NEC Tables · Grounding

Table 250.66 — GEC sizing

Service conductors in, grounding electrode conductor out — plus the three shortcut caps the notes hand you.

Free · NEC 2023 · Updated
2 AWG/kcmil
Over 3/0 through 350 · Copper
grounding electrode conductor size per Table 250.66
Exam trapTable 250.66 sizes the grounding ELECTRODE conductor. Equipment grounding conductors size from 250.122 — off the overcurrent device, not the service conductors. Mixing the two tables is the classic grounding fail.

Size the GEC from the largest ungrounded service-entrance conductor (or the equivalent area of paralleled sets). The notes matter as much as the grid: connections to rod, pipe, or plate electrodes never need to be larger than 6 AWG copper; to a concrete-encased electrode, 4 AWG.

Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors(AWG/kcmil)Size of Grounding Electrode Conductor(AWG/kcmil)
CopperAluminum or Copper-Clad AluminumCopperAluminum or Copper-Clad Aluminum
2 or smaller1/0 or smaller86
1 or 1/02/0 or 3/064
2/0 or 3/04/0 or 25042
Over 3/0 through 350Over 250 through 50021/0
Over 350 through 600Over 500 through 9001/03/0
Over 600 through 1100Over 900 through 17502/04/0
Over 1100Over 17503/0250
Notes that change the answer — 250.66

Notes:

1. If multiple sets of service-entrance conductors connect directly to a service drop, set of overhead service conductors, set of underground service conductors, or service lateral, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.

2. If there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.

3. See installation restrictions in 250.64.

Values derived from the 2023 National Electrical Code for exam study. Verify installations against the adopted code and local amendments.

Race your codebook — 3 lookups on this table

Q1

A house has both a ground rod and a concrete-encased electrode (Ufer ground) installed. The electrician needs to know the requirements for the conductor that ties these two electrodes together into a single grounding electrode system. Which section covers this jumper?

  1. A 250.53(D)(1)
  2. B 250.53(C)
  3. C 250.53(E)
  4. D 250.54
Show answer & explanation

Correct: B — 250.53(C)

250.53(C) covers the bonding jumper that connects the grounding electrodes together to form the grounding electrode system, including sizing per 250.66 and connection per 250.70. 250.53(E) only addresses sizing of the jumper to a supplemental rod/pipe/plate electrode, and 250.53(D)(1) covers continuity of a metal water pipe electrode, not the interconnecting bonding jumper itself.

Q2

A crew is installing a grounding electrode conductor to bond the non-current-carrying metal enclosures of a 4160V pad-mounted switchgear lineup to the grounding electrode system. Which section gives the table for sizing this conductor based on the largest ungrounded supply conductor?

  1. A 250.190(C)(3)
  2. B 250.190(B)
  3. C 250.186(A)(1)
  4. D 250.191
Show answer & explanation

Correct: B — 250.190(B)

250.190(B) directs sizing of the grounding electrode conductor for over-1000V equipment enclosures to Table 250.66. 250.190(C)(3) instead covers sizing of the equipment grounding conductor (a different conductor), and 250.186(A)(1) sizes the conductor grounding the service's grounded system conductor, not equipment enclosures.

Q3

An electrician is landing the grounded service conductor, the equipment grounding conductors, and the service enclosures all onto the same grounding electrode conductor at service equipment. Which section requires this conductor and points to how it must be sized?

  1. A 250.24(E)
  2. B 250.24(D)
  3. C 250.24(C)
  4. D 250.24(A)(1)
Show answer & explanation

Correct: A — 250.24(E)

250.24(E) is the section requiring the grounding electrode conductor to tie the equipment grounding conductors, service enclosures, and grounded service conductor to the grounding electrode(s), sized per 250.66. 250.24(C) covers the main bonding jumper (a separate connection at service equipment), and 250.24(D) covers sizing the grounded conductor brought to service equipment, not the electrode conductor.

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Grounding Conductor Calculator Free calculator GEC Sizing — the concept NEC guide Article 250 practice questions 77 questions
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