Services, Grounding and Bonding, Panelboards and Transformers
Grounding and bonding is the subject electricians most often get wrong in the field and on the exam, largely because two different conductors with similar names do completely different jobs. This chapter separates the grounding electrode system, which stabilizes voltage to earth, from the equipment grounding system, which clears faults, and then adds the service, panelboard and transformer rules that surround them.
Service Conductors, Clearances and Disconnects
Service conductors are the only conductors in a building that are not protected by an overcurrent device ahead of them, so the code compensates with strict rules on where they may run, how far they may extend inside a building and how quickly they must reach a disconnect. Overhead clearances are graded by what passes underneath, from pedestrian areas up to public roads. The service disconnect must be readily accessible and located nearest the point where the conductors enter the building.
The Grounding Electrode System
All grounding electrodes present at a building must be bonded together to form one system, which is why a house with a metal water service, a footing electrode and a driven rod ends up with all three connected. Each electrode type has its own qualifying condition, and the metal water pipe is unique in that it always requires a supplemental electrode. The grounding electrode conductor is sized from the service conductors, then capped by the electrode it serves.
Equipment Grounding Conductors and Bonding
The equipment grounding conductor is the low-impedance path that carries fault current back to the source so the overcurrent device opens, and the earth is explicitly not permitted to serve that purpose. It is sized from the rating of the overcurrent device ahead of the circuit, not from the size of the phase conductors, except when the phase conductors are upsized. The main bonding jumper is what ties the grounded conductor to the enclosure and equipment grounding system, and it exists at exactly one place in a premises wiring system.
Panelboards and Overcurrent Device Placement
A panelboard is protected as a unit by a device rated no higher than the panelboard itself, whether that device sits inside as a main breaker or ahead of it in the feeder. Beyond that, the code is concerned with clear identification, so that a future electrician can find the right circuit without guessing. Circuit directories must describe the specific load and its location rather than using vague or obsolete descriptions.
Transformers
Transformer questions usually combine a current calculation with a protection percentage. Start by finding primary or secondary full-load current from the kVA rating and the voltage, remembering the 1.732 factor on three-phase, then apply the protection percentage from the transformer article. A transformer secondary is also a separately derived system, which means it gets its own system bonding jumper and grounding electrode connection.
Last updated: July 2026