Grounding, Bonding, and Overcurrent Protection
What the exam tests
Grounding and bonding questions test electrode systems, conductor sizing, neutral separation, equipment grounding, and overcurrent protective device ratings under Article 250 and Article 240.
Core rules
- A grounding electrode system must be installed at each service; metal underground water pipe, metal frame, concrete-encased electrode, and ground ring are common electrodes.
- The grounding electrode conductor is sized from Table 250.66 based on the largest service conductor.
- The equipment grounding conductor is sized from Table 250.122 based on the OCPD rating, not the service conductor.
- The main bonding jumper connects the service neutral and equipment grounding conductor only at the service; downstream panels must keep neutral and ground separate.
- OCPDs must have an interrupting rating at least equal to the available fault current, and series-rated combinations must be listed.
Worked example
A 400 A service uses 500 kcmil copper service conductors. Table 250.66 requires a 1/0 copper grounding electrode conductor. The 400 A OCPD requires a 3 AWG copper equipment grounding conductor from Table 250.122. If the OCPD is increased to 600 A later, the EGC must be increased to 1/0 copper; the GEC does not automatically follow the OCPD.
Traps
Using Table 250.66 for the EGC; bonding neutral and ground in a subpanel; ignoring 250.122(B) when conductors are upsized for voltage drop; and accepting an OCPD with an interrupting rating below the calculated available fault current.