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Too many ground rods?

NatW's picture

Hi,


I am a Breaktime lurker who does a fair amount of DIY and charity work. I am currently moving the electrical service at my house in a suburb of Columbus, Ohio. Things are going well, but I was told today I shouldn't use two of my ground rods.


The project involves moving the 200 amp aerial service currently going to the house back to a new mast mounted on the garage, and then underground to two subpanels in the house. 


Outside the garage at the new service entrance I have buried two ground rods (only one required in Ohio) connected to the ground bus on the new main panel with one #6 bare copper wire. Each of the two feeds going to the subpanels in the basement has a green #4 aluminum ground wire running from the new main panel in the garage to the subpanels in the house basement.


Today the inspector approved the trench/buried cable and main panel/meter box with no problems, so I'm good to have the electric company move the service and started to fill my trench back in (water table averages around 30 inches - starting to have mosquitos). I have not hooked up the subpanels yet.


I am also burying conduit for someday moving cable/phone underground, which would be a metallic connection between buildings. My plan was to install two more ground rods outside the house (one is already driven), with a separate wire going to each subpanel. This is based on things I have read here and in books, and I have already driven one of the rods. However, the inspector told me not to connect ground rods at the house to the panels - I should only have one ground location at the main panel which is now in the garage.


The obvious thing to do here is to not hook up the ground rods outside the house. The inspector has been very good to work with and isn't asking me to do any more unnecessary work. However, current opinion here (and possibly NEC 2008??) seems to be that I should have ground rods for both buildings. I don't see how bonding to the house plumbing is any different than a ground rod at the house, except that I'd rather any stray current stay away from my plumbing.


On a related note, when I remove the current main panel in the house, I will install splice boxes for about six or seven circuits that are moving to the new subpanels. There is already a ground wire/rod in this location and I was thinking of connecting them to the circuits I was splicing for additional ground (hooking all the grounds together, so they would still be grounded back to new subpanel). I don't see any need to do this, but don't see how it could hurt, either.


So, would you leave it with ground rods just at the garage, or would you go back after final inspection and hook up the rod I've already driven for the house subpanels? Am I sacrificing any safety by not connecting the rods?


Thanks in advance for any input.


-Nate

(post #106605, reply #1 of 19)

There is a lot of confusion between "ground" and "bond" requirements, I would suggest going to the library and reading the NEC 250.32 of the locally adopted version and perhaps picking the brain of a local electrical contractor.


There is no "too much grounding", the NEC is the minimum requirements not the max, I'm working on a building that has well over 200 ground rods. Currently in the State of Washington you would need two ground rods at each building. The only time a single ground rod is used is on a temp. power pole.

(post #106605, reply #4 of 19)

pye,


Thanks for the reply. According to the power company service manual, Ohio only requires one rod, but a footnote states that other surrounding states require two. I drove two at the service entrance to play it safe. The inspector appeared a bit surprised to see two at the service entrance, but didn't say anything.


The inspector did not like that the 3rd rod was at the subpanels at the house, rather than connected to the service panel at the garage.


I thought that you were supposed to place ground rods at the subpanel when in a separate building, unless there is no metallic connection. I don't presently have a metallic connection other than the feeds, but plan to bury cable/telephone between the garage and house in the future so I figured I'd put the ground rods in now while doing everything else.


Is there a potential harm in having ground rods in different locations that I am not seeing?


Thanks again,


-Nate

(post #106605, reply #5 of 19)

My interpretation is that when the sub-panel is in a separate building, you need to have a second ground, besides connecting the ground back to the main panel.

Thus I'm with the OP, and not the inspector on this one.

IN addition, my interpretation is that the point of connecting the plumbing (and gas) supply lines to the ground is to ground the pipes, not have the plumbing act as a ground.

But I'm not an electrician.

(post #106605, reply #7 of 19)

Is there a potential harm in having ground rods in different locations that I am not seeing


Like Bill said, not there for 'stray current' whcih can result in possible accelerated corrosion.


First, do whatever the inspector says, then connect later, after analyzing the situation for potential corrosion problems.


