100 amp subpanel in detached garage/apt

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  #1  
Old 08-30-07, 08:15 PM
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100 amp subpanel in detached garage/apt

This forum is fantastic. I appreciate the time all the experts contribute.

I am building a detached garage/apt and want to confirm my wiring plan. From searching other posts I think I know what I need to do but am looking for confirmation.

My run from my main 200 amp panel to the garage panel is approximately 80'. My garage is new construction and will be grounded with 2 ground rods and to the foundation rebar. There is no metal connection between the garage and the house. I am planning to install a 100 amp subpanel and feeding it with 3-#2 copper cables (2 hots and a neutral) in a 2" diameter pvc conduit.

I sure would appreciate if someone who knows more than me could confirm that my plan is safe and meets code.

Many thanks in advance,
B W Swede
 
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  #2  
Old 08-30-07, 10:57 PM
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Smile

I am a novice and do not feel qualified to respond to any de-tached sub-panel questions. I would love to "field" a couple of things for practice though (lol) if I may until the pro's get here. I'm sure you know they are going to ask you why you don't run 4 wires instead of three.(perhaps in the future you will have a need perhaps for a metal connection of some sort) You might ask if this includes things like low voltage wiring etc.!! Anway if you run two hots and a neutral I have read many times here that the neutral and ground wires at your sub are to be married. If you run two hots, a neutral and a ground then they are isolated from one another at the sub. As far as voltage drop I'm pretty certain there is no issue until you get over a 100feet. (I could be wrong) I'm pretty sure they will recommend a main breaker panel. In other words you'll have a 100 A breaker in both the main panel and in the sub. If you have 6 or less breakers in the sub (double pole are considered as one) then I thought I read a main breaker in the sub is not required , but recommended. If there is no main breaker in the sub (if your panel is a main lug only) then I believe but am not certain that a disconnect at the location of the de-tached building would have to be made available. I'm guessing you'l get 10 points for your ground rod/ re-bar application. I'm also guessing you'll be asked what kind of loads you will have a the de-tached building and that you'll be reminded to have some GFI protection. Well let's see what responses there will be. This is so exciting(lol) There are so many safety issues. That's what its all about doing it right and safety, safety and more safety. Good luck
 
  #3  
Old 08-31-07, 04:20 AM
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Spend the extra money and run a ground wire. You won't regret it and your connection will be more flexible for the future.
 
  #4  
Old 08-31-07, 04:21 AM
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subpanel

I'd wire the detached garage the same as any feed out of the main panel. That is: running the two hots/neutral along with a ground wire from the main panel to the subpanel.
 
  #5  
Old 08-31-07, 04:45 AM
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Sidecutter's pretty much right. I'm not sure if he's telling you to bond the neutral and the ground in the subpanel or not. You are NOT to bond them at the subpanel. A ground wire from the main panel would be a good idea eventhough you have the groundrod as well. I would run #3 copper wires in at least an 1 1/2 PVC conduit, although the bigger you go, the easier the pull will be. Another tip would be to install a pullwire as you install the PVC. Reason being that if you have a hard time fishing it later you'll have to dig the pipe up. Use a #12 or #10 for a pull wire, NOT string. String can burn its way through the PVC and your wires will get stuck. You may want to purchase some wire lube too for greater ease of pulling (although I've used dish soap in a pinch). Sidecutter was also right about the GFCI protection. The lights and a designated (single recepticle) outlet for a garage door do not have to be protected, but general purpose outlets do. If the walls are open the cable drops will have to be in a protected raceway. If you're building an apartment above, that can be treated as a dwelling unit.
 
  #6  
Old 08-31-07, 05:47 AM
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If you run a three wire feed (no ground) you MUST bond the ground and neutral at the sub panel. If you run a four wire feed, then you DO NOT bond the neutral and ground at the sub panel.
 
  #7  
Old 08-31-07, 07:38 AM
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Guys (and gals?),

Thanks for all the great feedback. From the responses I gather that I should include a ground wire in with the 2 hots and a neutral, which I will do.

Brewcityc said that he would use #3 copper wire. Just to confirm, can I run the 100 amp panel for that distance with #3's?

What size should my ground wire be? (I think I read in the archives that the ground can be smaller than the feeds.)

Thanks again for all the great information and suggestions,
B W Swede
 
  #8  
Old 08-31-07, 08:04 AM
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Cool

Why not just run the same size neutral wire as the hots (whatever that winds up being). I believe a minor calculations would not have to be made if you reduce. If the difference in price would not be that great I would go with the same size as my hots!! Just my opinion.
 
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Old 08-31-07, 08:59 AM
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> Just to confirm, can I run the 100 amp panel for that distance with #3's?

Yes, the voltage drop using #3 copper, 100A, 80' is less than 1.6% which is well within the code recommended 3% limit; also, the 75 ampacity of #3 copper is 100A, so you're good on both fronts. You may also want to consider using #1 aluminum as it is about the half the price of copper. I agree with your 2" conduit; it's a few dollars more, but pulls so much easier.

> What size should my ground wire be?

#8 if copper, #6 if aluminum

> will be grounded with 2 ground rods and to the foundation rebar.

These should be installed with bare #6 copper wire.
 
  #10  
Old 08-31-07, 09:27 AM
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Talking

One should have coffee first when responding at 6 am. I read the neutral wire into your ground wire question. oops
 
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