Services and Feeders
Service and feeder questions test whether you can trace capacity, disconnecting means, and grounding from the utility connection to the panelboard. The IRC treats service equipment in Chapter 36 and feeder requirements in Chapter 37. Expect load-calculation results to be compared against minimum ratings and marking requirements.
Core rules
- Service conductors and equipment must have adequate capacity for the calculated load. A one-family dwelling service disconnecting means must have a rating of not less than 100 amperes.
- The service disconnecting means must be installed at a readily accessible location nearest the point of entrance of the service conductors, or outside. It must be marked and suitable for use as service equipment.
- A service must have a grounding electrode system. Metal water piping, ground rods, concrete-encased electrodes, and other electrodes are bonded together; the grounding electrode conductor is sized from the applicable table.
- Feeders must be sized for the load and protected at their ampacity. Feeder equipment must be accessible and listed for the application.
- Keep grounded and grounding conductors separate downstream. At a subpanel, the neutral bar must not be bonded to the enclosure; the equipment grounding conductor terminates on a separate bar.
Worked example
A load calculation for a one-family dwelling totals 88 amperes. What minimum service disconnect rating is required? The calculated load is 88 A, but the dwelling minimum is 100 A. Select a 100 A service disconnect and conductors with at least 100 A ampacity, not the next standard 90 A size. The minimum rating overrides a smaller calculated load.
Traps
- Reading nearest point of entrance as nearest panel location.
- Bonding neutral to ground at a subpanel.
- Using service-entrance conductors as feeders without overcurrent protection.
- Forgetting that service equipment must be marked suitable for use as service equipment.