Wiring and Protection
Core Articles
This domain covers Chapter 2: Article 200 (grounded conductors), 210 (branch circuits), 215 (feeders), 220 (calculations), 225 (outside branch circuits and feeders), 230 (services), 240 (overcurrent protection), 250 (grounding and bonding), 280/285 (surge protection), and 300 (general wiring methods).
Concepts to Master
- Grounded vs. grounding conductors: the grounded (neutral) conductor carries current; the equipment grounding conductor does not except during a fault. Do not confuse them.
- Overcurrent protection: know standard ratings, the next-size-up rule (limited to the next higher standard rating under specified conditions), and tap rules.
- Grounding and bonding: understand system grounding, equipment grounding, bonding of metal parts, and the grounding electrode system. Separate the grounding electrode conductor from the equipment grounding conductor.
- Branch circuit ratings: 15, 20, 30, 40, 50 A general-purpose circuits; specific rules for small-appliance, laundry, and bathroom circuits.
Traps
- Mixing up grounded conductor (neutral) and grounding conductor terminology.
- Applying the next-size-up rule where it is not allowed (e.g., where the load exceeds the ampacity).
- Forgetting that the service neutral is a grounded conductor and must be bonded at the service only.
- Overlooking that a 20 A small-appliance branch circuit may serve receptacles but not lighting in some contexts.
Worked Example
A 25 A continuous load needs overcurrent protection. Continuous loads are sized at 125%, giving 31.25 A; the next standard size up (35 A) may be permitted only if the conductor ampacity and conditions allow. Always check the load-versus-rating condition before applying next-size-up.
Rule of Thumb
Size conductors for ampacity, size overcurrent devices for protection, and keep grounded and grounding conductors conceptually separate.