Getting grounded

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  #1  
Old 12-07-07, 12:38 PM
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Getting grounded

We're running power out to a shed and a garage. These are separate structures both about 10' from the house and each other. Below is a diagram... ignore the dots, just look at the lines and the X's and O's . The X's are the grounding rod locations for the house. The o's are where I'd place the ground rods for the garage. The question is, assuming we would provide grounding electrode system (rods) at the space between the garage and the shed, can we use the same rods for both garage and shed or does shed need its own? They are being fed on different circuits, but it is possible to feed the shed from the garage if that'd make the difference in using the same ground rods for both.

Thanks!

.............|-----|......|-------|
.............|____|...o..|.........|
.........................o..|.........|
|-X--X -------|......|_____|
|...................|
|...................|
|____________|
 
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  #2  
Old 12-07-07, 02:42 PM
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You cannot share grounding rods between separate structures.

Tell us your whole setup. Are you really installing sub panels at both the garage and at the shed?
 
  #3  
Old 12-07-07, 04:24 PM
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OK, I've sort of spread my plans across several posts and we're still figuring out what is the best alternative. I will therefore give the BIG answer and present what we are trying to accomplish and the options we are considering.

House is a 200A service with normal modern loads and wiring for a 1600 SqFt space plus 800 SqFt unfinished basement.

Garage is in short run to be a woodshop. We have lots of bench and stationary tools and power tools, incl dust collector and air filter--heating is open issue at moment. At most there will be two people operating at the same time. Given calcs I've done it seems like 100A would suffice and we are running a 2-3 Cable/Wire to the garage for 240v service, which is about a 70' run.

Shed is a workout space and is 8x10' housing low amperage exercise equipment: stairmaster, elliptical trainer, ski machine--which now that I think of it I don't believe have motors in them but rather some sort of magnetic resistance to effort, but they do have some electronics as well to keep track of things. In addition, there'll be a LCD TV, a fan, some lights. Subject of a subpanel here is open to debate... at most it'd be a small one maybe 2 circuits and a cutoff.

Finally, we have a project to run power outside to receptacles from which we will power low voltage walkway lights. There are three 300W transformers likely to be maxed out.

In addition, there are five 75W (likely) lamps that will be in two sconses and one lampost. It is unclear where these lights will get their power.

We have already begun laying out our cable indoors. The 2-3 cable (which will transition to wire for outdoor run) is a done deal. We have run two 12-2 cables intending to run one to the shed and one to receptacles on 3-4 sides of house. These would be run at 20A.

However, we are considering (as posted elsewhere) changing the 12-2 run to the shed to be 12-3 instead (we can repurpose the 12-2 cable run to go to a dedicated circuit for the refrigerator which is currently on the upstairs lighting circuit and I don't like that). In that case of course it'd be 240v and not suffer too much voltage drop (the topic of a previous post).

Other options: We've considered running power from the garage to the shed, probably in an overhead run. At some point the garage will be torn down and rebuilt and we'd like to not lose power to the shed during that construction, but this consideration can be waived if there are more compelling factors.

We're currently planning to power the five 75W lamps from the shed circuit since two of these lights will be affixed to the outside of it and (without making a dedicated conduit run from the house to the lampost) the shed would be the nearest accessible source of power--also it provides an easy place to put a timer/switch for the lights; though it is feasible to run this lampost by tapping into the outdoor receptacle circuit.

And so based on info I've collected so far, it seems that we would change the 12-2 to 12-3 as feed to the shed. The shed and garage would be separate circuits, which seems cleaner to us. And of course would have separate subpanels. However, the grounding requirement is something I've kept stuffed in a back recess of my mind but I guess I have to admit it as a major complication at this point.

Suggestions please!
 
  #4  
Old 12-07-07, 04:40 PM
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The shed gets no grounding (other than the ground wire in the 12-2 or 12-3 cable).

The garage sub panel needs a ground (or two). It cannot use the same ground as the main house. I recommend a four wire run to the garage, which you might already be planning.
 
  #5  
Old 12-07-07, 04:48 PM
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Originally Posted by racraft View Post
The shed gets no grounding (other than the ground wire in the 12-2 or 12-3 cable).

The garage sub panel needs a ground (or two). It cannot use the same ground as the main house. I recommend a four wire run to the garage, which you might already be planning.
We are running #2 awg x 3 conductors and #6 ground to the garage.

Glad to hear the shed does not need a grounding system. I assume your reply does not depend upon which of the options we choose. Just for my continuing education... what is the reason it does not need the ground is it the size of structure, its usage, or just the lower amp circuit?

Thanks!
 
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Old 01-24-08, 09:24 AM
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One more question on this topic.... (one for now any way)...

