ECN Forum Forums Code Related Discussion NEC & other Code issues sub panel detached building

George...it IS a fine point, and one I've never seen contested- or even discussed.

I absolutely agree that the separate building needs a ground rod; lightning just might hit it, after all!

But, as I read the "25 ohm" requirement, that applies to the ground rod at the service drop, and every other ground rod- even on a seperate building- is a 'supplimental' rod for that requirement. I do not see a requirement for the "25 ohms" to be met for each and every rod, or structure.

How would you test it, anyway? I can't imagine you separating the two for testing, when they are required to be connected to each other.

Reno- When you need to check the resistance of a rod, you can disconnect it from the system and test it independent from the rest of the Grounding Electrode System. I've tested individual rods and gotten 40 to 50 ohms and then added a rod by tying it to another rod and in my testing the total resistance will drop well below 25 ohms. I think you and I will have to agree to disagree on the need for 25 ohms or less on a rod at the second building. As a side note, we always say the "resistance of a ground rod" when all ground rods have the same resistance. It not really the rod itself that we are measuring but instead the path of resistance offered by sending current through the earth using the rod's surface.

In the twisted things someone comes up with reading artice 250 I have a question.
Can I just bury a 10' stick of copper pipe and call it "A metal underground water pipe in direct contact with the earth for 3.0 m (10 ft) or more..." as in 250.52(A)(1). I don't even have a burial depth on that one.

Greg- 250.53(G) says 30" . You could call it a water pipe electrode only if it had water running through it IMHO.

Getting back to Reno's theory, where he thinks maybe one rod that don't meet the 25 ohms or less test is okay at a second building on the property, I have this added comment. If this were true than if I had say five houses on a block that were on the same transformer, only one of them would need to have their Grounding Electrode System meet the ground rod resistance requirement and the rest of the homes would only need to have a single rod electrode (assume water included) because they are only suplemental. And the beat goes on

renosteinke Wrote: Reno
I have copied the sections that would appear to apply. What are we missing? Were is the language that would allow us to treat the required grounding electrode system as a 250.54 electrode. Supplementary electrodes are "connected to the equipment grounding conductors specified in 250.118" and no I don't think connection via the Grounding Electrode Conductor is what the code making panel intended.

The last sentence of 250.32 reads in part "the grounding electrode(s) required in Part III of this article shall be installed." 250.54 reads in part
"250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted"
Supplementary electrodes are not "the grounding electrode(s) required in Part III of this article."
In short I think you are off your soundings on this one. The language is clear in requiring the installation of "the grounding electrode(s) required in Part III of this article." Since that means that single rod and plate electrodes must have a resistance to ground of twenty five ohms or less I have to believe that two electrodes are required.

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.
III. Grounding Electrode System and Grounding Electrode Conductor
250.50 Grounding Electrode System.
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system. Where none of these electrodes are available, one or more of the electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.
250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.
250.56 Resistance of Rod, Pipe, and Plate Electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

I agree with Tom, but think the following would apply and may be where Reno was going.

I would think you could say the rods at the house augment the one at the outbuilding but only if the EGC in the feeder is of sufficient size to qualify as an electrode bonding conductor. For a feeder with #8 ungrounded conductors, you need a #8 EGC in order to count it as an electrode bonding conductor. The #10 the OP used would not be large enough. I think the intent is that the augmenting rod be near the detached structure but no closer than 6' to the primary rod. But there is no code I can find that mandates a maximum limit on the distance.

Mark---by golly, I think you have it! I forgot about the reduced size of the ground wire that is likely to be run! I certainly follow your reasoning in that the ground wire needs to be at least the size of the wire form the "main" panel to the first ground rod.

As for all the homes on the block being served by one rod at one house...well, that kind of ignores the role of the service equipment, doesn't it? My position has been that the 25 ohm requirement, the "system" as it were... is defined as everything served by the main disconnect. Short form- one meter, it's one system.

Now, as for "all the homes...."- oddly enough, we can see something like this in apartment buildings, where multiple meters will be tied to the same ground rod. If it's a smaller building (6 or less meters), then there might not even be a main disconnect.

The 25 ohm requirement is something of a "red heiffer" (for non-bible scholars, I mean "paradox") in the code. One rod needs to met it- but add a second, and there is no requirement for the combination to meet it. I always wondered about that.

The original poster started off saying that , of course, he would have two rods. I see nothing in the code that ever would require two rods anywhere, if the first one meets the 25 ohm test. So that question still stands- why two?

I guess that is why "water pipes" have to be supplimented with another electrode. If you didn't pay your bill and they shut the water off, you wouldn't have an electrode anymore.

sorry

Greg, what are you sorry about?

George can afford to pay his water bill.

Besides I thought it was funnier than H***.

Actually the strange thing is we had people spec'ing 1/2" copper pipe as bonding conductors under computer room floors for a while. Some engineers felt that since high frequency transients only travelled on the surface of the wire a pipe was as good as a solid conductor of the same diameter