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Old 02-17-2010, 11:57 AM   #1
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Subpanel supply cable and other issues..


Hey guys,

I'm planning on taking on a decent sized project in about a month when the ground starts to thaw out. My girlfriend's father has a detached garage and another outbuilding right next to it, and they're both fed by the same 120V 20A line which was buried underground and installed before they bough the house. To add to that, that line doesn't even have its own circuit breaker in the house, it's attached to the same line that their kitchen and bathroom are on. Needless to say, they are tripping that circuit way too much.

What we want to do when the ground thaws is dig a trench to the garage, and run a 240V line from the main load center in the house and install a 100A "subpanel" in the garage.

I've installed a few subpanels before, but never of this magnitude. I've only done one up to 50 amps before, so I'm not experienced with this much amperage and wire sizing for it.

I'd like to use type XHHW 1/0 aluminum clad wire, ran inside PVC pipe for the outdoor parts of the run. Would this wire be acceptible, or should I consider going up to 2/0? Also, I'm afraid with wire sizes that big that I might have problems fitting the wire into a 100A QO breaker, as well as fitting the neutral wire into the grounding/nuetral bar in the main load center. I noticed that on the right side of where the main common neutral from the street comes in, there's a large unused lug that connects to the main neutral. Could I connect the neutral for a subpanel feed at that spot?

Thanks for any and all advice!

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Old 02-17-2010, 12:02 PM   #2
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Subpanel supply cable and other issues..


Here's a chart from the NEC that shows wire size limitations. How long is the run? You may have to account for voltage drop as well if it's long.

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Old 02-17-2010, 12:05 PM   #3
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Subpanel supply cable and other issues..


I don't think the run will be longer than 100 feet, but I haven't measured it yet. I'd have no problem upsizing the wire to 2/0 aluminum, my only concern is the wire fitting into the lugs in the main load center.
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Old 02-17-2010, 12:32 PM   #4
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Subpanel supply cable and other issues..


Try this calculator. http://www.electrician2.com/calculat...r_initial.html

Make sure you search this forum for sub panels/unattached structures. There is much more to this than running a 220 volt line out to a building, alot more.
Yes, the lug on the neutral bar is fine for landing the 100 amp feeder neutral. And that existing 120 volt circuit that is running out to the building now, must go.
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Old 02-17-2010, 01:16 PM   #5
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Subpanel supply cable and other issues..


Quote:
Originally Posted by J. V. View Post
Try this calculator. http://www.electrician2.com/calculat...r_initial.html

Make sure you search this forum for sub panels/unattached structures. There is much more to this than running a 220 volt line out to a building, alot more.
Yes, the lug on the neutral bar is fine for landing the 100 amp feeder neutral. And that existing 120 volt circuit that is running out to the building now, must go.
Yes, I absolutely plan to get rid of the existing 120V line.

I realize there is even more to this than I asked about, thanks for the heads up on the existing threads!
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Old 02-18-2010, 07:09 PM   #6
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Subpanel supply cable and other issues..


Does anyone have a recommendation of cable type for the run to the detached garage?

I originally thought I would OK running XHHW metal clad cable in PVC conduit but after reading some threads on here, I'm not sure that would work. Apparently even the best conduit can be flooded with water?

So I'm kinda lost as to which cable type to choose for the feeder line. I'm all but certain I'll be using either 1/0 or 2/0 aluminum, but I don't know what type.

Any advice?
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Old 02-18-2010, 07:15 PM   #7
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Subpanel supply cable and other issues..


Any wire run in conduit/underground must be wet rated
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Old 02-18-2010, 11:32 PM   #8
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Subpanel supply cable and other issues..


Well, I guess that rules out the metal-clad cable then.

Would URD quadruplex cable be appropriate? According to a dealer I found, I could use #2 aluminum URD in conduit (it says "in duct", I'm assuming that means conduit?) and be fine up to 115 amps? I figured I'd have to go up to 1/0 or 2/0 but not according to the site I found. Is this because URD is technically not NM-B stlye cable and so it counts as seperate conductors in conduit?

http://customcable.thomasnet.com/vie...-direct-burial

Also, is standard white schedule 40 PVC pipe acceptible for conduit usage? I'd prefer to use that if I could, it seems as thoguh it'd be easuer to make the proper bends and what not. The only reason I could think of why it wouldn't be okay is because it's not gray in color, because electrical PVC conduit is also schedule 40, right?

Thanks for all your help!
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Old 02-19-2010, 05:05 AM   #9
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Subpanel supply cable and other issues..


You can not use the white pvc. It must be electrical conduit.

You will have a hard time pulling urd inside conduit.
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Old 02-19-2010, 09:26 AM   #10
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#2 Alum will not work for 100a
You need to go by the NEC chart that has been posted
Seems for single wires you need #1 & for cable you need 1/0

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