Power to the shed

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Old 09-03-11, 02:57 AM
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Power to the shed

My questions first:

1) Does the current NEC still recommend 18" for PVC and 6" for metal with buried conduit?

2) For a 70-foot run attached to a 40A double breaker, would 8/4 be sufficient, or is 6/4 a better idea?

3) What's the best conduit size, for possible future expansion and heat dissipation?

4) Why should I use PVC over metal, or vice versa?

Now, a bit of further detail. After I got a price quote of $15K for a standby generator install, given my requirements, I put that off. With a bit of prodding from Irene, I recently bought a portable generator (power literally came back 5 minutes after I powered it up for the first time).

I really disliked the whole mess of running extension cords everywhere, so I looked around for a way to hook the generator into the whole house. What I found is a GenTran R3020 "power center", which sits outside between the incoming utility power and the main breaker panel. It has interlocked breakers to switch between 200A utility and 30A generator power, but also 12 breaker positions, for additional loads.

Now, power in my house is a bit strange, because the breaker panel was installed on the opposite side of the house from the service entrance (modular home that was wired on the assembly floor), so there's a 50-foot run of a huge cable from the meter outside to the panel inside. This made getting power to the shed annoying enough to put off for a few years. But putting this power center in will allow me to run electricity from it to the shed, knocking the distance down from about 120 feet to just 70 feet.

A licensed electrician will be doing the power center install, since that will be faster, and a numbered seal on the service entrance will need to be broken to access the power lugs (and me doing the breaking is likely to piss someone off). There's a main breaker right there, so the electrician won't even need the power company to kill the juice, which can only speed things up.

The rest, I'll be doing myself, hence the questions. I appreciate any answers, or suggestions if it seems I'm missing something.
 
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Old 09-03-11, 11:00 AM
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1. Electrical codes do not recommend, they mandate. That stated, the depth from the surface of the ground to the top of the conduit depends upon both the type of conduit AND where it is laid. EMT (thinwall) is not allowable for underground usage as it will rust out in a short time. Rigid or intermediate (RMC, IMC) may be as shallow as 6 inches but if it is under a public street or driveway it needs to be a minimum of 24 inches. Residential driveways or parking areas used only for dwelling-related purposes may be 18 inches.

2. The #8 conductors (copper) are sufficient. The equipment ground need only be #10 copper.

3. You may run as large a conduit as you desire. I have no idea what your future expansion needs may be. Your minimum conduit size is 3/4 inch for four #8 conductors. Remember that you may have only a maximum of four 90 degree bends between access points.

4. PVC conduit doesn't rust and is easier to work than is either RMC or IMC. Where it comes out of the ground I prefer to use IMC or RMC if it is in any way going to be "subject to damage" such as being hit by a string trimmer or something although schedule 80 PVC or some kind of mechanical guarding method is also acceptable.


If your meter feeds a circuit breaker before the cable running to the circuit breaker panel on the opposite side of the house then it should not be necessary to remove the meter to install the Gen-Tran power center.
 
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Old 09-03-11, 12:32 PM
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For a 70-foot run attached to a 40A double breaker, would 8/4 be sufficient, or is 6/4 a better idea?
Assuming you are referring to a cable assembly such as NM-B (aka Romex), the answer would be no. Type NM-B cable is not rated for any use outdoors and that includes in conduit underground which is a wet area. Type NM-B cable is for dry areas such as inside a house. I would recommend Type THWN or XHHW conductors in your conduit.
 
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Old 09-03-11, 05:22 PM
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Originally Posted by Furd View Post
1. Electrical codes do not recommend, they mandate.
Just want to correct a misconception on your part. The NEC is mandatory only when local building codes adopt it, and even then they may modify it.
 
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Old 09-03-11, 05:27 PM
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Originally Posted by CasualJoe View Post
Assuming you are referring to a cable assembly such as NM-B (aka Romex), the answer would be no.
I can't see why you'd make such an assumption, given that I referred only to gauge/count in the question. I didn't ask if normal indoor cabling would be OK to use in the conduit. I was inquiring about a suitable gauge for a specific current over a specific run length.
 
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Old 09-03-11, 05:54 PM
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Originally Posted by Thanny View Post
I can't see why you'd make such an assumption, given that I referred only to gauge/count in the question. I didn't ask if normal indoor cabling would be OK to use in the conduit. I was inquiring about a suitable gauge for a specific current over a specific run length.
I simply made that assumption based on the way you referred to the gauge/count. That terminology typically refers to a cable assembly.

I appreciate any answers, or suggestions if it seems I'm missing something.
You asked for suggestions, that was mine.
 
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Old 09-03-11, 06:12 PM
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Originally Posted by Thanny View Post
Just want to correct a misconception on your part. The NEC is mandatory only when local building codes adopt it, and even then they may modify it.
There is no misconception on my part, I'm usually the one that states that ANY model code, be it electrical, mechanical, plumbing, building or whatever is only advisory unless/until enacted into law by a local or regional political subdivision. The way that you worded your question implied that codes are generally advisory only when in fact they are MANDATORY wherever they are enacted into law. And yes, that political subdivision has the power to add to or delete from the model code via the enabling legislation.
 
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