Services, Feeders, and Branch Circuits
This domain is about the hierarchy of the distribution system and where each rule applies.
Definitions that drive the rules
A service conductor carries power from the utility supply to the service equipment. A feeder supplies a panelboard or other distribution point downstream of the service. A branch circuit supplies the final outlets. The same ampacity table may apply to all three, but demand factors, overcurrent rules, and grounding rules change with the category, so questions often hinge on identification first.
Concepts to understand
- Service equipment includes the main disconnect, service box, metering, and grounding/bonding point; the neutral is bonded to ground here and only here.
- Overload and short-circuit protection must be at the supply end of the feeder or branch circuit, or as permitted by tap rules.
- Continuous loads are calculated at 125 percent of their rating for conductor and overcurrent sizing.
- Voltage drop is a design issue: long feeders need larger conductors even though ampacity is adequate.
- Disconnecting means must be accessible, labelled, and within the distance allowed by the Code for the equipment served.
Traps
Confusing a feeder with a branch circuit; bonding the neutral again at a sub-panel; sizing a service from connected load without applying demand factors; ignoring that a panel's busbar rating, not just its main breaker, limits the load.
Worked example
A 60 A continuous feeder load on a 208 V three-phase system: 60 × 1.25 = 75 A, so select conductors and an overcurrent device rated at least 75 A-typically an 80 A breaker with conductors sized to the 75 °C column.