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Grounding and bondingJourneymanMasterResidential wiremanNEC 2026

Grounding electrode conductor sizing (NEC 250.66)

Short answer

The grounding electrode conductor (GEC) is sized from the largest ungrounded service-entrance conductor, or the combined area of one phase of parallel sets, using Table 250.66. If the GEC runs only to a ground rod it never has to be larger than 6 AWG copper; only to a concrete-encased electrode, 4 AWG copper; only to a ground ring, the size of the ring.

Updated Original problems, new numbers every time

Live practice

Practice: grounding electrode conductor sizing

Each problem uses new numbers, so you practice the method rather than memorizing an answer.

Formula

GEC size = Table 250.66 row for the largest ungrounded service conductor
Parallel sets: Equivalent area (cmil) = Sets × Area of one conductor (cmil)
Rod only: never more than 6 AWG Cu. Concrete-encased only: never more than 4 AWG Cu. Ring only: never more than the ring conductor
SymbolMeaningUnit
Service conductorLargest ungrounded service-entrance conductor, or equivalent area for parallel setsAWG or kcmil
CMCircular mil area, Chapter 9, Table 8cmil
GECGrounding electrode conductorAWG or kcmil

Worked example

The largest ungrounded service-entrance conductor is 900 kcmil copper. The grounding electrode conductor will be copper, run indoors, and it connects to a metal underground water pipe electrode. Using Table 250.66, what is the minimum GEC size?

  1. Size the GEC from the largest ungrounded service conductor, not from the service rating.
  2. In Table 250.66, find the row for 900 kcmil in the copper service conductor column.
  3. A water pipe electrode gets the full table size. Read the copper GEC column for that row: 2/0 AWG.

Answer: 2/0 AWG

Grounding electrode conductor sizing, explained

The grounding electrode conductor connects the service grounded conductor and equipment to the grounding electrode system: the water pipe, building steel, footing rebar, rods or a ground ring. Table 250.66 sizes it from the largest ungrounded service-entrance conductor, not from the service rating or the main breaker.

Read the row using the column for the metal of the service conductors (copper or aluminum), then read across to the GEC column for the metal of the GEC. When the service is run in parallel sets, add the areas of the conductors in one phase and use that equivalent area to find the row.

Some electrodes cannot carry much current into the earth anyway, so the NEC caps the GEC that runs only to them. A GEC to rod, pipe or plate electrodes only never has to be larger than 6 AWG copper. One to a concrete-encased electrode only never has to be larger than 4 AWG copper. One to a ground ring only never has to be larger than the ring conductor. A GEC that runs to a water pipe or building steel gets the full Table 250.66 size.

Aluminum GECs have installation limits in 250.64(A): not in direct contact with masonry or the earth, and not within 18 inches of the earth outdoors. Exam questions about aluminum GECs assume an indoor run.

How to solve it on the exam

  1. Find the largest ungrounded service-entrance conductor. For parallel sets, multiply the area of one conductor (Chapter 9, Table 8) by the number of sets.
  2. In Table 250.66, find the row for that size in the column for the service conductor metal.
  3. Read the GEC size in the column for the GEC metal (copper unless the question says aluminum).
  4. If the GEC runs only to a rod, pipe or plate, stop at 6 AWG copper; only to a concrete-encased electrode, stop at 4 AWG copper; only to a ground ring, stop at the ring size. Use the smaller of the table size and the cap.
  5. If it runs to a water pipe or building steel, use the full table size.

Common mistakes

  • Sizing the GEC from the service rating or the main breaker instead of the service conductor size.
  • Using Table 250.122, which is for equipment grounding conductors.
  • Using one conductor of a parallel set instead of the combined area of the phase.
  • Finding the row with the copper column when the service conductors are aluminum.
  • Running a GEC larger than 6 AWG copper to a ground rod when the question asks for the minimum.

Exam tips

  • Highlight the two column headings of Table 250.66 (service conductors and GEC) in different colors. Most errors are reading the wrong side.
  • The rod, concrete and ring caps are short sentences in 250.66(A), (B) and (C). Tab them with the table.
Calculator

Grounding electrode conductor sizing calculator

Result–

    Questions people ask

    How do you size a grounding electrode conductor?

    Use Table 250.66 with the largest ungrounded service-entrance conductor, or the combined area of one phase for parallel sets. Read the GEC size from the copper or aluminum GEC column.

    What size GEC goes to a ground rod?

    The part of the GEC that runs only to rod, pipe or plate electrodes never has to be larger than 6 AWG copper or 4 AWG aluminum, under 250.66(A). If Table 250.66 calls for something smaller, use the smaller size.

    What size GEC goes to a concrete-encased electrode (Ufer)?

    It never has to be larger than 4 AWG copper when it connects only to the concrete-encased electrode, under 250.66(B).

    Is the GEC sized from the service amperage?

    No. It is sized from the size of the largest ungrounded service-entrance conductor. The equipment grounding conductor is the one sized from the overcurrent device rating.

    Sources

    About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.