Checked against primary sources 2026-08-24

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.

What this conductor connects, and where it lands

Article 100 defines it as a conductor used to connect the system grounded conductor, or the equipment, to a grounding electrode or to a point on the grounding electrode system. Read that as the wire from the service to the earth.

At a service the connection is made at any accessible point from the load end of the service conductors to the terminal or bus where the grounded service conductor lands at the service disconnecting means (NEC 250.24(A)(1)). That is a range, not a single point, and items exploit the range by describing a landing spot and asking whether it is inside it.

One more permission is easy to miss. Where the main bonding jumper is a wire or a busbar running from the grounded conductor bar to the equipment grounding bar, the grounding electrode conductor is permitted to land on the equipment grounding terminal instead (NEC 250.24(A)(4)).

The table is the starting point, not the answer

Size comes from Table 250.66, entered with the largest ungrounded conductor supplying the service or, for a separately derived system, the largest ungrounded derived conductor (NEC 250.66). Parallel sets enter on their combined equivalent area.

Find the size, read across, take the value. That is the method, and on an item with a metal water pipe electrode it is the whole method.

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

The three ceilings

Each names a maximum, above which a larger conductor is not called for.

• To a rod, pipe or plate electrode, single or multiple, the conductor is not required to be larger than 6 AWG copper, or 4 AWG aluminum or copper-clad aluminum (NEC 250.66(A)).
• To a concrete-encased electrode, the one the trade calls a Ufer, it is not required to be larger than 4 AWG copper (NEC 250.66(B)).
• To a ground ring, it is 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, and that answer will be sitting there on the sheet because the item writer put it there.

The condition attached to all three

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

Work it against four cases and the rule stops being slippery.

1. A 400 ampere service with two driven rods and nothing else. The conductor stops at the rods, so 250.66(A) caps it at 6 AWG copper regardless of what the table says for the service conductors.
2. The same service with a metal underground water pipe and two rods, one conductor running to the pipe and continuing on to the rods. It serves a water pipe, which has no ceiling, so the table value governs the whole run.
3. The same service where a separate bonding jumper reaches the rods from the electrode system, and the main conductor goes only to the water pipe. The jumper to the rods is capped at 6 AWG. The conductor to the pipe is not. Two conductors, two answers, one building.
4. A footing electrode only. 250.66(B) caps it at 4 AWG copper. Add a ground ring later and the conductor that reaches both is sized on whichever electrode calls for more.

The tell in an item is a second electrode type. Read the sentence for whether one conductor reaches both of them, because that is the fact that switches the ceiling off.

One continuous length, and the exceptions with names

The conductor is installed in one continuous length without a splice or joint, subject to four carve-outs the section names by number: 250.30(A)(5), 250.30(A)(6), 250.30(B)(1) and 250.68(C) (NEC 250.64(C)).

Inside that, a short list of connections is permitted and behaves like an exception list. Irreversible compression connectors listed for grounding and bonding, the exothermic welding process, and busbar sections bolted together are on it. So are the bolted, riveted or welded connections in a structural metal frame, and the threaded, welded, brazed, soldered or bolted-flange connections in metal water piping.

A wire nut is not on that list, and neither is a split bolt. That is the whole exam item most of the time.

Physical protection turns on 6 AWG

At 6 AWG and larger, whether the conductor needs protection depends on whether it is exposed to physical damage, and if it is not, it may run along the surface of the building without a metal covering (NEC 250.64(B)(1)).

Below 6 AWG there is no such test. Protection in rigid metal conduit, intermediate metal conduit, Schedule 80 PVC, RTRC-XW, electrical metallic tubing or cable armor is required either way (NEC 250.64(B)(3)).

So the hinge is the wire size, not the location, and an item that describes a small conductor in a protected spot is testing whether you know that.

A steel sleeve makes it worse unless you bond it

A ferrous metal raceway, enclosure or cable armor around the conductor is bonded at each end to the electrode or to the conductor itself, creating an electrically parallel path (NEC 250.64(E)).

Left unbonded, the steel acts as a choke. The changing magnetic field induces an opposing current in it and the impedance of the path goes up, which is the opposite of what the sleeve was installed to do.

Nonferrous raceway does not have this problem and is not held to the requirement. When the jumper is needed it is the same size as, or larger than, the largest grounding electrode conductor inside (NEC 250.64(E)(3)).

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 an item 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 (NEC 250.64(A)).

One test for telling this conductor from the other one

Ask what it terminates on. An electrode, or a point on the electrode system, means this conductor and this table.

Anything that runs alongside the circuit conductors and lands on equipment is the other one, and it sizes from a different input entirely.

What this page cites

• NEC 250.66 Size of the ac grounding electrode conductor, and the three ceilings in the subsections beneath it. source
• NEC Table 250.66 The sizing table itself. Read it in your own code book. No value from it is printed here.
• NEC 250.64 Installation: continuity, the 6 AWG protection hinge, ferrous raceway bonding and the aluminum rules.
• NEC 250.24(A) Where the conductor connects at a service, and the permission to land it on the equipment grounding bar.
• NEC 250.52 What counts as a grounding electrode. Each of the three ceilings points back into it.
• NEC 250.30 Separately derived systems. Named inside 250.64(C) as one of the four carve-outs from the continuous-length rule.