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TheCowGod 03-14-2012 12:09 AM

Conduit questions in shed wiring
 
Hi guys. I'm Dan, and I'm in Knoxville, TN. I know running power to a shed is a very common topic here. I've been doing tons of reading and feel pretty comfortable with the requirements, but I have two questions I haven't been able to find a clear answer to.

First, my situation: I'm having a 10x16 shed built in my backyard, to be used primarily as a woodshop. I need more than a single circuit, so I know I need a subpanel and two grounding rods at least 6' apart, bonded to the subpanel with #6 bare copper wire.

My house has 200 amp service, and the main panel is in the garage at the opposite corner of the house from where the shed will go, so it would be pretty inconvenient to run a feeder from, there. But there's an existing 100 amp subpanel in the basement, presumably installed when the previous owner finished the basement. I'll be running a 60 amp feeder from that subpanel to the shed.

My total run will be something like 75', so I'm running #6 THWN for the two hots and neutral, and #10 THWN for the ground. I plan to bury it in 1.25" schedule 40 PVC, buried 18" deep.

My first question relates to how I need to run the wire when it's not underground. The basement is finished with a drop ceiling, so I plan to run the feeder across the drop ceiling, out of the house, down to the ground (it's a walk-out basement), then underground the 26' to the shed. I assume it still needs to be in conduit for at least the part that runs up the wall of the house. But what about inside, when it runs through the drop ceiling to the panel? Does it need to be in conduit for the whole run? Or would I just staple the four wires to the joists or something?

The other question relates to the trench. I've got a rain gutter downspout that discharges right at where the shed will soon be, so I'm running a 40' length of 4" PVC to pipe that water past the shed. It will be buried just about 6-12", just enough to run under the shed. Can I run the power conduit in the same trench (obviously, deeper)? Presumably I'd dig the 18" trench to the shed, lay the schedule 40, then add a few inches of dirt to bring it up to about 12" deep, and extend the trench past the shed at that depth, then lay the 4" drain pipe and backfill. Or do I need to dig two separate trenches? Is there a rule about how far apart they need to be if so?

If anyone sees any other flaws or concerns with my plan, please do speak up, I want to do this right. Thanks!

Dan

rrolleston 03-14-2012 12:25 AM

You will need conduit for the whole run from one panel to the other with threaded male adapters. Conduit has to be 18" below grade so about 20" deep. Schedule 80 wherever it could be hit by something. You may have to use a variety of LB's and 90's to get there. And I would use 50 amp breaker for #6 wire. And a main breaker in sub panel with any size main for a disconnect.

jbfan 03-14-2012 06:42 AM

Quote:

Originally Posted by rrolleston (Post 877197)
You will need conduit for the whole run from one panel to the other with threaded male adapters. Conduit has to be 18" below grade so about 20" deep. Schedule 80 wherever it could be hit by something. You may have to use a variety of LB's and 90's to get there. And I would use 50 amp breaker for #6 wire. And a main breaker in sub panel with any size main for a disconnect.

Why do you tell everyone to use a 50 amp breaker with #6 wire?

Thhn/thwn is rated at 65 amps in the 75 degree column.
It is rated at 55 amps when in romex, but can have a 60 amp breaker.

TheCowGod 03-14-2012 08:02 PM

Thanks for the responses. I spoke with the local electrical inspector and got my questions answered, so I thought I'd post the information I got, in case it's useful to anyone else. Of course this information isn't necessarily code or anything, it's just what my inspector said.

60 amp breaker is correct for a #6/#10 feeder consisting of single conductors in conduit. Schedule 40 is fine for underground use or anywhere it won't be in danger of getting damaged -- schedule 80 for places where it might, like next to a driveway or sidewalk.

The two pipes can't be in the same trench -- there needs to be 12" of compacted dirt between them.

Single conductor wiring does need to be in conduit all the way from one subpanel to the other, including above the drop ceiling etc.

The inspector also said he only needs one ground rod at the shed, not two of them 6' apart as I've seen here. He didn't say anything about needing to show it's under 25 ohm impedance or anything. I'll happily skip driving a second ground rod, but it sounded like most places do require that second one.

I'd read here that #6 bare wire is the appropriate one for connecting the ground rod(s) to the panel. The inspector says I need to use #4, though #6 is allowed if it's protected in bonded conduit. I hadn't come across that before, don't know if it's specific to this area.

Now that I know I have to dig two separate trenches, I'm considering spending the $135 on a trencher from SunBelt. I'm going to try my hand with a shovel first, but the soil here is mostly clay and I'm not sure how long I'll get before I give up and load up the trailer :)

Dan

rrolleston 03-14-2012 08:24 PM

Quote:

Originally Posted by jbfan (Post 877284)
Why do you tell everyone to use a 50 amp breaker with #6 wire?

Thhn/thwn is rated at 65 amps in the 75 degree column.
It is rated at 55 amps when in romex, but can have a 60 amp breaker.

keep forgetting that individual wires in conduit can be on 60 amp breaker.

Missouri Bound 03-14-2012 08:32 PM

Get the trencher. Save your back for pulling wire.

