Couple of detached building subpanel questions

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  #1  
Old 11-02-12, 09:08 AM
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Couple of detached building subpanel questions

Dad is preparing to build a detached workshop. Since I already have one we're basing much of his electrical needs on what I have, which is 100A 240V single phase service.

His workshop will be right next to the house, but unfortunately may as well be across a field as far as running the electric service is concerned. I'm pessimistically planning on 100' distance. I'm also planning on 100A service. Will 3AWG conductors suffice?

Like my shop, I'm planning on going with conduit from the main panel to the subpanel for his shop. Probably 2", will need at least one underground 90 bend to curve around the house, plus the curves for the verticals. Current plan is to trench for PVC. I'm already planning on installing an extra conduit for low voltage for a telephone or other needs like ethernet, and possibly an air line so that he can supply air from a compressor to his daily-driver garage for filling tires and the like, most likely another 2" conduit with a rubber hose pulled through it to be easily replaced if/when necessary. Should we go bigger than 2" for the electric service conduit?

I'm fairly sure that my shop is not completely in code. I think that my ground, instead of to a grounding rod at the shop, crosses the 5' walkway through the in-ground conduit and uses the main panel's ground. In my case, since the main panel is literally 5' from the shop and the subpanel is only about 20' by conduit pathway from the main panel I'm not extremely worried, but for his application I expect that we need to install grounding rods according to code near his subpanel, bonded correctly.

In his new shop I'm planning on just hard-piping all of the circuits. I plan to stub up from the subpanel into a wide junction box at several points, and to then run all of the individual conduits from that junction box. If he leaves the interior unfinished then the cables are protected, and if he finishes it then we have pathways.

Anything I'm missing?
 
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  #2  
Old 11-02-12, 10:52 AM
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Number 3 copper should be fine at 100 feet. I think it is extremely doubtful that your dad will ever come close to a 100 ampere load in his shop unless he has several people all using power tools at the same time.

2-inch PVC conduit would be way overkill in my opinion. 1-1/4 inch schedule 80 is rated for five #3 conductors and you only need three #3 plus a #8 equipment ground. To make the pull easier I would probably use 1-1/2 inch conduit. Remember that you can only have a maximum of four 90 degree bends (or equivalent) between access points. I would recommend using intermediate or rigid steel conduit where it comes out of the ground for protection against damage by string trimmers or other gardening tools although schedule 80 PVC is probably okay. You could also sleeve the schedule 40 with something else for protection at these points. You need to have a minimum of 18 inches of cover over the buried conduit although I prefer 24 inches.

Everything else sounds good but I'm still not fully awake.
 
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Old 11-02-12, 10:56 AM
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I'm also planning on 100A service. Will 3AWG conductors suffice?
Yes.

Should we go bigger than 2" for the electric service conduit?
No need. You could pull 9 or 10 3AWG conductors in a 2" pipe, and you only need 3, plus a smaller ground.

The rest looks good. You're right about the new ground rods, and should have those at your shop as well. Lightening is an equal-opportunity pathfinder.
 
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Old 11-02-12, 12:52 PM
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Thanks.

Right now I'm planning on three bends. Two from the verticals into the ground, and one within the ground. I'll put a junction box or LB-style entrance elbow if I have to curve again over into the workshop through the wall. I had also told him to plan on 24" deep. He'll be wanting it safe to pull vehicles over it.

Right now I'm planning on installing the PVC conduit first, then measuring before buying the cables. I can return any unused individual lengths of conduit, but obviously if I buy cables that are too long I can't exactly return the unused portion.
 
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Old 11-02-12, 01:42 PM
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Wires, not cables. A cable is an assembly of two or more individual wires with an overall jacket.

Also, don't skimp on length. Far better to scrap a couple of feet of wire than to need two feet more. If you buy 39 feet and need 41 feet you eat the 39 feet in addition to buying another 41 feet.
 
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Old 11-02-12, 01:51 PM
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Sorry, I do low-voltage for a living and it's cables all the way... *grin*

I'm not worried about buying five or ten feet too much, I just don't want to buy 100' lengths only to find out that I needed 75' lengths.
 
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Old 11-02-12, 03:25 PM
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It is best practice to use wires not cables in conduit. If you use cable minimum burial depth is 24". If you use wires in cable only need to go 18" deep. Except in special cases schedule 40 is fine. Schedule 80 is not needed.

