Grounding question for detached garage

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  #1  
Old 08-18-05, 12:52 PM
JayBay
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Grounding question for detached garage

Hey all - here's my situation. When we recently remodeled our house (built in 1921), we had most of the old knob & tube wiring replaced with new wiring. The only knob & tube circuit remaining is a 15 amp circuit running out to a detached garage, about 40 feet from the house.

The wires to the garage are not grounded, they are 2-wire only, and they run through a small (3/4" maybe) metal pipe underground into the garage.

Eventually I'd like to dig a trench and run all new wires to the garage, but I don't have the time or money to start that project now. In the interim I'd like to replace the existing 2 incadescent lighting fictures with 6 florescent fixtures.

I know without proper grounding I'm going to have trouble getting the florescents to work, so can I either:

A) Use a grounding rod, and if so what are the requirements for installing one?

B) Use the metal pipe that contains the wires running to the garage as a ground. It extends up into a wall cavity in the garage and I could attach a grounding wire directly to it.


Thanks in advance for any advice you can offer.

Cheers,
Jason
 
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  #2  
Old 08-18-05, 01:16 PM
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Is this "metal pipe" (ie, conduit) a continuous run all the way from the house to the garage? If so, you may be able to pull new wires (including a ground wire) through it.
 
  #3  
Old 08-18-05, 01:23 PM
JayBay
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It is continuous, but it's pretty small and has a couple bends at each end. I doubt I'd be able to pull the existing wires back out of it, and I'd hate to cut them and give it a go without being ready to dig the whole thing up if I fail.
 
  #4  
Old 08-18-05, 01:35 PM
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As a detached garge a ground ing rod is required anyway when you install a panel.
 
  #5  
Old 08-18-05, 02:00 PM
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If the metal conduit is continuous, I would think it could be used as a ground so long as it is tied to ground at the house. Perhaps you can run new romex from you main panel to the point where the conduit enters the house, then attach a metal junction box to the conduit and connect the new romex to the old wire there. That brings a good ground to the conduit, and allows you to eliminate the portion of K & T that runs to the conduit. Be sure to use one of those green grounding screws to bond the ground to the box, and make sure the connection from the box to the conduit is good. That should get your ground to the garage through the conduit.

Then do the same thing at the garage...J-box attached to the conduit, and tie the romex into the old wire there. Again, be sure to bond the ground wire to the box. Now you can run the romex wherever you want it. Be sure to provide GFCI protection to any/all recepticals.
 
  #6  
Old 08-18-05, 02:23 PM
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Whatever you do, do _not_ simply drive a ground rod in the detached garage and use it to 'ground' things. 'Grounding' without 'bonding' at best does nothing, and at worst is more dangerous than simply leaving the circuit ungrounded.

I believe that chirkware's approach is probably a pretty good idea, but I would _check_ the pipe first.

Shut off the circuit to the garage. Use a meter to make sure that it is off. Then run a wire along the ground, and attach it to one end of the pipe. Using a meter, measure the resistance between the other end of the wire and the other end of the pipe. If the resistance is as low as your meter can measure, then the pipe can reasonably be used as an equipment grounding conductor.

-Jon
 
  #7  
Old 08-18-05, 03:20 PM
montag
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Originally Posted by winnie
Whatever you do, do _not_ simply drive a ground rod in the detached garage and use it to 'ground' things. 'Grounding' without 'bonding' at best does nothing, and at worst is more dangerous than simply leaving the circuit ungrounded.
-Jon
Please tell me more about bonding. When you install the isolated ground buss is it considdered bonded?
 
  #8  
Old 08-18-05, 11:04 PM
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A) Use a grounding rod, and if so what are the requirements for installing one?
If you only have one 15 amp 120 volt branch circuit that supplies the building and it contains a equipment grounding conductor you do not need a ground rod. 250.32(a)exception


B) Use the metal pipe that contains the wires running to the garage as a ground. It extends up into a wall cavity in the garage and I could attach a grounding wire directly to it.
Caution: Just because this metal conduit appears to go into the ground it does not necessarily mean it is continuous. You need to check it as Winnie suggests. Also if this is individual wires coming out of the conduit do they terminate in a switch box or what?

You must provide a means of disconnect at the garage in your case all this needs to be is a single pole switch. You could simply install a metal switch box to the conduit and ground the switch to the box provided the conduit checks out for ground. The key here is do you have an equipment grounding conductor included with the garage wiring or is it knob and tube with no egc?

