i have a sub 50 amp sub panel that i want to put in a detached maint/gar building.
to save money on wire i am going to run two #6's for the lines and one #8 for the neutral(derating the neut)along with a #10 ground. my question is does this sound code compliant, we derate neutrals for the service feeders and this is going to be a feeder so it seems very complicit to me.

am i on the right page? any codes that i am missing which would forbid this derating?
thanks guys,
h20

oh yea it is a metal building which i will bond the beam structure and will drive two ground rods down at the building too.

By not knowing what your load is, I can't tell you that you can or can't derate the neutral. Also, I would be interested in how you terminate the neutral and ground wires in the sub panel. Where do you connect the Grounding Electrode Conductor in the sub panel?

There's no need to install ground rods if you run a separate equipment grounding (bonding) conductor (E.G.C.) to the detached garage.

See more about this topic here: https://www.electrical-contractor.net/ubb/Forum1/HTML/006874.html

And here: https://www.electrical-contractor.net/ubb/Forum2/HTML/002417.html

Hold on. You must ALWAYS install a grounding electrode system for a detached building with more than a single multiwire branch circuit. It doesn't matter if your have three or four wires; with or without an equipment grounding conductor.
see 250.32

The neutral may only be derated if there is sufficient straight 240 volt loads to warrant it.
see 220.22

If your neutral busses are specifically listed at 75C and you use 75C rated wire, then #8 copper is fine for a 50A feeder because that wire is still rated at 50 amps.

My bad.

Yes, a grounding electrode MUST be installed on any detached building.

...that is fed with a feeder.
If you only run a circuit no rod is needed.
See: 250.32

"Saving money" often turns out to be an expensive choice.

Compared to the costs of digging the ditch, running your circuits, etc., the cost of feeder wire is quite small.
At least run your wire in pipe- pipe at least one size larger than you need- so you can replace it easily if you ever need to. If you fail to run in pipe, or fail to bury it two feet down (minimum), my experience tells me you will be replacing it after a few years. Why dig twice?

Two ground rods? Why? One is plenty- though it would have been better to have placed a "UFER" in the slab. Oh, well, hindsight and all that!

My practice -and please note that there is room in the code for a few different approaches here- is to:
-Run a ground wire from the "main" panel to this "sub" panel, landing it on a ground buss (separate from the neutral buss); and,
-Plant the rod, and connect the wire from it to this same ground buss.

Of course, I make sure the neutral buss is NOT bonded to the case.

Hey Reno, what about 250.56?

i am in washington and two ground rods are required by WAC (washington administative code) unless you pay to get an engineering report testing the ohms to PROVE you have 25ohms or less and even then the AHJ's dont like it and you have to go to their supervisor to get a blessing...not worth the hastle.

george the garage will have little load on it lights (some flourescent mostly incandescent) few plugs for power tools 120 volt(drill,saw ect...)garage door opener.
and a small office in it more plugs a 2000 Watt heater and a computer in it. maybe a future rv plug down the road. are harmonics going to come into play here?

the gec will goto the ground bar(floating neut) in the panel with the rods and building bond, is this what they mean single point grounding?

if buss is rated at 75 degrees then its ok,
how/where is that listing?

thanks again guys,
h20

The 75 degree reference may be refering to 110.14(C) for termination temperature ratings.

What do you mean by "floating neutral"?

If your panel buss and breakers are rated for 75 degree C and the grounding buss is bonded to the metal enclousure and the neutral buss is "floating" (not bonded to the enclosure then it sounds like you are going to be code compliant. The grounding electrode needs to go to the grounding buss and the panel must have a main and the neutral must be full size based on what you said about the load. The grounding electrode conductor minimum #8 but most installers will run #6 because it can be installed without being in conduit. No concern about harmonics. Single point grounding is another topic all together and what you are doing is just plain old grounding.

250.56...the "25 ohm or less" requirement....good point, George!
And Irrellevant.

I sat "irrellevant" because this part of the code addresses the "grounding system." As the garage being discussed is a separate structure, and not a separate service....any ground rod driver would technically be supplimental to whatever there is at the main service.

Now, some may say 'but the garage is so far away.' In this case, "far" is a good thing. As documented in the American Electricians' handbook, as well as implied by the NEC requirement that supplimental electrodes be at least six feet away, the farther apart the ground rods are, the more evvective they are. Seems backwards, but there it is!

Reno- Read 250.32(A). Each separate Building or Structure fed from a common Service needs it's own Grounding system as discribed in Part III. I understanding your reasoning but every inspector I know asks for a Grounding Electrode system at each building where there are more than one circuit.

so george "the neutral must be full size"
then #8 would be full size since it doestn't have an * and is rated at 50 amps.
correct?


h20

Look for a label in the panel that says something like "Neutral terminations rated 60/75C". If you have that, then you can use the 75C rating of your wire.

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

I would like to put a little debate back in this thread if I might.

We are told in 250.32(A) Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

Now let’s look at 250.50.
250.50 Grounding Electrode System.
All grounding electrodes as described in 250.52(A)(1) through (A)(6) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.

Here we are told to bond any electrode found in 250.52 together and if none are present to install one that is outlined in (A)(4) through (7).

After reading both of the sections as referenced to from 250.32 I can not find anywhere that it says to keep going to 250.53 nor 250.56. With out being referenced to go there I ONLY need to fulfill what is mandated by 250.32(A).

So when gfretwell Made the statement to bury 10 foot of pipe and call it an electrode he was not to far from wrong. If the last 10 foot of water line was metal pipe then he would be right simply due to the fact that 250.32(A) only refers to 250.50 and neither 250.32, 250.50 nor 250.52 refers to any other section.

A proposal has been submitted to have the reference in 250.32 changed from 250.50 to part III of 250 for the 2008 cycle.

I agree George. THE rod at the second building or structure is Supplementary to the ECG and not Supplemental to the grounding electrode.