Background information:
- I’ve done a couple of whole-house re-wires, but have never done a detached garage. I understand the codes for in-house wiring.
- I’ve burried a Rigid PCV conduit between the house and the garage for the wiring.
- I’ve burried a grounding electrode at the garage and have carried it to the sub-pannel in the garage with unspiced #6 bare wire.
Questions:
I’m confused about how to treat the grounds in the garage. The code books I have are clear that the garage needs its own grounding electrode (which I’ve done). However, what about the following:
- Do I run a ground wire from the in-house ground electrode or water-service to the garage and connect it to the electrode connection there?
- Do I treat the garage like any other sub-pannel and issolate the neutral from the equipment ground?
Thanks, Jeff
Replies
Jeff,
Good question, and good going so far.
The safest thing to do is to run an equipment grounding conductor (ground wire) in the conduit from the ground/neutral bar of the service (or ground bar of the main panel) to the ground bar of the sub. The neutral of the subpanel should be "floating", that is, electrically isolated from the ground bar and the case of the subpanel. Terminate the grounding electrode conductor from the ground rod (or Ufer electrode) at the subpanel location to the ground bar in the subpanel. Also bond (connect electrically) any water piping in the garage to the ground bar of the subpanel. Ditto for the well casing if there is one, andy building steel, compressed air lines (if metallic) and so on. The point is to keep all metallic parts of the garage at earth (zero) voltage.
You have to do it this way if there are any metallic paths (water pipes, telephone, intercom, security system, or other wires) between the buildings. This is in case there's a fault to ground in the garage--you want the fault current running back to the main panel on a nice fat (low impedance) grounding conductor, NOT on a water pipe or telephone wire. A low impedance path will allow enough current to flow to open the feeder circuit breaker at the main panel, without endangering anyone in the process.
If there is no metallic path between house and garage, you can do without the grounding conductor from main to sub, and bond the ground and neutral bars together at the sub. You still connect the garage grounding electrode to the ground(and in this case, /neutral) bar of the sub--it's your lightning current drain in either case.
I like to run a grounding conductor with a floating neutral bus bar because that way you're sure that the zero voltage (ground) reference point is the same in the two buildings. Then if you (or a future owner) runs a phone wire to the garage, it's O.K., no danger. The only argument against running a grounding conductor is cost. And remember, you can run a smaller grounding conductor than your current-carrying conductors.
One more thing, most jurisdictions require two driven ground rods for each panel because the Code requires that you test and show the single ground rod impedance to be below 25 ohms--or to install two. If you're using rebar or wire in the footer (called a Ufer electrode), you're A-OK.
Best of luck.
Cliff Popejoy
I just want to add a layman's response to the question raised.
It probably says the same thing Jeff talks about.
Thanks, Jeff, for mentioning grounding water pipes, etc., which would be a good safety measure and I will incorporate that into future projects.
What I found out during a upgrade project several years ago is the neutral and ground can be joined only once in a system, which typically would be in the main breaker panel. However, when there is a main disconnect between the breaker panel and the meter, the ground/neutral is joined in the main disconnect and the main breaker panel becomes a sub-panel with separate ground and neutral bus bars. All other sub-panels, either fed from the main breaker panel or the main disconnect, would follow the same rule of separate neutral and ground bus bars.
By the way, I buried THREE ground rods for the main-disconnect, spaced 6' apart. The inspector really liked that. Only one is required in our area. In hindsight, I wish I had buried more.
Also, the main disconnect was not required for my project but I went ahead and spent the money on it. I think it's a good idea to have one.
Alan
Ahneedhelp,
I'll bet the inspector liked three ground rods! I recently advised a friend who replaced the service in his house and I suggested that he install a handhole box (an irrigation valve box) at each ground rod, so the top of the rod could be seen. The inspector was impressed, and was on his side from then on.
On the point of connecting the ground and neutral, your are right, it can only be done at the service to a building. In a single building, the service point is in part defined as being the point where the main bonding jumper (which connects the ground and neutral bars) is installed.
However, with separate buildings, if there is no metalic path between them (and no grounding conductor or ground-fault protection for equipment), you can in effect treat the panel in the second building as a separate service, and bond the neutral from the main panel to the grounding electrode at the second bldg., making the ground/neutral bar the same.
