Services, Distribution, and Grounding/Bonding
What this domain tests
Services, Distribution, and Grounding/Bonding tests service equipment, panelboards, feeders, bonding jumpers, grounding electrodes, and fault-current paths. Expect questions on neutral separation, main bonding, grounding electrode conductors, service disconnect location, and equipotential bonding.
Core rules
- Bond the neutral to the equipment bonding conductor at the service or at a separately derived system's source; downstream panelboards keep neutral and bonding separated.
- The equipment bonding conductor must provide a low-impedance fault path back to the source so the overcurrent device trips quickly.
- Grounding electrodes include rods, plates, concrete-encased electrodes, and metal water pipe where permitted; the grounding electrode conductor connects the system to earth.
- Size service and feeder conductors for calculated load, continuous load, and voltage drop; protect them at their ampacity or the next standard overcurrent rating where permitted.
- Keep service disconnects accessible and labelled; identify each circuit in the panel schedule.
- Bond metal raceways, enclosures, and equipment to the equipment bonding conductor; use listed bonding jumpers around concentric knockouts and flexible connections.
Worked example
A 120 V branch circuit has a total fault-loop resistance of 0.5 Ω, including the source, ungrounded conductor, equipment bonding conductor, and connections. The prospective fault current is 120 ÷ 0.5 = 240 A. A 20 A inverse-time breaker with a magnetic trip at 10 times rated current needs about 200 A to trip instantly, so 240 A should clear the fault quickly. The reasoning: grounding and bonding are not about earth as a current return; they create a metallic fault path that lets the overcurrent device operate. A high-resistance bonding path lowers fault current and can leave metal enclosures energized.
Traps
Bonding neutral to ground in a subpanel; relying on earth as the fault return path; using the neutral as an equipment bonding conductor; omitting bonding around a water meter or flexible connection; sizing the bonding conductor from the neutral rather than the service conductors; mixing grounded and grounding terminals on receptacles.
Key points
- Neutral and bonding are connected only at the service or source of a separately derived system.
- The fault path must be metallic and low impedance.
- Earth grounding does not replace equipment bonding.
- Bond metal raceways, enclosures, and fittings.
- Label and schedule every service and branch circuit.