We've run 4x10 awg THWN to the shed for 30A 240v service, which is less than 10' from the side of the house (wire run is much longer of course). Below we are told we do not need separate grounding rod system, and that we do not tap into existing ground rod for house... so no grounding for the shed (except via ECG).

Service panel is 60A capable, but will merely have two 20A 120v branch circuits. One will service lighting inside and outside the shed (exact lighting to be determined). One service will power exercise equipment, TV, and some fans.

Question:
1) Should the neutral bus bar in the shed's panel be bonded to the ground or not? I have them separated but I wanted to double-check; in light of no grounding electrode system I wonder if it affects the neutral bonding issue.

2) This is not a new question, just a re-asking: why doesn't the shed require a grounding electrode system?

Hmmmm... I wanted to re-edit my original post below, but I cannot do it. So let me here amend our plans as described below. As stated above, the shed power was in the end supplied by 10 awg wire for 30A 240v service. The shed is fed underground via PVC conduit, NOT an overhead feed. There is no metalic pipe/conduit between shed and house. Also run in a second PVC conduit is RG6/U and Cat5e cables for TV/internet service. The lampost mentioned is now fed by a dedicated 12 awg feed via PVC conduit from the house.

Thanks
 
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Old 01-24-08, 09:39 AM
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Since you are installing a sub panel in the shed, it needs to be grounded. Period.

Sine you have other cables to the shed (which I hope are in a separate conduit) you need a ground wire to the shed with the power wires, which you say you have.


At the sub panel in the shed, the ground and neutral buss bars must remain separate.
 
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Old 01-24-08, 09:49 AM
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I think I can answer that... NEC 250.32 requires normally that buildings and structures supplied by feeders and branch circuits need a grounding electrode system. However, there is an exception when "only a single branch circuit, including a mulitwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non-current-carrying metal parts of equipment".

If you limit the shed's supply to a single multiwire circuit, you don't need a grounding electrode system. If, however, you had more than one circuit running to the shed, it would need a grounding system.

Separate the neutral and ground. Neutrals and grounds get tied together at one spot only, in the main panel in the house.
 
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Old 01-24-08, 11:40 AM
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Arnie, does that exception apply? How are you defining a single circuit? I would think that the exception applies only if that multiwire branch circuit directly feeds devices in the shed. In my case it is a regular "feeder" to a subpanel which in turn has two branch circuits. It seems in this case the exception would not apply; otherwise ALL subpanel feeders would qualify for the exception, no?

Since the multiwire branch circuit feeder is going into a subpanel in the shed, and that is hosting two 120v branch circuits, it seems that it would require a grounding electrode system. -- Racraft, that is what you are saying right?

What is the gauge of the grounding wire to be used?

Also, the shed is only three feet from my garage which will also be getting a grounding electrode system. Does the proximity of the grounding rods for both structures matter? If so how far apart need they be from each other (that is, how far apart should the two grounding rods serving the shed be from the two serving the garage--I know within a pair to keep them 6' apart).
 
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Old 01-24-08, 11:54 AM
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Since you are installing a sub panel in the shed, it needs to be grounded. Period.
 
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Old 01-24-08, 12:12 PM
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Let me ask the question.

If you are only running two 20 amp circuits in the shed, why are you installing a sub panel? My advice is as follows:

Use the #10 wire if you want to allow for the panel at a later date, but for now simply use the four wires to make a multi-wire circuit in the shed. Protect the circuit with a double pole 20 amp breaker in the main panel, and install a disconnect (switch) in the shed. From this point run your two separate 12 gage circuits.

This will allow you to avoid the ground rod and avoid the sub panel, saving time and money.
 
  #12  
Old 01-24-08, 12:51 PM
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Racraft, your suggestion is a good one--I should have gone that route. Unfortunately, I have already installed the subpanel, which I don't think I can return at this point. But I'll see... maybe its still best option even if I have to undo some stuff.

However, if I have to finish the job by installing the ground rods...

-what size ground wire?
-does it matter how far/close they are to ground rods for garage?

Thanks again, you guys are a great help... I'm slowly learning.
 
  #13  
Old 01-24-08, 06:29 PM
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OK, I am going to try to do it using the cut-off switch. I have a non-fusable box. It has terminals for the two hot wires and a ground bus. The instructions also say that to use it for a "service entrance" one needs to purchase a grounding bus accessory... which I don't understand, nor could I find.

Is my application (as a multiwire branch circuit from house to shed w/ cut off switch) considered a "service entrance"? If so, please explain what the intent is w/ regard to the ground bus.


As for the neutral wire, I assume I am to simply pigtail the neutral feeder to the other outgoing neutral wires?

Thanks,
Dave
 
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