Ground rod installation for shed building required or not?

[Danny89]

Sunday funday. So to my understanding in a residential setting it is required to install a ground rod for a new sub panel install at the shed building where the panel will be installed. Am I correct?

Here's what I would do.
1) PVC underground dig trench dig it deep enough by referencing burial depth chart.

2) pull in 4 wires to match 50 amp load of new sub panel...(2 hots,neutral, and ecg)from main panel of home to subpanel at shed.

3)drive ground rod in ground 8 feet down til just above grade.

4) don't bond mbj at subpanel. It's already bonded at main panel.

5) run and connect ground from sub panel to ground rod at shed.

6) terminate 4 conductors from main panel to subpanel.(2hot,neutral and egc.)

7) distance is roughly 100 feet away from house to shed.

Let me know what you think. Happy Sunday to all.






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[infinity]

Sounds fine but you'll need two ground rods or prove that one is 25 Ω or less and the rod(s) need to at or below grade to the top.

 

[zbang]

So to my understanding in a residential setting it is required to install a ground rod for a new sub panel install at the shed building where the panel will be installed. Am I correct?

That's pretty much true for any structure that's fed by more than a branch circuit (residential or not, see 250.32). Otherwise, that all looks OK.

 

[LarryFine]

Sounds fine but you'll need two ground rods or prove that one is 25 Ω or less and the rod(s) need to at or below grade to the top.

Interestingly, I've never been required to provide more than a single rod for a detached building.

 

[infinity]

Interestingly, I've never been required to provide more than a single rod for a detached building.

You're not required to install any rods but you are required to connect to a GES or a single electrode is none are present. A single rod needs to be 25 Ω or less to qualify as an electrode or two rods.

 

[Danny89]

Awesome thanks guys!

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[Greg1707]

Common grounding electrode

Common grounding electrode

You're not required to install any rods but you are required to connect to a GES or a single electrode is none are present. A single rod needs to be 25 Ω or less to qualify as an electrode or two rods.

So, would it be permissible to run the GEC back to the house and tap the existing Grounding Electrode System?

 

[zbang]

So, would it be permissible to run the GEC back to the house and tap the existing Grounding Electrode System?

Essentially, you do, but it's an ECG from the house to the shed.

Possibly oversimplifying the whole thing (and some exceptions apply)--

every structure must have it's own GES, which is composed of grounding electrodes (rod or rods, ufer, plate, etc) and a GEC connecting them (house is a structure, shed is another structure, etc).

if the structure is fed by a feeder (not a service), that feeder must contain a ECG (more exceptions, not relevant to most new installations).

the structure's GES must connect to the ECG in the feeder.

In the end, run a 4-wire feeder from the house, pound two rods at the shed, connect the rods to the grounding conductor from the house and to the ground bar in the subpanel. Profit .

I'm sure there's a Mike Holt graphic of all this.

 

[Greg1707]

shared GES

shared GES

Can both structures share the same GES? For example, if the ground rods for the house are close by is there a need to install additional rods or can the nearby house rods be used?

 

[LarryFine]

They can be shared and, if close enough, should be.

 

[petersonra]

They can be shared and, if close enough, should be.

Well I am inclined to agree but I don't know that the code actually says that.

 

[infinity]

Well I am inclined to agree but I don't know that the code actually says that.

I agree the wording is a bit ambiguous because it does not say that a separate GES or electrode is required for each structure.

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

 

[LarryFine]

Well I am inclined to agree but I don't know that the code actually says that.

I based my statement on the "if available must be connected" and "all electrodes bonded together" rules.

 

[PaulMmn]

Sounds fine but you'll need two ground rods or prove that one is 25 Ω or less and the rod(s) need to at or below grade to the top.

All the way to the top? Or would a 9' ground rod with a couple of inches showing meet the requirements (ie with a clamped on ground wire back to the box)?

 

[hbiss]

Interestingly, I've never been required to provide more than a single rod for a detached building.

You're not required to install any rods but you are required to connect to a GES or a single electrode is none are present. A single rod needs to be 25 Ω or less to qualify as an electrode or two rods.

I agree with Larry. Does anybody know what year the Code was changed to allow two ground rods if a single is more than 25Ω ?

-Hal

 

[zbang]

I agree with Larry. Does anybody know what year the Code was changed to allow two ground rods if a single is more than 25Ω ?

Before 1984 (that book was handy, it requires the second). I'd wildly guess in the '50s or '60s, but that's before my time.

 

[infinity]

All the way to the top? Or would a 9' ground rod with a couple of inches showing meet the requirements (ie with a clamped on ground wire back to the box)?

Sure even a 10' rod could be used if you only wanted to pound in 8' and leave the rest sticking out of the ground. Good luck convincing the inspector that it's a 10 rod.

 

[mopowr steve]

Something tells me that it was written that the ground rod should be driven to below grade if not to a point below frost line (if you have that). Reason I say this is because I have noticed some rods above grade tend to be pulled up every season due to the freeze, albeit in small amounts but after 10 years all of a sudden you find 2’ sticking out of the ground.

 

[kwired]

Something tells me that it was written that the ground rod should be driven to below grade if not to a point below frost line (if you have that). Reason I say this is because I have noticed some rods above grade tend to be pulled up every season due to the freeze, albeit in small amounts but after 10 years all of a sudden you find 2’ sticking out of the ground.

If an 8 foot rod doesn't get you below frost line - it's too darn cold.

 

[kwired]

I agree with Larry. Does anybody know what year the Code was changed to allow two ground rods if a single is more than 25Ω ?

-Hal

Mentioning of 25 ohms and additional electrodes if over 25 ohms goes back a long way AFAIK. Oldest code book I have is 1987 and that is mentioned in there.

What did change somewhat more recently is in 2011 they rewrote things.
Before 2011 it always said something to the effect that if it didn't have a resistance of 25 ohms or less you needed to provide an additional electrode. 2011 and after it says you must have an additional electrode, but then they added an exception that says if a single electrode has resistance of 25 ohms or less the additional electrode is not required.

This ultimately eliminated having someone say "I'm not certain if my rod is 25 ohms or less" and not driving a second rod. Now with the wording like it is, you must use a second electrode unless you prove the first is 25 ohms or less.

I know most everyone around these parts only drove one rod never measured the resistance and moved on until that change in 2011. Now it is simpler and even less expensive to just drive two rods than to mess with proving the first one is low enough resistance.