Go to the 2002 NEC, Section 250.32 (B)(2) to see it with your own eyes. This has been the same for quite a few years. Just another example of how a little difference--like separate vs same building--has a big effect on what's OK and what's not. It also demonstrates the value of reading the Code yourself--too much important detail can be lost in the translation.
Cliff
Hi, Cliff -
RE - However, with separate buildings, if there is no metalic path between them (and no grounding conductor or ground-fault protection for equipment), you can in effect treat the panel in the second building as a separate service, and bond the neutral from the main panel to the grounding electrode at the second bldg., making the ground/neutral bar the same.
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Not being a licensed electrician and a scholar of codes, my approach is to overdo things and not try to second guess the inspector.
Just so I can make sure I understand what you are saying....
If I went with a properly sized grounding wire to the sub-panel in a separate building (basically a four wire run), then the separate ground and neutral bar would be the proper thing to do - correct ?
It seems to be the sensible and safer thing to do although code would allow what you are mentioning.
I'm thinking while the conduit and cable is being laid anyway, why not go ahead and run the ground wire also ?
I wish I had thought of the handhold box for the tops of grounding rods. I did draw a fairly accurate map of the locations of the rods for future reference but something more obvious would have been nice.
Alan
Alan,
You are absolutely right on: for a subpanel in a separate building, a 4-wire run between panels, floating neutral bar in the subpanel, ground rods at subpanel tied to ground bar at subpanel. This is what I prefer to do, even if there is no metallic path between the buildings.
And as far as locating the ground rods, you could always have an electrician use a cable locator to track them. Have him (or her) do it when he's out there for something else...
Best regards.
Cliff
p.s. I'm working with Fine Homebuilding right now on an article about installing a service. It looks like it'll appear in issue #150. Be on the lookout! Cliff
Hi, Cliff -
I'll be on the lookout for #150 in the mailbox.
RE - for a subpanel in a separate building, a 4-wire run between panels, floating neutral bar in the subpanel, ground rods at subpanel tied to ground bar at subpanel. This is what I prefer to do, even if there is no metallic path between the buildings.
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I'm a bit confused by the above statement....
I was talking about 4-wire between the panels and (multiple) ground rods where the ground/neutral is joined. In my case, at the main disconnect next to the meter. No ground rods buried for the sub panel in a separate building because there is a solid path for ground from the separate building via the properly sized ground wire to the main disconnect (through the main breaker panel).
Does that sound right to you ?
Alan
Cliff,
Thanks for the quick response. I will also look forward to the article.
Alan,
The code reads that if you have more than one branch circuit in the separate building, you MUST have grounding electrode(s) at the separate building. The ground conductor coming across from the main building is not sufficient. Here's the section that talks about this:
250.32 Two or More Buildings or Structures Supplied from a Common Service.(A) Grounding Electrode. Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), the grounding electrode(s) required in Part III of this article at each building or structure shall be connected in the manner specified in 250.32(B) or (C). Where there are no existing grounding electrodes, the grounding electrode(s) required in Part III of this article shall be installed.Exception: A grounding electrode at separate buildings or structures shall not be required where only one branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non–current-carrying parts of all equipment.(B) Grounded Systems. For a grounded system at the separate building or structure, the connection to the grounding electrode and grounding or bonding of equipment, structures, or frames required to be grounded or bonded shall comply with either 250.32(B)(1) or (2).(1) Equipment Grounding Conductor. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of (1) That required by 220.22 (2) That required by 250.122(D) Disconnecting Means Located in Separate Building or Structure on the Same Premises. Where one or more disconnecting means supply one or more additional buildings or structures under single management, and where these disconnecting means are located remote from those buildings or structures in accordance with the provisions of 225.32, Exception Nos. 1 and 2, all of the following conditions shall be met: (1) The connection of the grounded circuit conductor to the grounding electrode at a separate building or structure shall not be made. (2) An equipment grounding conductor for grounding any non–current-carrying equipment, interior metal piping systems, and building or structural metal frames is run with the circuit conductors to a separate building or structure and bonded to existing grounding electrode(s) required in Part III of this article, or, where there are no existing electrodes, the grounding electrode(s) required in Part III of this article shall be installed where a separate building or structure is supplied by more than one branch circuit. (3) Bonding the equipment grounding conductor to the grounding electrode at a separate building or structure shall be made in a junction box, panelboard, or similar enclosure located immediately inside or outside the separate building or structure.(E) Grounding Electrode Conductor. The size of the grounding electrode conductor to the grounding electrode(s) shall not be smaller than given in 250.66, based on the largest ungrounded supply conductor. The installation shall comply with Part III of this article.
