deep holes filled with 1/4” stone! The heavy duty steel entry door still opens and closes fine after 5 years because of the stones the shed has not settled not even 1 millimeter!

Don’t forget a heater for the winter! I installed a small wood pellet stove that I can turn on from the main house. No sense going out to a cold shed. :)
Also to help on heating in the winter and cooling in the summer I suggest reflectix foil on top of your shed insulation for a good thermal break!
See mine
https://www.hearth.com/talk/threads/shed-roof-thermal-relief-for-winter-summer.84824/#post-1112024
 

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Hello
All 8 cement blocks under the shed rest on 2 foot deep holes filled with 1/4” stone! The heavy duty steel entry door still opens and closes fine after 5 years because of the stones the shed has not settled not even 1 millimeter!
Mine is a shipping container. One thing I could do is to simply not cement the joint were the riser goes into the LB box: the inside of conduit is a "wet location", so doesn't seem like it really matters if a smidgen of water gets it there. The main point of the conduit, I believe, is to simply project the wire within; that would still be the case.

Don’t forget a heater for the winter! I installed a small wood pellet stove that I can turn on from the main house. No sense going out to a cold shed. :)
I'm in NC, not NH :)
 
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Yes the conduit is just to protect the wire from something bumping into it and hungry rodents like squirrels that chew the wires!
NC does not get as cold and snowy as up here!
 
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FWIW there are no expansion joints on our ~50' run from the house to the garage.
 
FWIW there are no expansion joints on our ~50' run from the house to the garage.
I think it's the vertical risers where they might be needed. Even Carlon says you don't need 'em for buried horizontal runs, because the soil temperature is so nearly constant.
 
Yes, understood, I should have been more specific. Our vertical risers before the garage LB and panel end don't have them. Our climate is mild like yours.
 
i've never spliced in a LB because there is not much room and you don't bend the wires to sharp that there could be a weak spot. a box is a good thing it's where the romex can splice into the uf cable without a code violation. some codes are dumb and we have to follow them anyway but putting a expansion fitting where a pvc comes out of the ground is a good thing. more than half the time i come across a pipe without one it usually breaks the knock outs out of a meter socket or breaks the box
 
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a number 10 wire spliced into a box or anything grounds count as one wire but rest is 2.5 cubic times 6. white red and black are 3 and the other white red and black are 3 different wires. so that splice is 17.5 cubic fill
 
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