Overcurrent Protection, Grounding, and Bonding
Overcurrent Protection, Grounding, and Bonding combines Articles 240 and 250. The exam tests standard OCPD ratings, conductor protection, tap rules, the grounding electrode system, grounding electrode conductor and equipment grounding conductor sizing, and the separation of grounded and grounding conductors beyond the service.
Overcurrent protection
- Standard ratings (240.6): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A, and so on.
- Conductors are generally protected at their ampacity, with the specific permissions of 240.4(A) through (G).
- Tap conductors must meet one of the 240.21(B) rules: length, ampacity ratio, and physical protection all matter.
- Series-rated systems require the marked combination; do not substitute a random breaker.
Grounding and bonding
- Grounding electrode system (250.50): all electrodes present must be bonded together.
- GEC sizing uses Table 250.66 and is based on the largest ungrounded service conductor.
- EGC sizing uses Table 250.122 and is based on the rating of the OCPD ahead of the circuit.
- The main bonding jumper connects the grounded conductor to the equipment grounding conductor only at the service or separately derived system. At a subpanel, neutral and ground must be separated.
Worked example
A 200 A service uses 2/0 copper ungrounded conductors. Table 250.66 gives a 4 AWG copper GEC. The EGC for a 200 A circuit from Table 250.122 is 6 AWG copper. These are different tables because they answer different questions: GEC sizing is based on service conductor size, while EGC sizing follows the overcurrent device rating.
Traps
- Using Table 250.122 for the GEC or Table 250.66 for the EGC.
- Bonding neutral to ground at a subpanel, creating parallel return paths.
- Forgetting that a metal water pipe is only one electrode and must be supplemented.
- Missing the 20 ft of rebar or 10 ft of rod electrode rules.
- Applying tap rules without checking the ampacity ratio and length limits.