SD515 03-14-2012 08:39 PM

Quote:

Originally Posted by TheCowGod (Post 877759)
I'd read here that #6 bare wire is the appropriate one for connecting the ground rod(s) to the panel. The inspector says I need to use #4, though #6 is allowed if it's protected in bonded conduit. I hadn't come across that before, don't know if it's specific to this area.

Unless your local codes amend this section of the NEC, ground rods only need to be connected with (up to) #6 Cu. Protection is required when subject to damage. I usually use ½” or ¾” PVC conduit when needed for that protection. Don’t have to bond plastic, as you would if it were metal conduit (EMT, Rigid, etc.)

rrolleston 03-14-2012 08:41 PM

And once you know how much wire you need add some extra to it. Too long and you can cut it off too short and you have to splice it.

SD515 03-14-2012 08:45 PM

Quote:

Originally Posted by rrolleston (Post 877775)
keep forgetting that individual wires in conduit can be on 60 amp breaker.

Individual #6 THHN/THWN-2 Cu in conduit can be on a 70A breaker. #6 Cu in NM cable can be on a 60A breaker.

Ravenworks 03-14-2012 08:52 PM

You should rent a Mini- excavator with a 8" bucket and do it all in one hole. The price difference between the two is little.

TheCowGod 03-14-2012 09:46 PM

Quote:

Originally Posted by Ravenworks (Post 877801)
You should rent a Mini- excavator with a 8" bucket and do it all in one hole. The price difference between the two is little.

Hmm, at least at my local Sunbelt, the 24" trencher is $135/day and the smallest mini excavator, a 2000 lb unit (Bobcat 316) is $235. Is there a smaller/cheaper option I'm not aware of? I don't know much about excavators. Thanks.

Dan

Missouri Bound 03-14-2012 09:53 PM

See if they have a "Dingo" available. It's basically a walk-behind / ride on that takes various attachments....back hoe, blade, bucket. Versatile and may be more in your price range.

Ravenworks 03-14-2012 10:22 PM

Quote:

Originally Posted by TheCowGod (Post 877846)
Hmm, at least at my local Sunbelt, the 24" trencher is $135/day and the smallest mini excavator, a 2000 lb unit (Bobcat 316) is $235. Is there a smaller/cheaper option I'm not aware of? I don't know much about excavators. Thanks.

Dan

wow that is high, someone said something about a dingo,it;s up you.
I don't know what your yard looks like,but I was thinking about footprint.
A lot of times when I do jobs I will lay down plywood, put my spoil pile onto, when it's said and done you would hardly know I was there.
If you have a nice yard you need to see the big picture,think about when the pipe is in the ground what comes next.
When I did my under drains at my house I laid 500' of pipe 30" deep and tamped the dirt every foot with a helper and was drinking beer in 8 hours,but I've been doing it 30 years
Maybe you can do a couple of things around the house with the excavator or ask a neighbour if they need something done. Around here if you get the machine on a Sat it goes back on Mon. Split the rental.

TheCowGod 03-15-2012 04:49 PM

1 Attachment(s)
Quote:

Originally Posted by Ravenworks (Post 877880)
wow that is high, someone said something about a dingo,it;s up you.
I don't know what your yard looks like,but I was thinking about footprint.
A lot of times when I do jobs I will lay down plywood, put my spoil pile onto, when it's said and done you would hardly know I was there.
If you have a nice yard you need to see the big picture,think about when the pipe is in the ground what comes next.
When I did my under drains at my house I laid 500' of pipe 30" deep and tamped the dirt every foot with a helper and was drinking beer in 8 hours,but I've been doing it 30 years
Maybe you can do a couple of things around the house with the excavator or ask a neighbour if they need something done. Around here if you get the machine on a Sat it goes back on Mon. Split the rental.

Looks like a Dingo is $200/day, and then a trencher attachment is $64 more. I do plan to put in a fence soon, and so I'll have a bunch of post holes to dig, but adding an auger attachment would be ANOTHER $64, totaling around $330. Looks like it's still cheaper to get the dedicated $135 24" trencher, and then later when I work on the fence, splurge and rent the tow-behind one-man auger for $98/day.

Ah well, thanks for the tips -- I'd never heard of a Dingo and it's definitely good to be aware of it. I plan to put in a concrete sidewalk to this new workshop down the road, and the Dingo may be useful at that point.

I started digging the first, shallower trench (the 8" deep, 37' long trench for the drainage extension, see attached picture), and man, just removing the sod wore me out. I'm a sheltered computer programmer who isn't used to doing manual labor out in that big blue room outside of my front door :) I was trying to avoid the expense of the trencher, but when I think about how tiring it was doing this (relatively minor) work, and how much harder it'll be digging 18-20" down for the other trench for the electrical conduit, I think I may have to bite the bullet. Ah well.

Sunbelt's closed Saturday and Sunday, so if I rent it tomorrow, I get it till Monday for one day's price. How ironic, since (from what I've read) it'll get these trenches dug in like 20 minutes :)

Dan

jbfan 03-15-2012 04:56 PM

Theres always the HD parking lot!!!!

Just kidding.

I had a customer hire a lawn guy to trench his yard for the shed.
It took him longer to unload and set up then to dig the trench.


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