Own house has concrete encased rebar below water table, 2 well casings > 100 ft away, a shed footing with welded rebar, and a few buried steel cables.  All tied together, plus a buried car chassis or 2 (which were used as shed footings) also tied in.  Did put a microammeter datalogger on the connection to the well  casings to make sure they were not going to get any accelerated corrosion, could care less about the buried chassis<G>

(post #106605, reply #11 of 19)

junkhound,


I am not planning to have any current on the system except when things go wrong - "stray current" was a bad choice of words and should have been ground fault. I don't have a microammeter, and haven't used dataloggers in about 10 years, so I'll have to assume there isn't much current running around.


Ground rods are in a clay/loam with the water table around 30 inches. I don't have the equipment to measure resistance, but have to assume I'm doing pretty well. House was built around 1924, and garage has no foundation worth talking of, so grounding to rebar isn't an option. City water - no well. (There is rumor of an old cistern under the fence between me and the neighbor). Cars are old, but no desire to bury them presently!


Thanks,


-Nate

(post #106605, reply #2 of 19)

Bump for the better informed.


Read post about 4 times, situation as I understand it:


Proposed:


200 amp SEP at detached garage, aerial feed from POCO pole. Currently has 2 ground rods driven and attached to SEP with #6 bare copper. (Note: Here, I would be required to use #4 bare copper for 200a service.)


Garage SEP feeds 2 subpanels at house via underground feed of some sort - wiring method not mentioned - metallic conduit? PVC? #4 aluminum ground conductor is a part of this wire run from SEP to the subpanels. (Two - 100a runs perhaps? Al 2-2-2-4?)


Metallic conduit run installed underground for TV, phone, etc. ("metallic connection between buildings").


All of the above is OK with your local inspector.


You propose driving 2 ground rods at house and connecting subpanels - inspector says "do not connect these rods to the subpanels"


Somehow plumbing factors into this situation as does at least 1 additional ground rod (at the house) connected to some splice boxes.


Clarify for those who follow?


Jim


Never underestimate the value of a sharp pencil or good light.


Edited 7/11/2008 4:53 pm ET by JTC1

Never underestimate the value of a sharp pencil or good light.

(post #106605, reply #3 of 19)

Thanks for the bump:


You are correct: 200 amp SEP at detached garage, aerial feed from POCO pole. Currently has 2 ground rods driven and attached to SEP.


Underground feed is via 2-2-2-4 aluminum, buried 30 inches. Two-inch PVC conduit taking it vertically to depth on each end of run with sweep on bottom, no conduit in between.


Approximately 12-18 inches above this is one-inch PVC (not metallic) conduit running the entire length of the trench. I plan to run telephone and cable through these someday. No plans for plumbing to garage.


Inspector wants ground rods only at SEP - where I already have two. We are OK there. I had also driven one rod near the house subpanels and was ready to drive the second, but he told me not to connect them.


He wants plumbing in the house grounded (presumably bonded to subpanels which are then connected to SEP and ground rods at garage).


Main question: is it correct/safe/unsafe to only have ground rods at SEP in garage, and none at the house? I thought the rods were supposed to be in both locations when you have a subpanel in a separate building.


On the plumbing: if I have no ground rods at the house, wouldn't any stray house current that found its way to the ground be more likely to travel through the copper water service entrance pipe than back to the SEP at the garage and then to ground rods?


On the splice box: is there any useful purpose for the old ground rod from the old SEP at the house? It is not close to the new subpanels. Can I connect it into the house ground wires away from a panel, or should I just cut it off and forget it's there? This question is, of course, irrelevant if I shouldn't have any ground rods at the house.


Hope this clarifies.


Thanks again,


-Nate

(post #106605, reply #6 of 19)

"He wants plumbing in the house grounded (presumably bonded to subpanels which are then connected to SEP and ground rods at garage).

Main question: is it correct/safe/unsafe to only have ground rods at SEP in garage, and none at the house? I thought the rods were supposed to be in both locations when you have a subpanel in a separate building.

On the plumbing: if I have no ground rods at the house, wouldn't any stray house current that found its way to the ground be more likely to travel through the copper water service entrance pipe than back to the SEP at the garage and then to ground rods?"

The purpose of a GROUND ELECTRODE SYSTEM is not to handle "stray currents" (whatever they maybe).

