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Old 11-28-2011, 08:55 PM   #1
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Sub panel wiring and voltage drop questions


Hello,

First of all, let me thank you all for the information I've gotten from this site so far. I done a lot of searching on here and there are obviously some very knowledgeable people here! I've got a few questions regarding adding a subpanel in a garage which is roughly 150 feet (total cable length) from the main building. I want to be as educated as I can be about this before I get quotes from electricians. The main building has a 5 to 10 year old 100amp BR style main panel. I'm planning to put a 100amp sub panel in the garage, and the wire will be direct buried with a trencher.

1) Can I use 2-2-4 aluminum triplex URD (I don't know if that is the correct terminology, that's what I've always called it) direct buried at this distance (150ft) for a 100amp panel? This would seem heavy enough, but I don't know if it would have unacceptable voltage drop at that distance. If I wanted to "future proof" the installation, would running 4/0-4/0-2/0 aluminum instead be enough for a 200amp panel if I ever wanted to upgrade my service?

2) I know the subpanel needs an independent ground run also, (and the neutrals and grounds separated) but what wire type and size should I use for this to be able to directly bury it? Is bare wire acceptable for direct bury or does it need a coating?

3) Will the electrician be able to add "outgoing" neutral and ground lugs on to my BR style main panel, or would this require a new main panel?

4) I plan to protect the cable going in and out of the ground with 2" Sch. 80, and then going directly into the buildings and up to the panels which are on the interior side of the exterior walls of each building. Is this acceptable, or do I need any boxes outside the building as shutoffs or transition points? Is URD fine all the way to the service panels from a code standpoint? What writing does the cable need on it to make it suitable for this application?

Thank you for your time and guidance. I want to be sure the install is done right the first time, but also that I'm not paying for things I don't really need.

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Old 11-29-2011, 12:24 AM   #2
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Sub panel wiring and voltage drop questions


Where are you located? Canada? US?

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Old 11-29-2011, 05:54 AM   #3
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Sub panel wiring and voltage drop questions


A couple of questions.....

1. Why so much current? I have a 'good sized' garage....I'm feeding mine with 50A....more than enough power....the most I pull is if I'm running my compressor AND a table saw at the same time. I have a 100A sub-panel....but it does not mean you have to give it 100A. Just asking since you want to save money....

2. Why not conduit? Since your going to trench it....PVC conduit would give you a bit more protection and you could use cheaper THWN wire.

Regardless of which way you go....a bare ground is fine. That is how I pulled mine...(in conduit)

And, yes....you are right about the ground rods and connection...ground rods (2) at the garage...ground wire connected to your sub panel and main panel....but your neut buss and ground buss NOT connected together in your sub panel.
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Old 11-29-2011, 08:09 AM   #4
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Sub panel wiring and voltage drop questions


I'm in the US - in an area currently under the 2008 code.

Ddawg16, you make a great point that I hadn't realized. So it is perfectly acceptable to have a larger sub panel than the wire will handle, so long as the breaker feeding it is of the correct amps? I guess I had just thought of 100amps as the minimum typical panel size, so I thought I'd run cable capable of 100amps.

I'll have a compressor, saw, and some other power tools, and I'd also like to put in provisions for a welder down the road. In any of those cases, those tools (except the compressor) wouldn't likely run at the same time as any of the others.

The 200amp reasoning was that I actually have a like new scratch and dent 200amp panel sitting around that I already had for a project that never happened. Would it be OK to use the 200amp panel as the sub panel and then just supply it with a smaller wire and smaller breaker? Also, I own several lots around the garage, and I could build on them in the future and want to be able to pull power from the there. Of course, trying to outsmart my "future self" and plan for whatever I might do in the future is impossible and could just be a waste of money!

As for conduit, is the THWN so much less expensive than direct bury that it pays for the conduit and is still less expensive? I just thought not putting it in conduit the whole way would be less expensive. I guess if I put it in a conduit and went with smaller wire for now, I could always pull larger wire if I ever really needed it.

Thanks for the input!
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Old 11-29-2011, 08:29 AM   #5
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Sub panel wiring and voltage drop questions


I would run the conduit (2") as it offers a little more protection and allows for upgrading in the future. Use the 2-2-2-4 Aluminum Mobile Home feeder that is readily available at most home centers and you can use a 90A breaker on the main panel. You can use the scratch and dent 200A panel you have.
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Old 11-29-2011, 01:32 PM   #6
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Sub panel wiring and voltage drop questions


Thanks for the tips. When you recommended the 2-2-2-4, I presume the 4 awg is the ground, and one of the 2awgs is neutral, correct? Question, do I NEED a 2 awg neutral, or do you just recommend it because that is how it commonly is available as mobile home feeder? I just wondered, since I'd been thinking about a 2-2-4 urd with a separate ground, would 4 awg not be enough for the neutral?

I had always thought 2 awg was good for a 100 amp panel, why is it only good for 90 amps? Too much distance? I'm just curious.

Also, back to some of my original questions, do I need any exterior shutoffs for code purposes, or can I go directly to the panels? Also, are the ground and neutral lugs easily added to my BR main panel?


Thank you to everyone who has commented so far, I really appreciate it!
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Old 11-30-2011, 07:52 AM   #7
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Sub panel wiring and voltage drop questions


Also, when it comes to wire, would cable labeled XHHW be suitable for burial if it is in conduit?

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