The grounding electrode conductor, and the caps most people miss

Sizing straight off the table gives the wrong answer on any rod-only or concrete-encased system. Three short subsections cap it, and they are where the question lives.

The table is the starting point, not the answer

The grounding electrode conductor is sized from a table indexed by the size of the largest ungrounded service conductor (NEC 250.66).

Find your service conductor size in the left column, read across, take the value. That is the method, and on a question with a water pipe electrode it is the whole method.

But the section that sends you to the table also says except as permitted in the three subsections that follow it. Those three subsections cap the result, and which one applies depends entirely on what kind of electrode is at the other end.

The three ceilings

Each one names a maximum, above which a larger conductor is not required (NEC 250.66).

• To a rod, pipe or plate electrode, the conductor is not required to be larger than 6 AWG copper, or 4 AWG aluminum (NEC 250.66(A)).
• To a concrete-encased electrode, the one the trade calls a Ufer, not required to be larger than 4 AWG copper (NEC 250.66(B)).
• To a ground ring, not required to be larger than the conductor used for the ring itself (NEC 250.66(C)).

These are not small adjustments. On a large service the table can call for a conductor several sizes above the ceiling. A candidate who reads the table and stops will confidently answer with a conductor two or three trade sizes too big.

The condition attached to all three

Every one of the three ceilings carries the same qualifier, and it is the part that gets skipped: the ceiling applies only where that conductor does not continue on to another type of electrode that requires a larger conductor.

So a conductor running to a ground rod and stopping is capped. The same conductor running to a ground rod and then continuing to a water pipe is not, because it is now serving an electrode that has no ceiling.

Exam questions test this by describing a grounding electrode system with more than one electrode in it. The presence of a second electrode type is the tell.

Why a rod gets a ceiling at all

It is worth understanding rather than memorizing, because the reasoning is the same one from the bonding article.

A driven rod has high resistance to earth, tens of ohms in ordinary soil. The current that can flow through it is limited by that resistance, not by the conductor feeding it. Past a certain point, a larger conductor to the rod accomplishes nothing at all.

A concrete-encased electrode has lower resistance than a rod, which is why its ceiling sits one size larger. A metal water pipe has lower resistance still, which is why it gets no ceiling.

The ceilings track how much current the electrode can actually pass. Once you see that, remembering which electrode gets which cap stops being arbitrary.

Installation rules that get tested alongside sizing

A few short rules from the installation section come up often enough to know.

• The conductor runs in one continuous length without splices, with a short list of permitted exceptions including irreversible compression connectors and exothermic welding (NEC 250.64(C)). A wire nut is not on that list.
• At 6 AWG and larger, physical protection depends on whether the conductor is exposed to damage. Below 6 AWG, protection is required regardless (NEC 250.64(B)).
• A ferrous metal raceway around the conductor is bonded at both ends (NEC 250.64(E)). Left unbonded it acts as a choke and raises the impedance of the path it was meant to protect. Nonferrous raceway does not have this problem and is not held to the rule.

That last one surprises people. A steel sleeve protecting a grounding electrode conductor makes things worse unless it is bonded, because the changing magnetic field induces an opposing current in the steel.

The aluminum rules pull in opposite directions

If the conductor is aluminum or copper-clad aluminum, two distance rules apply and they measure from different surfaces (NEC 250.64(A)).

One permits a termination within 18 inches of the bottom of an outdoor enclosure that is listed and identified for the environment. The other forbids terminating within 18 inches of the earth.

Same number, opposite direction, and a question can quote either one. Read which surface is being measured from before you answer.

Aluminum also cannot be in direct contact with concrete, or installed where conditions are corrosive, unless it has an extruded polymeric covering.

A separate table you will also be asked about

The grounding electrode conductor is not the equipment grounding conductor, and they are sized from different tables.

The grounding electrode conductor runs from the service to the earth electrode, sized by service conductor size. The equipment grounding conductor runs with the circuit conductors, sized by the rating of the overcurrent device ahead of it.

Reaching for the wrong table is a common error because both are in Article 250 and both produce a conductor size. Ask what the conductor connects to. If it terminates on an electrode, it is the grounding electrode conductor.

What this page cites

• NEC 250.66 Size of the ac grounding electrode conductor, and the three ceilings in the subsections beneath it.
• NEC Table 250.66 The sizing table itself. Read it in your own code book.
• NEC 250.64 Installation, including continuity, physical protection and the aluminum rules.
• NEC 250.52 What counts as a grounding electrode. Referenced by each of the three ceilings.