Section 250.24/Grounding Service-Supplied AC Systems
NEC 250.24 requires a grounding electrode conductor and main bonding jumper at the service to connect the grounded conductor to the equipment grounding system and earth.
For AC systems required to be grounded, the grounding connections must be made at each service. Section 250.24(A) requires a grounding electrode conductor (GEC) to connect the grounded service conductor (neutral) to the grounding electrode system. Section 250.24(B) requires a main bonding jumper to connect the grounded conductor to the service equipment enclosure. The GEC must be sized per Table 250.66 based on the size of the largest ungrounded service-entrance conductor. Section 250.24(C) requires that the grounded conductor be routed with the ungrounded conductors and sized not smaller than specified in Table 250.102(C)(1). Section 250.24(D) prohibits grounding connections (neutral-to-ground bonds) on the load side of the service disconnecting means.
Why this section exists
The service grounding connection is the single most critical point in a building's electrical safety system. The main bonding jumper creates the low-impedance fault return path that allows overcurrent devices to clear ground faults quickly. The grounding electrode conductor connects the system to earth, stabilizing voltage during normal operation and providing a path for lightning and surge currents. The prohibition on downstream neutral-to-ground bonds prevents objectionable current on equipment grounding conductors and metallic piping systems.
What plan reviewers look for
Plan reviewers verify that the one-line diagram shows the main bonding jumper at the service equipment and the grounding electrode conductor to the grounding electrode system. They check the GEC size against Table 250.66 based on the service-entrance conductor size. They verify that no neutral-to-ground bonds exist downstream of the main disconnect, particularly in subpanels. For separately derived systems (transformers, generators), they check that grounding complies with Section 250.30 instead of 250.24.