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Old 12-31-2007, 08:16 AM   #1
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Detached Garage Sub Panel Grounding Q


Hello all. I have read threw this detached garages page about grounding ( http://forums.mikeholt.com/showpost....4&postcount=13 )
and had a few questions about it. Im working with the same sistuation as example #3. I have a 200 amp main and I am running a 100 amp sub in a detached garage useing PVC and have no water, gas or other metal lines running to it. The garage is 40ft one way from the main panel. I see I should run only 3 wire out to the garage add a ground rod and connect the netural and the grounding rod together in the panel, same way as in the main.

My question is what problems will arise if and when a gas line or water line is ran out the garage? A gas line my be added for a heater in the future. Will the electrical system have to be changed or re done? Also for the record the money of adding an extra wire for 40 foot is not a big deal and 1 1/2 conduit was run so there is plenty of room. Im just trying to the find the safest and best all around way of doing it for now and for the future if gas and water is run out there. I dont want to run into any issues of metal boxes carring current.

Can someone help anwser the question and also explain or direct me to a diagram that helps better understand the 2 different ways of doing it the cause and affects of each.
The other way would be to run 4 wires out to the garage and keep the ground and netural seprate unlike the main. Would a grounding rod also be used in this case?
Thanks for you help, JIM

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Old 12-31-2007, 08:41 AM   #2
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Detached Garage Sub Panel Grounding Q


Run the four wires and install a ground rod. The 2008 code removes the option of running three wires to a detached structure, so go ahead and comply with the new code and be safe.

I am sure our master illustrator will post a diagram for you soon (your cue Stubbie).

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Old 12-31-2007, 08:44 AM   #3
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Detached Garage Sub Panel Grounding Q


If the detached building has more than one circuit (in your case a panel that will feed more than one circuit) you will need a ground rod.

The safest installation is using a full size ground from the main building to the sub panel in the detached building. The other way is allowed because it is cheeper. (starting with the 2008 code it will no longer be allowed)

Sorry I have no spiffy picture links.

The hazzard comes in when the neutral (which carries current under normal conditions) and the ground wire (which is intended to only carry current under a fault condition) are bonded at the sub panel.

If the neutral wire were to fail, the electricity would attempt to return to the source via the ground wire or earth. There now exhists a hazzard that if you were to touch the metal parts of any part of the system (panel enclosure, metal frame of appliance or box etc) you could become a path to ground.

If the ground and neutral are not bonded at the sub panel, and the neutral comes open, your stuff just will not work.

To answer the next most obvious question: Does this create the same hazzard at the main pane? The answer is yes. In some countries the electrical systems are ungrounded. In this country we choose to offer the protection from lightning provided by this bond over the open neutral hazzard. This connection is mostly to protect the utilitie's and your equipment from lightning strikes down the line.

Nothing will protect your home from a direct strike.
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Old 12-31-2007, 09:50 AM   #4
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Detached Garage Sub Panel Grounding Q


Thanks for the fast replys guys. So im going to run the 4th wire to a ground bar. Im also going to run a ground rod hooked up to the ground bar and keep the netural wire seprate on its own netural bar, In the Sub Panel but not in the main. In the main I will have my netural and ground bar connected.

Now also the 4th ground wire I run will that be the same size as the other 3 wires? So 4 wires all the same?

I was going to run 4 gauge even thow I go with 6 or even maybe even 8 guage. The cost isnt much different and the conduit size will hold that easy correct. What size gound should I run from the grounding rod? Also the wire running from the rod into the panel is not insulated correct? Do or should I run it in conduit to the panel. I was thinking it would look kind of tacky drilling a hole in the stucco and running it up the inside of the wall without and conduit. Is that how it should be done? Or whats the correct procedure? Thanks JIM
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Old 12-31-2007, 11:29 AM   #5
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Detached Garage Sub Panel Grounding Q


I may need to post twice to get all the illustrations that are relative....

