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New Panel & Feeder Cable for Accessory Building - Novice Questions

New Panel & Feeder Cable for Accessory Building - Novice Questions

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  #1  
Old 08-05-08, 08:11 PM
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New Panel & Feeder Cable for Accessory Building - Novice Questions

I am trying to upgrade the wiring to an outbuilding at our new home in New York. The previous owner had brought a 10/2 UF overground from the house's panel to a four plug receptacle in the outbuilding. I want to bury the line and add a few circuits and a subpanel for them with a feeder cutoff. I have a very rocky path to the house and am trying to minimize the depth of burial required. I will have 5 - 15 amp circuits (GFI outdoor outlet, AC - 15 Amp required, interior outlets, computer and lights). I have run #10 wire for all circuits.

What I would like to know is if I can use a 120 volt / 30 Amp feeder line? I had been hoping for 30 AMP b/c I understand that if I use a GFI breaker in the house for the feeder I might only need to go down 12" with UF. Am I correct on the code for this burial? Can I use the preexisting 10/2 UF as my feeder? Is 30 amp the correct size for this service panel or should I increase it?

Any other comments? Thanks for any thoughts.

Ben
 
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  #2  
Old 08-05-08, 08:31 PM
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I will have 5 - 15 amp circuits (GFI outdoor outlet, AC - 15 Amp required, interior outlets, computer and lights).
If you need 5 circuits, you'll certainly need to run a subpanel. I'd question though if you really need that many circuits for what you described. You may be able to get away with only two 20A circuits (presuming you're not anticipating much future expansion)

I have run #10 wire for all circuits.
That's fine, but understand that it's a bit overkill. 20A is the most you can run to general purpose receptacles and lighting, and 20A can be run with 12ga wire (unless you're talking about long distances). The 10ga is fine as you can always upsize, but you can NOT connect these to a 30A breaker.

What I would like to know is if I can use a 120 volt / 30 Amp feeder line?
You can. With your current setup you can either run 4 wires (hot/hot/neutral/ground) so you can have 30A @ 240v, or you could run 3 wires (hot/neutral/ground) and have 30A @ 120v.

I would consider running a 20A Multi-Wire Branch Circuit (MWBC). This will basically give you two 20A circuits at your building using 12/3 + ground wire (again presuming you're not talking long distances). It's quick, simple, and inexpensive. It will also give you more power than you could get from a 30A 10/2 circuit with a subpanel.

If on the other hand you are planning future upgrades (shop tools, etc), you should consider a more significant service, maybe 60A.

if I use a GFI breaker in the house for the feeder I might only need to go down 12" with UF.
I believe this is true - someone else will confirm. In the above case, you'd use a 20A dual pole GFI breaker to protect the wire, and use 12/3 UF and bury it at least 12" deep, unless you're going under a driveway).

Hope this helps!
 
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Old 08-05-08, 08:39 PM
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The 15/ 20 amp 120 volt circuit for underground run protected by GFCI receptale or Breaker. Yes you can bury shallow as 12 inches with the UF cable only or in conduit that can be 12 inches.

But for myself I rather keep it at 18 inches or deeper that just me

Merci,Marc
 
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Old 08-05-08, 08:47 PM
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Definitely use PVC conduit in that rocky ground and think about putting two conduits down at the same time. One for cable, telephone, etc. and one for the power. It is cheap and you won't be sorry! 3/4 or 1" for the power and 1/2 or 3/4 for the others.

Easy to work with and makes a nice installation.
 
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Old 08-06-08, 07:17 AM
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These responses are great.

FYI: The run is ~ 90 feet. The room is 200 sq ft and will used for an office/auntuncle holiday overflow. The #10 was overkill. Had I known I would neary blow a gasket getting these wires into the outlet boxes I would have not done this. :-0 My 5 Circuit scenario is likely overkill as well.

I don't plan future expansion of this building. Based on the needs described it sounds like the 2 - 20 Amp Circuits (MWBC or not) would be plenty. Do you suggest keeping the AC on a separate circuit and bundling all of the other needs (lights, computer,outlets)? Or would you suggest another configuration?

If I went the 2 - 20 Amp route would I forego the need for a subpanel and instead have the breakers back in the house? Further, what would be required to connect the circuits up with the various needs in the outbuilding, simply junction boxes? Would the MWBC choice require different connection needs in the outbuilding?

Regarding the ditch I am glad to hear 12 inches will be OK. In some spots I'd have to break up a rock the size of my house to get below that, so I will just fill above it to get the 12". Possibly I will run this section in conduit to be safe. In other spots I will be going to 18" assuming the Ditchwitch I will be renting on Friday will be ok with the roots and smaller rocks. I will definately be using conduit for the low voltage (phone, internet and cable) and conduit/LB fitting to come from underground to enter the house and outbuilding.

If I run the 2 - 20 amp circuits with the 12/2 wires are there seperation requirements for the trench (e.g must be seperated by 6")? Is there a preference to fill with sand, gravel or the dirt given my situation?

Thanks again for all of the responses!

Ben
 
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Old 08-06-08, 09:00 AM
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Ultimately if you're looking for a shallow installation, you can do a 6" depth if you use rigid metal conduit or intermediate metal conduit. Granted this is more expensive and somewhat difficult to work with, but could solve your problem if the soil is exceptionally rocky.

