Can someone quote the NEC about grounding to water pipes. I have a proposal on my desk from a “consultant” to install cell phone antenna on the roof of one of my buildings, and I’m not comfortable with one part of the drawings. It’s a one story building, flat roof with short parapet walls, and they will build a raised work platfor to hold the cabinets and the antenna. There is a note on the drawings that says to run a X ga (I forget what size) insulated ground wire down the side of the building and bond to the plumbhing. There is also several nice details about ground rods, but nowhere do they show on the site plans so I’m guessing they would rather bond to the pipe than punch through the pavement.
“Put your creed in your deed.” Emerson
“When asked if you can do something, tell’em “Why certainly I can”, then get busy and find a way to do it.” T. Roosevelt
Replies
It really needs to be connected to some point on the buildings grounding electrode system.
The water pipe may or may not be a ground electrode.
From the 2005 NEC
"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."
"250.52 Grounding Electrodes
(A) Electrodes Permitted for Grounding
(1) Metal Underground Water Pipe A metal underground water pipe in direct contact
with the earth for 3.0 m (10 ft) or more (including any metal well casing effectively
bonded to the pipe) and electrically continuous (or made electrically continuous by
bonding around insulating joints or insulating pipe) to the points of connection of the
grounding electrode conductor and the bonding conductors. Interior metal water piping
located more than 1.52 m (5 ft) from the point of entrance to the building shall not be
used as a part of the grounding electrode system or as a conductor to interconnect
electrodes that are part of the grounding electrode system.
Exception: In industrial and commercial buildings or structures where conditions of
maintenance and supervision ensure that only qualified persons service the installation,
interior metal water piping located more than 1.52 m (5 ft) from the point of entrance to
the building shall be permitted as a part of the grounding electrode system or as a
conductor to interconnect electrodes that are part of the grounding electrode system,
provided that the entire length, other than short sections passing perpendicular through
walls, floors, or ceilings, of the interior metal water pipe that is being used for the
conductor is exposed."
Other ground electrodes are
metal building frame when in contact with earth or encased in concrete in the ground.
Concrete encased electrode *Ufer - wire or rebar in the footer)
Ground rings
Rod and Pipe electrodes
Plate electrodes
Other metallic underground structures.
Then it goes on to say that a water pipe can't be the sole ground electrode because of the possibility that the pipe is replaced with plastic.
And if ground rods are used that they either need to be tested (rarely done) or 2 rods used.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I have no experience with cellphone installations on top of existing buildings, but I've run into them on water towers a number of times (I do electrical engineering for water treatment plants.) I suspect they will want to put in their own separate grounding system that will be pretty extensive, and they just want to bond it to the existing building system. At water towers, they typically bring in a small prefab building (maybe 12 x 20 or so) for their equipment and park it adjacent to the tower. Their ground system consists of a heavy (like 4/0 copper) buried ground ring surrounding the building, bonded to multiple ground rods, with all that tied to big copper bus bars inside their prefab building. I'd suggest talking to their designer to find out what they have planned.
You're close. They show a grounding wire bonded to several places of the metal platform structure, and there are details of 10 ft ground rods with inspection covers in the ground, cad welded to the wire. It says to use two ground rods. It's a fairly detailed set of drawings, very neatly done, so that's why I was a little surprised when I didn't see any ground rods located on the site plans.
Thanks for the info Bill."Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
commercial services get bonded to everything
building steel, water, Ufers, ground rods.. why can't the antenna bond be run back towards the service inside and attached there? it should be a simple matter of tracing the conduit to the water
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While in the end everything gets tied togehter the better option is to run t he ground from the antenna as direct and straight as possible to the earth.Possibly what they are getting to is to install ground rods directly below the tower and run the ground to it. Then bond that the building ground electrode system, ie the metalic water pipe outside or withing 5 ft of entering the build..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
that makes more sense and explains the two rods.
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