To save money you could use quadplex aluminum "mobile home" cable.
 
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Old 11-02-12, 03:44 PM
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Yeah, I had already planned on just buying individual wires. I might find a distributor with white and red insulation available on their 3AWG cable, so that I could not have to label it with a different tape for identification.

And I know, no cutting bundles open to use the wires in conduit unless the individual wires are labelled, which they probably aren't.
 
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Old 11-02-12, 06:14 PM
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an air line so that he can supply air from a compressor to his daily-driver garage for filling tires and the like, most likely another 2" conduit with a rubber hose pulled through it to be easily replaced if/when necessary.
I am no expert on this, but I don't think I'd be pulling a rubber hose through conduit for air. Why couldn't you just run a 3/4" PVC or CPVC line underground between buildings for air?
 
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Old 11-02-12, 06:23 PM
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NEVER use PVC (or CPVC) piping for air lines! The compressor oils interact with the plastic and it can explode sending shards of plastic several feet, even when buried two feet deep.

Washington State industrial safety codes specifically prohibit PVC in ANY compressed air system. I suspect that many states or the federal OSHA codes also prohibit PVC usage in compressed air systems.


T-W-X, I suspect that you will find it all but impossible to find #3 wire with an insulation color other than black.
 
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Old 11-02-12, 07:03 PM
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I might find a distributor with white and red insulation available on their 3AWG cable
You seem to be confusing cable with wires. Wires are single conductors. Cables are two or more wires in a sheath.
 
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Old 11-03-12, 05:20 AM
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NEVER use PVC (or CPVC) piping for air lines! The compressor oils interact with the plastic and it can explode sending shards of plastic several feet, even when buried two feet deep.
Well, I said I was not an expert, thanks for that good information, Furd. I have never used PVC for air lines and would prefer copper or galvanized steel pipe, but underground copper joints would have to be silver soldered if I am not mistaken and galvanized steel would require a lot of cutting and threading. What would be the best material for underground air lines in your opinion?

I suspect that you will find it all but impossible to find #3 wire with an insulation color other than black.
Yes, extremely difficult to find colors in that size unless you order them.
 
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Old 11-05-12, 05:48 AM
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Yeah, I do low voltage, we install cables, not wires, so I tend to use the word cables in my vocabulary a lot more than the word wires. I'll make sure that I don't do that when I get a distributor on the phone.

My shop actually has air in PVC, and it's leaking at the glued joints badly enough that it depressurizes from 120PSI in about five minutes. I'm getting ready to replace all of those lines with copper, but I don't have to go underground and there's nowhere for me to need to trench. Dad, on the other hand, will be trenching from the wall at his existing garage to the wall at his shop, so to me it's a no-brainer to add air. Trouble is, no permanent solution is rated to handle the combination of pressure cycling and the stresses of being in the ground, so despite some time looking for a better solution, it seems that the best way to do it is to just run a pathway and then pull an air hose through that pathway.

Since we're in absolutely no hurry I'll see if I can order other colors of #3 copper wire. If not, white electrical tape it is.
 
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Old 11-05-12, 07:13 AM
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Why not galvanized pipe or epoxy coated steel gas pipe for the air?
 
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Old 11-05-12, 10:53 AM
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Mainly because I don't have any pipe threading stuff, and I simply do not know about the long-term durability with the water that will inevitably condensate out of the compressed air. I can replace a hose by just tying a new hose to the old hose with a coupler and pulling the old one out.

For all I know, an epoxied black pipe will last as long or longer that the system will be in service, but I'm just not familiar enough with it.
 
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Old 11-05-12, 11:21 AM
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Furd. I have never used PVC for air lines and would prefer copper or galvanized steel pipe, but underground copper joints would have to be silver soldered if I am not mistaken and galvanized steel would require a lot of cutting and threading. What would be the best material for underground air lines in your opinion?
Some jurisdictions may require that underground potable WATER joints be silver-soldered but many do not. I wouldn't bother with using silver-solder on compressed air piping, even underground. You could use soft temper (flexible) copper and that comes in 60 foot coils and use flared (not compression) fittings without problems.

I like the idea of using a conduit and pulling the air line through. I probably wouldn't use a rubber air hose but instead nylon tubing specifically designed for compressed air usage or else flexible copper. There ARE certain plastics that are designed for use in compressed air systems but you won't find them at the corner hardware store or mega-mart homecenter.
 
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