The purpose of local ground electrode system is to provide a path for external surges from lightning or surges on the high voltage transmission lines.

Several things can be used for a ground electrode system. For residential construction the most commonly used are water pipes, ground rod(s) and concrete encased rebar or copper wire in footings for new construction.

For the water supply pipe to be used it must at least 10 ft long and metallic for the underground portion. Because it might be replaced in the future with plastic piping it the code requires that it must be used it available, but can't be the only ground electrode.

The commonly used seconard ground electrode are ground rod(s). The code say that unless the ground rod is tested to be less than 25 ohms then 2 rods must be used. The test proceedure is speicalized and not commonly done. Thus it is common to use 2 ground rods for a ground electrode.

As you can set the resistance of a ground rod can be vary high and much to high to trip a breaker from a ground fault. The clearing of ground faults is the purpose of the equipment grounding conductor.

The purpose of bonding is to insure than any metal parts of a building that might become electrified are all at the same potential. Thus water pipes that are metallic inside, but don't qualify as ground electrode are bonded to the "ground system". In this case the equipment grounding conductor at the sub panel.

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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
. William the Geezer, the sequel to Billy the Kid - Shoe

(post #106605, reply #8 of 19)

Bill,


Thanks for the reply. By "stray currents" I was referring to ground faults, which would be cleared by the ground conductors. The conductors would take it to earth following the easiest path - either through the plumbing to earth or back through the subpanel feeds to the service entrance panel and then the two rods in the earth.


Surges from power transmission lines would be intercepted at the service panel rods. That makes sense to me.


I'm still a bit perplexed as to what happens to surges at the house. I have two ground rods at the service entrance at the garage. How would an additional ground rod (or two) installed at the house be any different than connecting to the copper water supply pipe?


The inspector doesn't want ground rods at the house, which is detached from the garage. Is there a danger that surges from the power transmission lines could enter the service panel, but follow the underground feeds to rods at the house rather than going straight to the rods at the service panel?


Thanks,


-Nate

(post #106605, reply #9 of 19)

"I'm still a bit perplexed as to what happens to surges at the house. I have two ground rods at the service entrance at the garage. How would an additional ground rod (or two) installed at the house be any different than connecting to the copper water supply pipe?"

It isn't any different. Depending on the local ground conditions, water line depth, etc the water pipe is probably as good or better than the ground rods. Years ago just the water line was used.

The reason for also using ground rods or other electrodes is that the code realizes that at some point in the future that the water line might get replaced with plastic. And done by an plumber and with a HO that has no idea about the grounding system.

"The inspector doesn't want ground rods at the house, which is detached from the garage. Is there a danger that surges from the power transmission lines could enter the service panel, but follow the underground feeds to rods at the house rather than going straight to the rods at the service panel?"

First of all I suspect that the house is fairly close to the garage and with the run under ground there is not much that would affect one differently than the other.

So, in practice, it is not that mcuh different.

And if strike hit the ground between the 2 buildings there would be no way to prediction what would happen.

But there is absolutlely nothing wrong with having more ground elecrodes (ground rods in this case). There is no logic, code, or engineering that I know of that supports the inspector.

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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
. William the Geezer, the sequel to Billy the Kid - Shoe

(post #106605, reply #10 of 19)

Thanks, Bill. That makes sense.


You are correct. The house and garage are only about 50 feet apart. There is around 75 feet between ground rods because I dug around a crawlspace addition to the original basement.


I think I will go ahead and connect to the third ground rod after final inspection. I hate to think I hammered that thing in for nothing.


Thanks again,


-Nate

(post #106605, reply #12 of 19)

The NEC requires a ground electrode at each building, the only exception is if it is only fed by a single branch circuit. Your inspector is wrong or you misunderstood him. They get tied together via the grounding conductor in the feeder to the second building.
If you are worried about stray current you want as much grounding and bonding as you can get. As for surges. Be sure all your incoming utilities are tied to the same ground electrode system. Don't let the cableco or telco drive a separate rod, not connected to the one your poco uses. Any differences in potential will be reconciles in your equipment that connects both together (TV/Modem etc)
The ground electrode has nothing to do with clearing faults, that is the EGC in your wiring method, through the main bonding jumper in the main panel, back to the XO on the utility transformer (tripping the breaker)

Greg

(post #106605, reply #14 of 19)

gfretwell,


Thanks for the post. When the inspector started talking about it I thought maybe he just meant it wasn't necessary, so I specifically asked him if I should connect the ground rod to the subpanel. He said no, everything should go back to the service entrance. I saw no reason to push him since it didn't cost me any more time or money, and he's the inspector, but wanted to post on BT to be sure I wasn't missing something and should hook it up later.


