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Grounding and Bonding Practice for the California Exam

Two different tables. The grounding electrode conductor sizes off the service conductors (250.66). The equipment grounding conductor sizes off the breaker (250.122). On the California exam, calculations are part of the Determination of electrical system requirements domain. Here is the pattern, one worked example, and a question to try.

Last reviewed June 2026

One worked example

A service with 3/0 copper service-entrance conductors. Size the grounding electrode conductor.

  1. 1
    Name the problem

    Grounding electrode conductor, so Table 250.66, sized off the service conductors.

  2. 2
    Pick the rule

    Table 250.66 takes you from the service-conductor size to the GEC size.

    GEC ← Table 250.66 (service size)

  3. 3
    Pull the numbers

    Service is 3/0 copper.

  4. 4
    Run the arithmetic

    Read across Table 250.66. 2/0 through 3/0 copper service lands on a 4 AWG copper GEC.

    3/0 Cu service → 4 AWG Cu GEC

  5. 5
    Check it

    4 AWG copper. Different table than the EGC, do not grab 250.122 by mistake.

Now try one

Your turn. A branch circuit protected by a 60-amp breaker.

Using Table 250.122, what copper equipment grounding conductor does it need? Enter the AWG number.

  1. 1
    Run the arithmetic

    Read Table 250.122. A 60-amp circuit calls for a 10 AWG copper EGC.

    60 A → 10 AWG Cu EGC

  2. 2
    Check it

    10 AWG copper. The breaker drives the EGC, the service drives the GEC.

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