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This will be handled by an electrician, but I wanted to get info on what's permissible as my first stab at talking to an electrician led to a conclusion that seems disallowed and unsafe (entirely isolated ground system for an antenna).

We have a 1950's house with the service entrance at a back corner and the water main entrance near the center of the front of the house. It looks like the primary grounding for this house is a 4 gauge conductor from the panel through conduit buried in the slab and bonded to the water main a foot or so after it enters the house.

A new antenna connection penetrates the house near the water main and has a lightning arrester in-line, currently connected to nothing.

Questions:

  1. Does it make sense that that water main (about 50-80 feet away from the main panel) is the primary GEC for the house?

How to bond the lightning arrester bus bar:

  1. New ground rod outside, > 16 feet from water main, bonded via approved clamp to water main outside

  2. New ground rod outside, > 16 feet from water main, with 4 ga conductor running inside and then about 10 feet and also clamped separately to water main near entrance.

or #2/#3 without the new ground rod.

The first electrician who I talked to said "just drive a ground rod, at least 8 feet, and call it a day," but didn't have anything to say about tying it to the house electrical grounding system.

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  • I would say that the water main does not make sense as the primary ground for the house. It's too far away to provide a low impedance path to ground for lightning protection. I'll defer to the electricians to answer specifics about what's code and what isn't. I also suggest looking for a copy of the ARRL Grounding and Bonding book if you want to learn a lot more about antennas and ground systems. If you don't care about the electrical theory and just want an answer, you can skip the book.
    – mrog
    Jan 4 at 23:07
  • I must not have read the entire question, did not realize the distance. Communications grounding is allowed to use a local rod this is allowed to be smaller 5’ and can even be 1/2” pipe (a standard driven pipe is 3/4” and 8’ long . We want the grounding path to be as short as possible max 20’, #6 copper is the largest ground required in this case.
    – Ed Beal
    Jan 5 at 14:39
  • Thanks, @EdBeal - by "grounding path" you mean antenna->ground rod and by "local," you mean not tied back into the household ground? For shielded coax coming into the house, this seems like a problem as you'll have the case of some equipment at a difference ground potential than the case of others.
    – Lios
    Jan 5 at 17:50
  • They still require bonding but for a remote communications ground for an antenna this is the easiest method, I brought it up here a year or so ago and I don’t think anyone had ever heard of a 5’ grounding rod. Grounding is all about lightning and clearing a fault, unfortunately many residential ground systems are not below the value required to clear a fault (6 ohms on a 20 amp breaker) this is why the neutral is required to be bonded to ground this is where the fault is normally cleared. The grounding rod is really for lighting and a direct strike vaporizes #6 for several feet.
    – Ed Beal
    Jan 5 at 18:10
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The water main is a grounding electrode, but not a grounding electrode conductor

You are correct that a metal water service line is considered a primary grounding electrode when present; however, not the least due to the potential for the utility to replace said metal water service line with a nonconductive, non-grounding plastic water service line, the NEC requires a supplemental "made electrode", usually a pair of ground rods 8' long and 6-8' minimum apart, when the primary electrode is a water service line. For these rods, NEC 250.66(A) lets you use 6AWG copper to tie them back to the main grounding electrode conductor.

However, you must tie those rods back to the grounding electrode conductor in addition to tying the lightning arrestor grounding wire to said ground rods. (This is required by NEC 250.50 and 800.100(B)(2) or 800.100(D).) Furthermore, the reason you see the Grounding Electrode Conductor tied to the grounding electrode at the service entrance point is because NEC 250.68(C)(1) requires that connection to be made within 5' of where the water service enters the house. (This requirement exists so that a plumber doesn't accidentally leave your house afloat in the electrical sense of the term.)

So, I would use plan #3, but modified with the addition of a second rod, and more importantly modified to use either a split-bolt or a T-tap connector (Ilsco GTT-2-2 or equivalent) to connect the bonding jumper to the GEC directly without having to sever the latter. You can connect the lightning arrester's bonding wire the same way for now; generally speaking, though, you'll want to put in an Intersystem Bonding Termination device at the main panel if at all possible, as that provides a central place for future communications grounding wires to land.

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  • Thanks for the correction and the detailed explanation. My whole concern about the current system (communications aside) is the relatively long distance between the service entrance/first panel and the water main. I wonder if the conduit through the slab is part of the system then or it simply doesn't matter - that thick, stranded copper is low impedance enough. Other question, though - would it be better to add any new made electrodes to the panel/service entrance side, but still bond the antenna system to the GEC attached to the water main?
    – Lios
    Jan 5 at 18:03
  • @Lios -- if it's a metal conduit in the slab, then it needs to be bonded into the system at both ends to avoid "choking" the grounding electrode conductor. You could do as you describe in your comment, by the way, as it'll make the rods somewhat more effective Jan 5 at 23:33
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The new rod has to be 6’ away but further is better. the rod and pipe electrodes bonding jumper (the wire going from the electrode to pipe is required to use the wire size or largest ungrounded service entrance conductor. If only 2-3 awg you could get away with #8 copper as that is the smallest size allowed however I would encourage you to use #6 that is the largest size required for residential 250.66.a and although aluminum #4 is allowed it can’t be terminated within 18” of earth so copper is really the only way to go. If you use #8 it has to be protected where copper #6 doesn’t so 6 is the best way to go with an approved pipe to copper clamp (for the size of your pipe) and an approved clamp for the rod to copper. Inspectors like the tags do leave them on or on the ground next to them. But they can tell as they are usually bronze with stainless screws. 6’ apart 350.53.3 or B both state 6’, 18” from earth & aluminum 250.64.A.

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