Grounding, Bonding and Overcurrent Protection
What the exam targets
Article 250 grounding and bonding plus Article 240 overcurrent protection. You must keep straight the grounded conductor (neutral), grounding electrode conductor (GEC), equipment grounding conductor (EGC), bonding jumper, and grounding electrode.
Concepts to own
- 250.50 grounds the electrode system as a whole: all available electrodes must be bonded together. A ground rod is at least 8 ft long; 25 ohms or less, or add a supplementary electrode (250.53).
- GEC size comes from Table 250.66 based on the largest service-entrance conductor. EGC size comes from Table 250.122 based on the overcurrent device rating.
- The main bonding jumper ties neutral to ground at the service or at a separately derived source only. In a subpanel, neutrals and grounds are separated and the neutral bar is not bonded.
- Bond metal water piping, gas piping, structural steel, and jumper around water meters.
- 240.4 protects conductors; 240.4(B) allows the next standard size up to 800 A; 240.21 tap rules cover 10 ft and 25 ft taps.
Traps
Sizing the GEC from the load or breaker rating; sizing the EGC from Table 250.66; forgetting 250.122(B), which requires the EGC to be increased proportionally when phase conductors are upsized for voltage drop; a second neutral-to-ground bond in a subpanel; using the neutral as an EGC on new 3-wire dryer or range circuits.
Worked example
A 400 A service with 600 kcmil copper service conductors: Table 250.66 requires a 1/0 AWG copper grounding electrode conductor. For a 100 A feeder, Table 250.122 requires an 8 AWG copper EGC. If the phase conductors are increased two sizes for voltage drop, the EGC must be increased in the same proportion.