Garage needs electric

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  #1  
Old 11-28-04, 05:04 PM
Heatmiser
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Garage needs electric

I want to run electric into my detached 2 car garage. I buried 3/4" PVC conduit and ran 10/3 wire from house into garage. Total distance from main panel to subpanel is 75ft. I want 2 circuits, 1 for lighting(3-4 ceiling mounted flourescent fixtures) and 1 for receptacles(2, maybe 3 outlets). What type/size/capacity subpanel should I buy?
Jim
 
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  #2  
Old 11-28-04, 06:23 PM
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It sounds like you purchased a cable assembly (three wires plus a ground in a cable). This implies NM or UF cable. This would be incorrect. For conduit you need individual conductors.

How deep did you bury the conduit, and what type of conduit is it? Let's hope that it is deep enough and the right type.

If all you want is two circuits for lighting and receptacles, then I recommend a multi wire circuit. That would mean a 20 amp dual breaker (240 volts) and the proper cable. For that distance I would use 10 gauge wire.

If you want a a sub panel for future expansion then I would run larger wire now. You can always have a multi wire circuit at 20 amps for now and upgrade to a sub panel later. Or you could put in the sub panel now. However, a sub panel means a grounding system at the garage and more expense plus more work.
 
  #3  
Old 11-29-04, 06:15 PM
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I buried 3/4" schedule 40 PVC conduit at roughly 20" under earth/grass.
The conduit run is continous, from inside basement to inside garage with a junction box at each end.
Inside the conduit is NM-B 10/3 wire with ground.

Are you saying i have to pull the wire out, remove the jacket material and run the individual wires through the conduit loose? If so, is this so individual wires can be pulled in/out later if necessary?

Inside the basement, from the junction box to the main panel(not tied in), is NM-B 10/3 with ground.
 
  #4  
Old 11-29-04, 06:26 PM
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The problem is that NM-B isn't rated for underground use, even in conduit. You can leave it there and wait until it fails, or you could remove it now (if you can even get it out) and run dual-rated THHN/THWN instead. The NM-B will fail eventually. And if you installed it as you were laying the conduit piece by piece, it will fail sooner rather than later.

You boxed yourself into a corner when you selected #10 wire. That limits you to a 30-amp breaker. That's enough for simple tools, but not enough for a freezer and more sophisticated tools. But since you're going to pull it out anyway, you get a chance to start over again with your design. You can't design this on the basis of having two or three receptacles. Receptacles use zero power. You need to design it based on what you plan to plug into those receptacles. Most detached garages are wired with #6 copper wire and a 60-amp double-pole breaker.
 
  #5  
Old 11-29-04, 07:44 PM
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OK. I can remove the 10/3 NM-B from the conduit and run THNN/THWN #10 wires. It's just a very wide "U" shaped run of conduit coming up to LB boxes where it enters house/garage.

My primary interest is lighting, the receptacles are for convenience(and assumably code requirements when bringing power into a detached garage) things such as radio, simple tools if any, but definitly no heavy and/or fixed appliances now or in the future. 30A should b enough for my foreseeable needs, i'm going to stick with 30A and #10 wire.

As for the sub panel, I need a floating neutral bus as well as grounded bus bonded to panel? With only two circuits running out of sub panel, can I wire both circuits bare grounds to the #10 bare ground (wrapped on a green screw into panel back) or must it be a fixed grounded bus bar?
 

Last edited by Heatmiser; 11-29-04 at 08:37 PM. Reason: info add
  #6  
Old 11-29-04, 10:39 PM
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There are a lot of options. You specifically said you wanted a subpanel, so we'll go with it. Run a black, red, white, and ground #10 copper wires to the subpanel from a 30-amp double-pole breaker in the main panel. Then you can run as many circuits as you want from the subpanel (you said you only needed two, but you can have more if you want). The grounding wires and neutral wires in the subpanel must remain electrically isolated (throw away the green bonding screw that comes with the subpanel, and add a grounding bar kit if the subpanel doesn't come with one). For the branch circuits in the garage, I would suggest using #12 cable off 20-amp single-pole breakers in the subpanel. You'll need a grounding rod for the subpanel (if the garage is detached). You won't be making wire-to-wire connections in the subpanel. You'll be attaching the feeder to the appropriate lugs, and the branch circuits to the breakers, neutral bar and grounding bar.

