Ground rod at detached metal building?

[racerdave3]

I have an upcoming job where the homeowner has installed a metal building that will be used for a garage on his property. The garage is for personal use and the calculated load for what he has proposed is moderate at best. I am running a conduit from the main panel in the house (200 amp) out to the building and installing a 100 (sub) panel out there. Of course because of the application I will be running 4 conductors with a separate ground and neutral. Now my question in in regard to grounding out at the building. Should I drive a ground rod at the panel location and hit the panel, or drive a ground rod and hit building steel, or hit the panel and the steel as well for bonding purposes? Along the same thought process, is one rod enough or would it be better to hit 2 corners of the building? I have heard much debate about these situations, when running a 4 wire service out to a building that has the separate ground, then driving a rod and creating a parallel grounding situation.

 

[mdshunk]

You can drive an extra rod where ever you want to. Since you're running 4-wire, make sure that auxillary 250.32 required GEC lands on the ground bar, though. The building's tin and framework might already by bonded by the fact that your panel is bolted to it. I often put a lug on the tin of these metal outbuildings and run my auxillary GEC through that lug on the way to the rod. See 250.32 for your requirements for this outbuilding grounding electrode.

 

[infinity]

If the building is metal clad but without a structural steel frame it does not qualify as a grounding electrode. You will need to provide a grounding electrode if one doesn't exist. This could be a ground rod if you so choose.

 

[George Stolz]

Dave, you have many options for the grounding electrode system (GES) routing. See 250.64(F).

Is the building steel you're referring to structurally holding the building up, or is it a wood structure with steel siding?

If it is strucural steel complying with 250.52(A)(2)(1), then no other grounding electrode is required. If it does not, then it can become a grounding electrode by installing a ground rod (or two, 250.56) and bonding the building steel to it.

Bonding the steel siding is not required, but you can if you'd like.

I have heard much debate about these situations, when running a 4 wire service out to a building that has the separate ground, then driving a rod and creating a parallel grounding situation.

Parallel grounding paths enhance the grounding electrode system. The overall resistance of the system drop, because resistors in parallel have less resistance than one single resistor.

Parallel paths for neutral current (called objectionable current in 250.6) are a bad thing. We want neutral current to flow on the conductors we intend.

By installing a fourth conductor (the EGC) out to your detached building, you have all but precluded the possibility of a parallel neutral path. If you keep the neutrals entirely seperate from the EGC's, GECs, and bonding jumpers, then you should not see any neutral current flowing on the EGC out to the building, or on a common water pipe between the structures.

 

[George Stolz]

Morning Trevor, I type slow. And I took a phone call from the missus in the middle of the post.

 

[racerdave3]

Is the building steel you're referring to structurally holding the building up, or is it a wood structure with steel siding?

This is a complete steel building, it has the structual steel for the frame and is clad in the metal roofing and siding, it's one of the "kit" garages you can buy complete with everything to finish it. The building is sitting on an "Alaskan slab" that is loaded with rebar, that I would have loved to hit with a GEC before they poured it, but since the guy contatced me after the slab was in, that was not an option.

 

[George Stolz]

I wouldn't say that too loud.

Under the 2005, an AHJ could insist that you jackhammer into the foundation to connect to the CEE.

Here is a picture I just worked up to elaborate on the parallel grounding paths thing.

In the image above, the overall resistance of the equipment grounding conductor above is augmented by the interconnected metallic water pipe.

 

[antenna2001]

I was just looking at this problem the other day

I was just looking at this problem the other day

250.32 summarizes what is needed. If you were using only one hot, one neutral, and ground to supply the other structure, then a grounding electrode would not be required. However, because you have more than one hot going to the other building, the code requires that "Where there are no existing grounding electodes, the grounding electrode(s) required in Part III of this article shall be installed."

If you have the NEC 2002 Handbook, see Exhibits 250.17 and 250.18 on page 191. They clarify the problem.

 

[George Stolz]

250.32 summarizes what is needed. If you were using only one hot, one neutral, and ground to supply the other structure, then a grounding electrode would not be required.

A multiwire branch circuit is considered one circuit in applying this requirement. It has two hots.

 

[don_resqcapt19]

antenna,

If you were using only one hot, one neutral, and ground to supply the other structure, then a grounding electrode would not be required.

Not correct. If you have only a single branch circuit, no grounding electrode is required, however if you install a feeder with one hot, one grounded and one grounding conductor, a grounding electrode is required.
Don