The information on only having one electrode ground system is good. I'll keep that in mind when I get the other utilities moved undergound.


-Nate

(post #106605, reply #13 of 19)

Nate,


I knew your post would catch Bill's eye. He is better informed than I and could explain the rational better than myself - he performed admirably, as usual.


I know what will pass inspection and what won't on a local basis, but cannot always explain the "why".


For example, my own home: 200a underground service; two rods at the SEP, connected with #4 bare copper. Additional #4 bare copper from SEP ground bar to city water entrance pipe (copper, connected to pipe about 2" inside the foundation); #4 jumper bypassing the water meter; another #4 jumper between the hot and cold pipes bypassing the hot water heater.


My sub-panel at the other end of the house (same building) has no ground rods of it's own.


If there was another sub-panel in a separate building, I would need run #xx-3 with ground cable to the other building (ground & neutral isolated at the sub-panel - neutral "floats" inside of the sub-panel). Sub-panel enclosure would be connected to the "incoming ground" from the SEP via the ground buss in the sub-panel (as would all of the ground wires from circuits contained in the sub-panel). In addition, I would be required to install 2 ground rods at the sub-panel and connect the sub-panel ground buss to them via #6 bare copper (assumes a 100a sub-panel).


In your case, I would have a tendency to follow the advice already given by others: Connect your sub-panels without any additional ground rods driven at the house - pass inspection - get your sticker - then promptly drive and connect the ground rods as per your original plan. The inspector does not live there.


I would not engage the inspector in a debate on this topic. Not all inspectors are created equal - I once had an inspector require a #6 ground wire to be "connected" to CPVC piping in a house. I just did it while he watched and affixed his inspection sticker. Ten minutes later the wire and clamps were gone.


My guess is that your inspector knows how to run a sub-panel within the same building, but has missed learning the appropriate method for one in a separate structure.


Jim


Never underestimate the value of a sharp pencil or good light.
Never underestimate the value of a sharp pencil or good light.

(post #106605, reply #15 of 19)

JTC1,


Thanks for the info, and the bump. When I created the thread it was gone from the list on the side before I even got to the home page, so I'm glad it got some attention.


What you would do is essentially what I planned to do here. Only two changes are that I have #6 bare copper instead of #4, and the subpanels will be on the other side of the basement from the water supply entrance and nearer the hot water heater. I will connect at the water heater, bypass there, and then bypass at the meter.


Ground and neutral are isolated at the subpanels. That's one of the reasons for this project. The original service to house prior to addition in 60s or 70s became a subpanel and was not wired to current standards by any stretch of the imagination. The picture shows what I mean. (Electrical tape over some of the breakers are circuits I've already disconnected). Had to install a new subpanel the day after we bought the house for the appliances to run properly. I've since replaced nearly all of the knob and tube in the old part of the house (some of it buried under blown in attic insulation).


-Nate

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(post #106605, reply #16 of 19)

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(post #106605, reply #17 of 19)

Looks like that Pushmatic panel was overdue for replacement.


Carry on!


Jim


Never underestimate the value of a sharp pencil or good light.
Never underestimate the value of a sharp pencil or good light.

(post #106605, reply #18 of 19)

I got wrote up by the county inspector for having more than one ground rod, so I just drove it below ground and once it was approved I dug up the head and hook back to it

(post #106605, reply #19 of 19)

I don't understand why these inspectors are having so much trouble with this.
Here is the picture from the 2002 NEC handbook, the 2008 makes the 3 wire feeder illegal so this is the way it has to be when it is adopted. This is still how you have always had to run the 4 wire feeder.

http://members.aol.com/gfretwell/subpanel/image3.gif

Greg

Greg