Grounding, Bonding, and Overcurrent Protection
Article 250 is the most heavily tested article on most journeyman exams, and it pairs with Article 240: the grounding path must be able to carry the fault current that the overcurrent device is expected to clear.
Grounding and bonding rules
- 250.24(A) requires the service grounded conductor to be bonded to the service enclosure and connected to the grounding electrode conductor at the service.
- Table 250.66 sizes the GEC from the largest ungrounded service conductor; a connection to a rod, pipe, or plate electrode never needs to be larger than 6 AWG copper (250.66(A)).
- Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it, not from conductor size.
- 250.122(B): if the ungrounded conductors are increased above the minimum for voltage drop, the EGC must be increased proportionally.
- A metal water pipe is an electrode only if it is in contact with the earth for 10 ft or more, and it must be supplemented by another electrode (250.52(A)(1), 250.53(D)(2)).
Worked example
A 400 A feeder uses 500 kcmil copper. Table 250.122 gives 3 AWG copper. The designer upsizes to 750 kcmil for voltage drop, a 1.5× increase in circular mils, so the EGC must also grow 1.5×: 3 AWG (52,620 cmil) × 1.5 = 78,930 cmil, which requires 1 AWG (83,690 cmil). Answer: 1 AWG.
Overcurrent protection
- 240.4(D) caps 14, 12, and 10 AWG at 15, 20, and 30 A except where 240.4(E) through (G) permit otherwise.
- 240.4(B) allows the next standard size up only where the ampacity does not match a standard rating and the next size does not exceed 800 A.
- 240.21 tap rules: the 10 ft and 25 ft rules each carry their own conditions on ampacity, protection, and enclosing.
Traps
- Using Table 250.66 for a feeder EGC.
- Running a neutral to a subpanel and bonding it there.
- Oversizing a GEC to a ground rod "because the service is big".