Minimum number of Grounding Electrodes

[pierre]

What is the minimum number of Grounding Electrodes required at premise wiring systems that are required to be grounded?
One scenario:
A small dwelling that utilizes a Ufer (concrete footing)grounding electrode system. I then 'bond' to the cold water and any other system that requires bonding.

Another situation could be a storage building that has no water and utilizes the Ufer ground.

In those and any similar situation, am I REQUIRED to have additional grounding electrodes? Am I required to drive ground rod(s)?

Pierre

 

[charlie]

Re: Minimum number of Grounding Electrodes

"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."

If you do not have a water pipe, building steel, or a concrete enclosed electrode that meets the requirements then you are permitted to use "one or more" of a ground ring, rod or pipe, plate, or other local metal underground systems or structures.

Pierre, 250.53(D)(2) only requires the water pipe to be supplemented by an additional electrode. None of the other electrodes are required to be supplemented. This rule is not because the water pipe is not sufficient, it is, but it is likely to be rendered useless with someone disconnecting the clamp or a plastic union being inserted.

With all this said, the answer to your question is no and yes if you do not have any electrodes, no if another one exists unless it is water.

 

[tom baker]

Re: Minimum number of Grounding Electrodes

If you install a ufer ground, then you are done, unless there are other qualifying electrodes such as strucural steel, which then is bonded to become part of the GES, as charlie points out.
If it is a time and materials job, you can drive two ground rods, but they are not required with a Ufer ground system.

Ground rods are often the only electode available, hence we often "drive two and go home".
A ground rod is frequently (not required, it could be a ufer ground )used to supplement a metal underground water pipe. Most dwelling units today use plasic water lines to the building, so
two ground rods are installed, but the requirement is for a supplemental grounding electrode. If you really want to install a time and materials electrode, put in a ground plate (you have to bury it, not drive it).

 

[ryan_618]

Re: Minimum number of Grounding Electrodes

I agree with the other gentlemen's thoughts. One of the hardest things I try to convince electricians of is that "two forms of grounding" requirement is a myth.

 

[trickett]

Re: Minimum number of Grounding Electrodes

Word of caution.... When a vapor barrier, such as plastic, is used between the concrete and the earth, the concrete is no longer in direct contact with the earth and the value of the earth to concrete connection is destroyed. In this case it could not be used as a concrete encased electrode.

Electrical Grounding 6th edition Ronald P. O'Riley

 

[ryan_618]

Re: Minimum number of Grounding Electrodes

How often do you see a vapor barrier underneath a footing?

 

[jimwalker]

Re: Minimum number of Grounding Electrodes

"How often do you see a vapor barrier underneath a footing? "
Very common in Fl.And i believe is required now.

 

[roger]

Re: Minimum number of Grounding Electrodes

Jim, when did this start and in what parts of FL?

Roger

 

[jimwalker]

Re: Minimum number of Grounding Electrodes

A rough guess would be 1o years ago.Keep in mind Fl has far more moisture problems.As to it being a rule im not sure but i see it all the time.The visqueen goes over the slab and into the footer ditch.We even tape the seems.But when the form boards are removed the side will have dirt up to it,so i think it still helps as a ground.And as of about 6 years ago Collier county (Naples) only used the rebar,and no rods.
Might add that the 6 x 6 road mesh in the slab is attached to the rebar.A pretty healty ground i think
As to where i have only wired in NAPLES an Tampa,so can't say if others doing it

[ February 08, 2004, 06:19 PM: Message edited by: jimwalker ]

 

[Guest]

Re: Minimum number of Grounding Electrodes

Are you guys saying that the Class A bond to the water pipe is not required to be supplemented by the common 2 outside rods? In a dwelling? Now iM CONFUSED

 

[iwire]

Re: Minimum number of Grounding Electrodes

Originally posted by charlie:
250.53(D)(2) only requires the water pipe to be supplemented by an additional electrode. None of the other electrodes are required to be supplemented. This rule is not because the water pipe is not sufficient, it is, but it is likely to be rendered useless with someone disconnecting the clamp or a plastic union being inserted.