To answer your question about the addition of a gas line or water line to the building is about a thing called objectionable current. Here is an illustration from Mike Holt that shows the problem and the hazard if a bond is made to the ground and neutral when installing 4 wire feeds. In this illustration from mike holt they are using metal conduit as the ground return for fault current vs. installing a ground wire in say pvc conduit. As you can see the neutral return current under normal operating conditions splits and travels on both the metal conduit and the feeder neutral. Touching the energized metal conduit could electrocute you. If I remove the bond and install a ground bar kit landing the grounds of the feeder and branch circuits to it and the neutrals to the neutral bar then I have effectively prevented neutral current from flowing on the feeder equipment ground back to the source and it is now safe to touch the metal conduit.
Now lets look at a 3 wire feed where only a neutral is ran with the two ungrounded conductors to the sub-panel. Using the illustration as a guide picture a 3 wire feed in pvc conduit. Remember if I run the wires in metal conduit I have effectively ran a 4 wire feed and I would not bond the neutral and ground. Using pvc or direct burial the only neutral current return and fault current return is the feeder neutral. The feeder neutral plays double duty so to speak. Now if I open that feeder neutral I will lose all my 120 volt circuits possibly placing some 120 volt circuits on high voltage and some on low voltage due to the loss of the neutral. If I were to have a ungrounded wire fault to ground occur on a branch circuit in the sub-panel it will energize all the metal on the load side of that open neutral to line voltage and no breaker will ever trip as I have lost my fault current return back to the source due to the opened neutral. Your first thought is going to be "what the Heck" walk over and grab the metal door of the sub-panel to check your breakers. Unknown to you that metal is live with voltage and current looking for a way to ground. So you can see the danger. 4 wire feeds reduce greatly this from ever occurring because you have that 4th wire in place. So if you add water or gas line to the building they are effectively connected to the electrical system of both structures via connections to the equipment ground at the metal housing at the house furnace and garage furnace. This provides an unwanted return path (parallel path) outside the feeder between buildings for objectionalble current flow in the event of an open neutral fault. Hope that was clear. The last two drawings are 3 wire and 4 wire feeds underground to detached structures. These drawings are my drawings and are not intended as a "how to" application as a complete understanding of all aspects are required for a safe installation. The drawings are intended as a guide and touch on a few of the related codes. Click on each thumbnail image to enlarge.

A very good article to read is located here....

http://www.selfhelpandmore.com/homew...rage/index.htm


Attached Thumbnails
Detached Garage Sub Panel Grounding Q-3-wire-feeder-detached.jpg   Detached Garage Sub Panel Grounding Q-4-wire-subpanel-detached.jpg   Detached Garage Sub Panel Grounding Q-open-neutral.jpg  

Last edited by Stubbie; 01-01-2008 at 12:07 PM.
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Old 12-31-2007, 01:59 PM   #6
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Detached Garage Sub Panel Grounding Q


Quote:
Originally Posted by jwhite View Post
If the detached building has more than one circuit (in your case a panel that will feed more than one circuit) you will need a ground rod.

The safest installation is using a full size ground from the main building to the sub panel in the detached building. The other way is allowed because it is cheeper. (starting with the 2008 code it will no longer be allowed)

Sorry I have no spiffy picture links.

The hazzard comes in when the neutral (which carries current under normal conditions) and the ground wire (which is intended to only carry current under a fault condition) are bonded at the sub panel.

If the neutral wire were to fail, the electricity would attempt to return to the source via the ground wire or earth. There now exhists a hazzard that if you were to touch the metal parts of any part of the system (panel enclosure, metal frame of appliance or box etc) you could become a path to ground.

If the ground and neutral are not bonded at the sub panel, and the neutral comes open, your stuff just will not work.

To answer the next most obvious question: Does this create the same hazzard at the main pane? The answer is yes. In some countries the electrical systems are ungrounded. In this country we choose to offer the protection from lightning provided by this bond over the open neutral hazzard. This connection is mostly to protect the utilitie's and your equipment from lightning strikes down the line.

Nothing will protect your home from a direct strike.
I have a similar situation in where I only have three wires run to a sub-panel on a pier. Are you saying it's safer to leave the neutral and ground bar unbonded? There is a ground rod attached approx 110' from the sub-panel connected to the grounding bar.
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Old 12-31-2007, 02:21 PM   #7
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Detached Garage Sub Panel Grounding Q


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Originally Posted by jerryh3 View Post
I have a similar situation in where I only have three wires run to a sub-panel on a pier. Are you saying it's safer to leave the neutral and ground bar unbonded? There is a ground rod attached approx 110' from the sub-panel connected to the grounding bar.
Absolutely not!!!!!!!!!!

I am saying that it is safer to run a four wire feeder to replace the current 3 wire feeder. However the current code allows what you have to stay.
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Old 12-31-2007, 02:28 PM   #8
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Detached Garage Sub Panel Grounding Q


Quote:
Originally Posted by jwhite View Post
Absolutely not!!!!!!!!!!

