Texas Electrician ExamTexas journeyman & master

You do not choose an electrode. You use what the building already has.

The rule is not pick one. It is bond together everything present at the building, then add if what is present is insufficient.

On this page
  1. The idea people get backwards
  2. The concrete-encased electrode
  3. The water pipe, and the supplemental requirement
  4. Rods, and the twenty-five ohm question
  5. What does not qualify
  6. What this page cites

The idea people get backwards

Every grounding electrode present at a building has to be bonded together into one system The rule is not pick one. If a metal underground water pipe, a concrete encased electrode, a ground ring, a rod or pipe, or metal building frame is present at the structure, it is part of the system and must be bonded in. A rod is only permitted as a standalone electrode when none of the others exist. The grounding electrode conductor runs from the service to the system, and bonding jumpers tie the electrodes to each other. service GEC grade metal water pipe in contact 10 ft or more concrete encased electrode rod or pipe building steel all of them bonded together, not a menu to choose from a rod stands alone only when none of the others exist at the building
Present means required. The rod is what you use when nothing else is there, not a default. Drawn against NEC 250.50, 250.52(A) and 250.53. Sizing is Table 250.66, in your book.

A grounding electrode system is not a selection. Every electrode present at a building becomes part of it and gets bonded together (NEC 250.50).

So a building with a metal underground water pipe, a concrete-encased electrode and a driven rod has all three in its system. Not the best one. All of them.

The list of what qualifies is in the article, and the ordering in that list is not a preference ranking.

The concrete-encased electrode

Often called a Ufer. It is rebar or conductor embedded in the concrete footing in contact with the earth.

Where one is present it has to be used. That is the sentence that surprises people, because it means a builder cannot skip it and drive a rod instead when the footing already qualifies.

It also has the lowest resistance to earth of the common electrodes, which is why its grounding electrode conductor ceiling is one size larger than a rod’s.

The water pipe, and the supplemental requirement

A metal underground water pipe in contact with earth for a specified length qualifies, and it is the one electrode the code will not let stand alone.

It has to be supplemented by an additional electrode, because water pipe is outside your control. Somebody replaces a section with plastic and the electrode quietly stops existing.

That supplemental requirement is a favourite exam item precisely because the reason for it is not obvious from the text.

Rods, and the twenty-five ohm question

A single driven rod that does not meet a specified resistance to earth has to be supplemented by a second electrode.

In practice most installers drive two and skip the measurement, which is permitted and is why you rarely see anyone testing.

On the exam the question is usually whether you know the threshold exists and what happens when it is not met. Read the section rather than relying on the field habit.

What does not qualify

Underground gas piping is specifically not permitted as a grounding electrode. That prohibition is worth knowing because the intuition runs the other way.

Aluminum is excluded as an electrode material as well (NEC 250.52(B)).

What this page cites

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Every rule here is cited to the section it comes from, and corrections are logged in public with the date. This is study material rather than electrical advice, and it does not replace your own code book or your inspector.