Remember with the 12" trench, you are allowed no more than a single (or multiwire) 20A GFCI protected circuit. This means it cannot supply a subpanel. With the rigid metal option, there is no restriction on amperage or voltage.

Had I known I would neary blow a gasket getting these wires into the outlet boxes I would have not done this. :-0 My 5 Circuit scenario is likely overkill as well.
If the boxes were really that hard to work with, then you probably have boxes that are too small to accommodate #10 wire. This is a code called box fill. If you look at a typical plastic box, it will have info stamped like #14 15, #12 10, #10 8 or something like that inside the box. This means you can have no more than 8 #10 wires entering the box -- all of the bare grounds together count as one and the receptacle or switch counts as two.

Is there a preference to fill with sand, gravel or the dirt given my situation?
Yes, in areas with rocky soil the code requires a "clean fill" for a few inches around the cable or conduit. This could be sand or smooth rock like pea gravel.
 
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Old 08-06-08, 11:33 AM
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Based on the needs described it sounds like the 2 - 20 Amp Circuits (MWBC or not) would be plenty.
Note that you'll need to use a MWBC if you want two circuits. Code only allows one circuit to an outbuilding (which a MWBC falls under). Using two 12/2 wires is not appropriate.

Do you suggest keeping the AC on a separate circuit and bundling all of the other needs (lights, computer,outlets)?
That's the way I would do it. You could use a little space heater in the winter off the AC receptacle if you wanted.

If I went the 2 - 20 Amp route would I forego the need for a subpanel and instead have the breakers back in the house?
Correct, you'd have a double pole, 20A GFI breaker in the house. You'd also need a disconnect means in the outbuilding. Most people would accomplish this with a double-pole 20A switch immediate inside the outbuilding which you could use to shut off all the power to the building. You could also use an air conditioning disconnect for this so it's not as easy to accidentally turn off, but that's your call. Most people would install the switch either lower or higher than a normal light switch so it's not easily confused.
 
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Old 08-07-08, 10:41 AM
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Thanks.

Let me get this straight about the boxes. The smallest box I used was a metal 3" x 2" x 2.5" box. I have a book that shows that for this size box I can have 5 #10 wires. I used 10/3. In a number of cases the box held the wires for an outlet served there and a pass through for the dedicated computer outlet down the line. Am I doing the math correctly: 1 set of grounds, 3 wires (10/3) from the panel (Black, red (for the computer), white and ground) and another set of 3 wires to the outlets down the line? Is that 7 or 5 (assuming the outlet is 2)? I might have to do some replacing it seems.

Just to be safe I have set up for an electricition to do the preliminary walk through and the final connection to the panel. So that should help keep the do-overs to a minimum from this point forward.

Ben
 
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Old 08-07-08, 11:08 AM
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Originally Posted by Benny20 View Post
Am I doing the math correctly: 1 set of grounds, 3 wires (10/3) from the panel (Black, red (for the computer), white and ground) and another set of 3 wires to the outlets down the line? Is that 7 or 5 (assuming the outlet is 2)? I might have to do some replacing it seems.
I count 9, which means a box of 22.5 cu in or larger. 2 black, 2 red, 2 white, 1 grounds, 2 receptacle. That requires a minimum box size of 4" x 4" x 2-1/8".

If you want to do the "long form" calculation, each #10 wire requires 2.5 cubic inches of box space. Take your wire count, multiply by 2.5 and that must be less than the volume of the box (height x width x depth).

(Black, red (for the computer), white and ground)
So the computer is already on a multiwire circuit?
 
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Old 08-07-08, 12:14 PM
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Well its better to know now than later. (If there is time enough to do it twice, there is time enough to do it right the first time.)

Can I get away with ganging another 2 x 3 x 2.5 box to the side? That will make it 4 x 3 x 2.5 or 28 cubic inches. This is more than the 22.5 cubic inches required by the configuration described below.

And yes the computer is on the multi wire circuit.

I have made the decision to keep the panel and the multi circuit arrangement, and run a feeder to the sub panel. I would have more than 8 devices on the non-AC circuit, and the wiring has already been done to the panel. I talked to a local inspector and he said that as long as the wire is buried at 18" in rigid nonmetallic conduit for most of the run, and is encased in cement over the two rocks that are less than 18" below the surface, he is likely to not "bust my b$%#^". Considering that this is a residential area (my back yard) he thought that would be fine.

So now for the feeder wire. I had bought 4 - 15 amp 1/2" breakers for the panel and 1 - 20 amp breaker for the AC (which matches the outlet although the AC calls for only 15 amps). If I left the 5 circuits (albeit underutilized) could I get away with a # 8/3 UF wire? Is the breaker width and amps OK? I am not clear when one would use a 1/2" vs a 1" breaker. What are the code requirements for panel size and feeder amps based on my needs? I have bought a 125 amp subpanel which I assume is OK, but am not clear about the size of the breaker I should use as the power supply cutoff in the outbulding. Asked another way, what amount of amps should I bring over? Should it be 120 of 240 volts?

Jeez I don't know what I would have done without this forum.

Ben
 
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