Grounding, Bonding, and Overcurrent Protection
Articles 250 and 240 produce a large share of exam questions because they combine rules with tables. The key skill is knowing which table governs which conductor.
System grounding at the service
250.24 requires the service grounded conductor to be connected to the grounding electrode system, and 250.28 requires a main bonding jumper at the service. Load-side of the service disconnect, the grounded conductor must not be bonded again (250.24(A)(5)), so subpanels get separate neutral and equipment grounding terminations. 250.32 covers separate structures, where a grounding electrode is required and an equipment grounding conductor is run with the feeder.
Electrodes and grounding conductors
250.52 lists permitted electrodes; a rod must be at least 8 ft long with 8 ft in contact with soil. 250.53(A)(2) requires a supplemental electrode for a single rod unless it measures 25 ohms or less. Table 250.66 sizes the grounding electrode conductor from the largest service conductor. Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of the circuit - not from the phase conductor size. 250.104 bonds metal water and other metal piping; 250.148 bonds boxes and raceways.
Overcurrent protection
240.4(D) caps small conductors at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG, with the motor and air-conditioning exceptions of 240.4(G). 240.21 limits tap conductors by length and ampacity.
Worked example
A 200 A service uses 2/0 Cu service-entrance conductors. Table 250.66 for 2/0 Cu requires a 4 AWG Cu grounding electrode conductor to the rod and water pipe. A 60 A subfeed requires a 10 AWG Cu equipment grounding conductor under Table 250.122. The supplemental rod electrode is bonded at 6 AWG Cu - 250.66 never requires more than that to a rod.
Traps
Sizing the equipment grounding conductor from the phase conductor; treating 25 ohms as a mandatory measurement rather than an alternative to a supplemental electrode; bonding neutral and ground at a subpanel; forgetting 250.104(B) bonding for metal gas piping.