I am saying that it is safer to run a four wire feeder to replace the current 3 wire feeder. However the current code allows what you have to stay.
Thanks for the reply. I'll leave them bonded. I plan on running a seperate EGC sometime in the spring. Got a 150' fiberglass tape you can lend out??? Besides that, whoever installed this mess jumped off one of the legs and ran a GFCI outlet for a 12A pool pump. I need to run a seperate circuit for that too.
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Old 12-31-2007, 02:42 PM   #9
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Detached Garage Sub Panel Grounding Q


Jerry,

The key is, after you run the EGC, is to remember to unbond the ground and the neutral.

Fishing 150' through conduit with wires in it is no fun. If I was doing it I would sacrifice a wire in the pipe and use it to pull all my new wires in as well as my pull wire's replacement. Of course, I'm typically not buying the wire.

Great post, as usual Stubbie.



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Old 12-31-2007, 03:47 PM   #10
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Detached Garage Sub Panel Grounding Q


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Originally Posted by Andy in ATL View Post
Jerry,

The key is, after you run the EGC, is to remember to unbond the ground and the neutral.

Fishing 150' through conduit with wires in it is no fun. If I was doing it I would sacrifice a wire in the pipe and use it to pull all my new wires in as well as my pull wire's replacement. Of course, I'm typically not buying the wire.

Great post, as usual Stubbie.



Andy
Thanks again. I've been reading Mike Holt's Understand the NEC, grounding and bonding chapter for the last two days and I'm starting to go in circles from touch potential, surface voltage gradiants, fault return paths, etc. Seems like minimum code and best practice are not always the same.
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Old 12-31-2007, 03:53 PM   #11
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Detached Garage Sub Panel Grounding Q


No, they are not. Even if I ran rigid metal conduit all the way to the detached subpanel I would still run an EGC, even though I know the rigid is actually a better path all by itself. Grounding and bonding confuses experienced electricians.

As noted above, the '08 code will eleminate the exception for having the neutral and ground bonded in the subpanel if no other metal paths exsist.
The NEC is definitely a design guide.
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Old 01-01-2008, 05:14 AM   #12
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Detached Garage Sub Panel Grounding Q


Hey guys - This is neat, especially the diagrams Stubbie found. A further question about a ground rod in the detached building. In my case the feed is 4-wire (70 ft) from the garage to the house. I'm off grid and originally had the inverter system in the house with a feed to the garage, but then I moved all the panels/batteries/inverters to the garage, in effect switching what was the main vs subpanel.

Question concerns the ground rod at the house (now subpanel). I always assumed it was better to have ground rods at both ends as long as they were tied together. But, years ago, several electricians (and one inspector) told me, no, do not use a second ground rod in the detached structure, so I haven't been doing it. But now I see you guys saying that it's not only a good idea but code requirement. Right? That's 2005 code? Wow, I missed it completely. Thank you. So, I will reconnect the house (subpanel) ground rod this afternoon (keeping neutral and ground completely separate at the house end, subpanel box grounded etc etc.). FYI there are copper water pipes underground between the two structures.

Further question: Since the subpanel grounding electrode can't clear faults to ground, what purpose does it serve? I can see it maintains system reference ground at same potential everywhere. Does that help avoid stray voltage problems? Or is it mainly insurance against surges during a lightning strike (or near miss)? How does that work? This is a high-lightning area. I have arrestors everywhere on the DC side of the system and a surge protector on the main panel but I have (fortunately) never seen the effects of a strike. Any hints or even guesses as to what sort of mess I might end up with if a I do get a hit, and what further precautions I might take would be much appreciated.

Incidentally, aerial rods on both structures, each rod tied to yet another grounding electrode directly beneath the rod, which are both tied to the respective panel grounding electrodes.

Thanks

Phil
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Old 01-01-2008, 05:24 AM   #13
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Detached Garage Sub Panel Grounding Q


The rods are for lightning and POCO high voltage occurences.
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Old 01-01-2008, 06:22 AM   #14
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Detached Garage Sub Panel Grounding Q


Andy -- Yup, I know that. And, thanks, it sounds then like everything I'm doing is OK. And there is no POCO so that leaves lightning. So then what would be helpful is any thoughts on how a lightning strike might actually act. Is the idea that, if I get a direct strike on one structure, the ground on the other structure might dissipate enough current to avoid at least some damage there? Maybe same if lightning nails a tree near both structures? Anybody point me to a good article on examples of lightning-caused damage to residential electric systems? Hey, maybe I answered my own question (with your help)-- that finally looks like something I could Google. If I find anything good I'll post it.

Best wishes,

Phil

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