Overcurrent Protection, Grounding, and Bonding
Overcurrent protection
Know the standard ratings in 240.6 and the small-conductor rules in 240.4(D): 14 AWG to 15 A, 12 AWG to 20 A, 10 AWG to 30 A, unless a specific article permits otherwise (motor circuits under 240.4(G) are the classic exception). The next-size-up rule in 240.4(B) allows rounding up only where the device does not exceed 800 A and the load does not exceed the conductor ampacity. Feeder and branch taps follow 240.21. Panelboards must be protected at or below their rating (408.36), with no more than one conductor per terminal unless listed otherwise (408.41).
Grounding and bonding
The single most tested distinction: the grounding electrode conductor is sized from the service conductors using Table 250.66, while the equipment grounding conductor is sized from the overcurrent device rating using Table 250.122. The service neutral is bonded to the enclosure and electrode system at the service only; at a subpanel the neutral and equipment grounding conductors stay isolated. All grounding electrodes present must be bonded together (250.50), and where the only electrode is a single rod, pipe, or plate, a second electrode is required unless 25 ohms or less is achieved (250.53).
Traps
- Using Table 250.66 for an equipment grounding conductor or Table 250.122 for a grounding electrode conductor.
- Bonding neutral and ground at a subpanel, creating parallel return paths.
- Forgetting 250.66(A): the grounding electrode conductor to a rod, pipe, or plate need not be larger than 6 AWG copper.
- Treating conduit as an adequate ground path where a wire-type EGC is required.
Worked example
A 200 A service uses 2/0 copper service conductors. Table 250.66 gives a 4 AWG copper grounding electrode conductor, but if the only electrode is a ground rod, 250.66(A) caps the requirement at 6 AWG. The 200 A feeder equipment grounding conductor is 6 AWG copper per Table 250.122.