Sharing ground rod

[makalroy]

Ive got fifty metal buildings supplied buy a generator. The gen supplies a MDP that feeds five sub panels, each sub panel feeds ten buildings, each building has a 63 amp load panel. Can I ground multiple buildings to one ground rod, or does each building need its own rod?
System bonding jumper installed at the gen. and EGC's run with all feeders

Thanks

 

[mcclary's electrical]

250.32 says each building need it.

 

[raider1]

250.32 says each building need it.

I agree.

I would also ask how many circuits or feeders are run to each building?

225.30 and 225.31 might also be a problem.

Chris

 

[Dennis Alwon]

I agree.

I would also ask how many circuits or feeders are run to each building?

225.30 and 225.31 might also be a problem.

Chris

Chris although I agree with you in principle I am not sure the op cannot do what he proposes. The NEC does not say how close the ground rods must be to the structure so why could you not run an gec back to the main building's ground rods-- totally expensive and probably a bad design but I think it is allowed. NO???

 

[raider1]

Chris although I agree with you in principle I am not sure the op cannot do what he proposes. The NEC does not say how close the ground rods must be to the structure so why could you not run an gec back to the main building's ground rods-- totally expensive and probably a bad design but I think it is allowed. NO???

Good point.

Also if each building is only supplied by a single branch circuit then the exception to 250.32(A) would permit us to omit the grounding electrode system at the buildings.

Chris

 

[makalroy]

250.32a

250.32a

once I set the first building and install a grounding electrode for it, I have an existing grounding electrode for the other buildings. Thats the way I read 250.32a

 

[raider1]

once I set the first building and install a grounding electrode for it, I have an existing grounding electrode for the other buildings. Thats the way I read 250.32a

Actually, 250.32(A) states that buildings or structures supplied by a feeder shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250.

I read this to mean that each building shall have it's own grounding electrode or grounding electrode system.

If these buildings have footing steel that qualifies as an electrode in accordance with 250.52(A)(3) or any of the other electrodes specified in 250.52 then you MUST use them as a grounding electrode unless the building is supplied by a single branch circuit in accordance with the Exeption to 250.32(A).

Chris

 

[makalroy]

Actually the buildings are already set and I installed a ground rod at each building. My problem is I have high ohm reading on the ground rods ( 35-80 ohms). I want to bond the ground rod at building one to the ground rod at building two, and the rod for building three to the rod at building four, and so on. That would give me two ground rods per building and fix my problem with out driving a second rod for each building.

What do you think, any code issues im not seeing?

 

[raider1]

Actually the buildings are already set and I installed a ground rod at each building. My problem is I have high ohm reading on the ground rods ( 35-80 ohms). I want to bond the ground rod at building one to the ground rod at building two, and the rod for building three to the rod at building four, and so on. That would give me two ground rods per building and fix my problem with out driving a second rod for each building.

What do you think, any code issues im not seeing?

I think you can do that provided that you comply with 250.53(C).

Chris

 

[augie47]

I agree with Chris.
(also, check your private messages)

 

[winnie]

Just how close together are these buildings?

How are you going to jump ground rods together?

Would the jumper between ground rods actually count as a 'ground ring' electrode?

-Jon

 

[makalroy]

The buildings are in rows and from row (a) to row (b) is about 20'.
So twentyfive, twenty foot, 6awg jumpers would be a lot simpler than 50 new ground rods.

 

[Smart $]

Would the jumper between ground rods actually count as a 'ground ring' electrode?

250.52(A)(4) requires the bare copper to be not smaller than 2 AWG and must encircle the building...

 

[SG-1]

Currently all the ground rods are bonded together via the EGC correct ?

When you are testing one rod the connection to the EGC is removed.

What is the resistance if you just remove the EGC at the first rod and leave the others connected ? I am thinking this will be very near the same value as after the bare 6AWG is installed. It will be stronger if you can run a continuous 6AWG between rods, or with as few breaks as possible.


IMO
I do not think the installation meets the letter of the NEC, two rods per structure would be required for that. I do think it meets the spirit, as this should be one killer grounding electrode system. With the close promixity of each building & rod, this installation is not specificly addressed by the code.

 

[iwire]

Chris although I agree with you in principle I am not sure the op cannot do what he proposes. The NEC does not say how close the ground rods must be to the structure so why could you not run an gec back to the main building's ground rods-- totally expensive and probably a bad design but I think it is allowed. NO???

Using that argument we would never have to install any electrodes as there is always one 'up stream' already.

 

[Dennis Alwon]

Using that argument we would never have to install any electrodes as there is always one 'up stream' already.

You would have to run an electrode conductor between all the rods which is probably more costly than driving the rods.

I just want to know where it says this cannot be done. I would not install it as such but it was brought up to me and I could not show that it wasn't compliant.

 

[winnie]

I believe that Dennis makes a key point: a GEC follows different rules than an EGC.

These rods are already connected together by EGCs. However only the rod directly connected to a building via a GEC counts as a grounding electrode for that building.

I don't see any prohibition from using one electrode to serve multiple structures, as long as suitable GECs are providing the connection.

It is not clear to me, for the particular situation described by the original poster, that running 20' for conductor, _buried to the proper depth_, would in fact be easier than adding an additional ground rod.

On the other side of the coin, it is known that increased spacing of ground electrodes lowers resistance; ground electrodes that are too close together share the same 'sphere of influence' and thus are less effective. (Consider the limit of two electrodes side by side; they act as a single somewhat fatter electrode.) Using the 'shared electrode' approach, the result is electrodes that will be better spaced than adding a new electrode at each location.

-Jon

 

[makalroy]

_buried to the proper depth_,

_buried to the proper depth_,

I dont think GEC's have a min. depth. If so where in the code can i find that.
Thanks

 

[raider1]

I dont think GEC's have a min. depth. If so where in the code can i find that.
Thanks

There is no NEC specifed burial depth for a GEC.

But GEC's need to be protected where exposed to physical damage.

Chris

 

[tom baker]

Good point.

Also if each building is only supplied by a single branch circuit then the exception to 250.32(A) would permit us to omit the grounding electrode system at the buildings.

Chris

But 225 requires a disconnecting means, if you work thru the rules it requires the disconnecting means to be suitable for use as a service disconnecting means, or a panelboard or fused disconnect.

Once you install a panelboard, its no longer a branch circuit, and then a GES is required.

The exemption in 250.32(A) is only allowed in dwelling unit outbuildings or garages, where a snap switch can be used as a disconnect.

Perhaps I'll make a proposal for the 2014 NEC for a FPN to 250.32(A) for the 225 rule.