Note that this is a fairly sophisticated DIY project, with lots of ways to screw it up if you're not fully prepared. Be careful.
 
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Old 11-30-04, 01:36 PM
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Yes 2 circuits. The sub box i have has only 2 slots available for breakers, it has a floating neutral bus bar(seperated with a plastic sheet), but no grounding bus.

I thought it was suggested that all grounding take place back at the main panel, but I can install a rod if need be. If this is the case, then the #10 ground wire coming from the main panel will tie into the ground bus bar(in sub box, along with 2 circuit gnds) which will also tie into a rod sunk in the earth.

Yes, it's a bit more involved than what one might envision it to be, but I have time to do research.
 
  #8  
Old 11-30-04, 01:43 PM
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You confuse system grounding with equipment grounding. The ground rod is required for the former, and the grounding wire between panels for the latter. The two grounding systems serve different purposes.
 
  #9  
Old 12-01-04, 07:05 PM
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Hi John, My subpanel saga continues.......................
I installed an 8' copper grounding rod down to about 4" left sticking out of grass. Connected a #10 ground wire to it and ran it into the sub panel.

The grounding connections are........... the 2 circuit ground wires, grounding rod, and ground wire from main panel all connect to grounding bus bar which is bonded to the sub panel box.

I am having trouble finding a small grounding bus bar for my few circuits, the Home depot only has bus bars for 10 or more circuits(too big).

Circuit neutrals(white) connect to the floating neutral bus bar which goes(floats) back to main panel where it is tied in.

Under no circumstances can the neutrals(white) and grounds(bare) be put on the same bus bar in the sub panel like you can sometimes find in the main panel. I ask this since the sub box came with a flat fork that can be used for bonding the neutral bus to the sub box.
Jim
 
  #10  
Old 12-01-04, 09:26 PM
SkyKing
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John, why do neutral and ground have to be isolate in a subpanel?

HeatMiser:

If I were you, I would run something bigger than #10. It is a good idea to run #6 or even #4. As far as a load, if you expect to use 30 amps worth of draw, you should probably go to the next biggest wire size due to the voltage drop over 75ft you speak of.

You can still hook up your 30 amp disconnect to the larger gauge wire. Less headache, more room to grow if you ever want to.
 
  #11  
Old 12-02-04, 04:40 AM
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Heatmiser,

Why are you insisting on using what you happen to have available? Small sub panels are not expensive and will increase your flexibility in the future.

SkyKing,

It is necessary to isolate the ground and return at a sub panel to avoid a voltage from developing on the ground bar (and wires) at the sub panel.

If you connect the return and ground wires at the sub panel and at the main panel, then the return current will flow through both wires. Because of voltage drop and because they are joined there will be some voltage other than 0 volts at the sub panel return and ground bars. This is not good, you want the ground to have no voltage on it.
 
  #12  
Old 12-02-04, 06:53 AM
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The entire 8 feet of the grounding rod needs to be underground. Pound it in those last four inches.

There are conditions in which the neutral and ground can (and must) be interconnected. You must only run three (not four) wires between the buildings, and there can be no other metallic paths (no pipes, no phone lines, no coax, no nothing) between the buildings. But this is not the preferred solution. Surely you can figure out how to separate them.
 
  #13  
Old 12-02-04, 03:20 PM
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Let there be light!!!!! yippie!!!!!!!!!!

Using whats available may of been somewhat foolish since the wire is the only real cost and more is always better, but that part is done now. More concentrated planning would of helped too. My only next upgrade will be to a newer house with a bigger garage. I'm not finished running the 2 circuits but now i can work in light instead of by flashlight, these short days don't help either and snow is on the way.

Thanks to all
 
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