You must have a light over the entrance door and a light inside plus at least one gfci receptacle in the garage for minimum requirements.
 
  #9  
Old 08-19-05, 06:50 AM
JayBay
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Thanks for your replies everybody. This has been a great help.

To answer your question Roger, the conduit does not contain a grounding conductor. There are only two wires running through it. I'll test the conduit itself as winnie suggests, to see if I can get a good ground from that, and if so I'll do the following:

- Use conduit as a ground, such as chirkware suggests with metal boxes on each end of the conduit, eliminating the K & T in the house that runs from the panel to the start of the conduit.

- In the metal box on the garage end of the conduit, I'll add a single-pole switch as a garage disconnect.

- There will be lights over the entrance door, and I'll also add a GFCI receptacle to the garage.

Did I miss anything? Again, all of the above is assuming the conduit will provide a solid ground. Keeping my fingers crossed there.

If the conduit fails to pass the test, what would be the best way to proceed?
 
  #10  
Old 08-19-05, 08:19 AM
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Jay you sound like you have the correct understanding. If the conduit doesnt show a good ground....I would try to use the two existing wires to pull in two new conductors and an equipment ground. This would be very small in costs. I'm not so sure I wouldnt do this anyway. You can use a pulling lubricant to assist getting the wires thru the conduit. Just post back if you decide to pull new wires and someone here can describe the process.
I would suggest using one of the existing wires to verify ground for the metal pipe. Power off of course.
 
  #11  
Old 08-19-05, 08:33 AM
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Do not use one of the existing wires to try to verify continuity of the pipe. This dangerous and unreliable. Use a separate piece of wire that you provide. This way you know it is intact and that nothing else it is connected to will interfere with the test.
 
  #12  
Old 08-19-05, 08:34 AM
JayBay
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My first thought was to pull new wires throught the conduit, but the conduit is only about 3/4 inch pipe, and there are a couple of bends on each end. I doubt I'd be able to pull the wires through, and I hate to attempt to do it without being prepared to go all the way and dig a trench and lay a new conduit if I fail.

Using an existing wire to verify ground sounds easier than running a seperate wire out from my basement to the garage. I'm just not sure I understand how to do that. I guess I would (with the power off) take one end of the wire where it comes out of the conduit, disconnect it from whatever it's currently tied into and physically attach it to the conduit itself... then test the resistance on the other end of that loop, between the wire and the conduit on the other end. Is that correct?
 
  #13  
Old 08-19-05, 09:45 AM
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I'm not exactly sure what Bob means by dangerous. There is a slight chance for unreliable. However, if you take both wires in the conduit and at one end put them in a wirenut, then go to the other end and check continutiy between them you can prove if they are good end to end. I thought this was a working circuit where we werent guessing?? Maybe I'm missing something. I believe that Jay knows to check for power on the wires even after turning the breaker off. I'm not exactly sure why power might be present at the other end or the wire connected to something else on a singe 120 feed to a garage. I'm not throwing caution to the wind here but Jay knows what he has there. If he tests properly I dont see the danger in his situation. There is an equal danger of the pipe being energized since we are dealing with no ground wire in the circuit and if something else is connected to the feeder wires.
If I was in a warehouse and had 20 wires in the conduit and 50 loads being served I might take a different approach. I see no reason for him to run and buy 50 feet of wire to do a continuity check on this metal pipe. If we are going to make him do that he might as well tell him to buy the rest of the wire to pull a new circuit and forget about the pipe being ground.

Jay you are correct in your thinking on the continuity test either way you choose. I suppose if you think there is some possiblity that this 2 wire feed has something going on that you dont understand then you can go get that 50 foot of wire but I'm just not seeing the need to do that.
 

Last edited by Roger; 08-19-05 at 10:10 AM.
  #14  
Old 08-19-05, 10:06 AM
JayBay
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Roger - yes the circuit to the garage is live. I can see how using one of them to test the conduit ground would be potentially less reliable. I do have quite a bit of wire kicking around from our prior remodeling and electrical work, so if I can find a piece long enough it shouldn't be too big a hassle string that out for the resistance test.

Now, I have a pretty clear plan of how to proceed if the conduit will work for a ground. If I may alter the discussion a bit...

If I were to try and pull new wires through the existing conduit, assuming I can get them through, what size wire would I need to use? Would the wire running through the conduit have to be UF, or is it better to use individual conductors? My guess is based on the small size of the conduit I'd be better off using individual wires. What is the standard procedure for pulling wires through an existing underground conduit?