Thank-you, Jeff, for posting the code section covering this topic.
The wording is very confusing and (C) seems to be missing.
It will take some time for me to decipher the Latin.
There is frequent reference to grounding electrode...would this be the grounding rod outside ?
In my scenario, the detached building would be fed from the main breaker panel because I don't have circuit breakers on the main disconnect. Would this code apply ?
Also, if I used a main disconnect from which the main breaker and the detached building were fed directly, would a 4-wire run to the detached building be sufficient ?
I'm printing out the code you posted to discuss with a friend who is quite a bit into this sortof stuff.
Take care,
Alan
Alan,
Sorry for the missing 'C' section. I was given that section on another forum. You can always go to the library or book store to look at it. I'm sure it must be online somewhere also.
Cliff mentioned that the electrode can be either a grounding rod or a "Ufer" electrode, which is making use of existing rebar or wire in the footer for the building. See http://forums.taunton.com/n/mb/display.asp?webtag=tp-breaktime&msg=19722.2
I also found some pictures at: http://www.ci.saint-paul.mn.us/depts/liep/pdf/Building/Electric/garwir00.pdf . This is clearly specific to St. Paul, MN and you should check your local codes, but the pictures were helpful to understand what Cliff was talking about.
-Jeff
Hi, Jeff -
This thread is becoming very informative.
I appreciate the links and the PDF file will be added to my collection.
Just need to print out some of this stuff and spend some time alone from the kids.
I have made several visits to the library in the past and looks like it's time for another visit.
(I do have a meeting with the town inspector tomorrow concerning some other electrical matters.)
Cheers,
Alan
> I suggested that he install a handhole box (an irrigation valve box) at each ground rod, so the top of the rod could be seen.
Where do you get those boxes? OSH and HD don't carry them, or if they do, they don't employ anybody who knows what they are.
A friend of mine used to sink ground rods for radio and TV transmitters. His favorite technique was to use 3/4" type K copper water pipe. He'd hammer the bottom end flat and poke some holes it it, then solder a few sections together and put a female NPT fitting on the top. With the pipe standing vertically, water from an ordinary garden hose was enough to erode the dirt away ahead of the pipe. He got rods 40 - 60 feet deep that way, and resistance under an Ohm.
-- J.S.
John -
Speaking of sinking ground rods, I used a tip picked up from one of the FH issues several years ago.
Dig a funnel shaped hole maybe 5-6" in diameter and about the same depth. Fill the hole with water.
Hold the ground rod vertically into the funnel hole with the water still in it and start tapping away at it with a up/down motion, little bit a time first.
Increase the up and downward thrust as the rod starts sinking.
Add more water into the hole if it drains dry.
With this technique, I sunk three rods most of the way with only the last foot or so driven home with a sledge hammer.
The last couple of strokes gets kinda long so you have to stand up and sit down as the rod starts sinking deeper.
It took only couple of minutes to get each of them sunk this way and it does work.
However, I was lucky and did not run into any rocks.
Alan
Alan --
Yes, that method will usually work. The advantage of your method over the pipe method is that it uses a lot less water, and therefore makes a lot less mess.
Do be sure to get more water in the hole quickly if it needs it. Getting it to re-soften later can be a problem. Clamp a piece of rebar to the top of the rod, and use it as a handle to save the squatting as you get near the end. If you hit a rock, you can also (on these short rods) just use the water pipe idea along side the rod, and pull the pipe when you're done. The water pipe will eventually erode enough dirt out to make room for the rock to get pushed to one side, unless the rock is bigger than a basketball. If you go deep enough with the pipe method, interesting things may wash up, like sea shells and fossils from deeper strata.
-- J.S.