Also, if it comes to pulling new wires...
I'd eventually like to get enough power into the garage to run all the lights, plus 2 garage door openers, and a handful of receptacles (possibly running a compressor or the occasional power tool). Based on my future requirements, I would think I'll need to dig a new trench anyway just to run enough wires for the (at least) 2 circuits I'd need. So in that case I might be better off forgetting about the old conduit and just running a new one altogether.

So there's where I'm stuck. If I can use the old conduit to get a decent ground to just run new florescent lighting (and the one GFCI receptacle), it would be a cheap, easy upgrade to make my garage more usable. But, if it becomes too much of a hassle I'm probably better off waiting until I have the time/money to do it all and do it right.

Sorry for the long post, but I want all of you to know I greatly appreciate your comments and suggestions.
 
  #15  
Old 08-19-05, 10:07 AM
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I would prefer to 'ring out' this circuit with an external wire. If there is a neutral to ground fault in the conduit, then there could still be current flow in the neutral even with the power to this circuit off. In order to use the wires inside the conduit to test the ground, you would need to disconnect them from the supply conductors, and there will be risk associated with disconnecting the neutral.

1) Turn off the power to this circuit.

2) Use the volt meter to check for voltage at the circuit. Use a clamp on current meter to see if any current is flowing in the individual wires going into the conduit. If you detect no voltage and no current, then proceed.

3) Disconnect the hot and neutral connections on both sides of the conduit.

4) Connect one of the wires to the metal conduit at one end of the conduit, then measure the resistance between wire and conduit on the other side.

-Jon
 
  #16  
Old 08-19-05, 10:14 AM
JayBay
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Thanks Jon - I will follow that procedure for testing the ground this weekend.
 
  #17  
Old 08-19-05, 10:14 AM
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You would need to make a better measurement of the conduit for us to actually assess the size; perhaps it isn't even a conduit at all, but some other sort of wiring method.

It is likely, however, that you have a trade size 1/2" conduit of some sort. If the inside is not rusted or otherwise blocked, then it should be reasonably easy to 'fish' new conductors through it.

In this case, it is best to use single wires rather than any sort of cable.

Because this is an underground conduit, it is considered a wet location. You would need to use wet rated conductors, eg THWN or XHHW insulated wire. The standard wire sold for wiring is usually THHN/THWN dual rated, and is fine.

For pulling in conduit, stranded wire is generally easier to work with...but a bit harder to terminate at either end; it takes more practise to get a wire-nut to work correctly with stranded wire.

When pulling the wire, you use some sort of rope or fish tape and wire pulling lubricant. Ideally you would pull the rope in when you pull the old wires out, and if possible you want to pull some sort of cleaning sponge through the conduit.

Finally, if you run a 'multi-wire branch circuit' using 3 circuit conductors and a ground conductor, you can most cheaply get quite a bit of power through this small pipe. MW branch circuits are harder to get right, but worth it in this case since they avoid the requirement for a subpanel or grounding electrodes. You will want to study them carefully before proceeding.

-Jon
 
  #18  
Old 08-19-05, 11:31 AM
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I have no argument with Winnies advice it is very complete. I apologize if my efforts to explain were not as clear as they should have been as to the circuit wires being disconnected at both the panel and garage. It seemed that that was apparent in my explaination.

I agree of the risk on the neutral to ground fault but you also have the risk of the conduit being energized. There is always risk... I'm not sure in every situation you can eliminate all of it but you do have to deal with it sooner or later.
It flat scares the heck out of me about the risk of DIY messing with wiring.

What is more risky is to give them bad advice since they look to the forum for the right advice or they wouldnt come here. If my advice was bad or dangerous then shame on me. I do not believe that disconnecting the circuit at the panel was a high level of risk or dangerous. To me it eliminated higher risks.

What is interesting is Winnie explained the proper testing of this circuit to eliminate risk of neutral current flow even with the breaker off. My hat is off to him. That isnt present on a bunch of posts similar to this that I have read.

Problem I see though is now we have to rely on the diyer to understand the use of an amp meter and to go buy one because he probably doesnt
own one. Money is a big deterent for many DIY and understandably so ....have you filled your gas tank lately?

IMO it is less "risky" to disconnect at the panel and garage then do a simple voltage test with inexpensive testers before conducting the continuity test with an existing wire or running your own.
 
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