PC Grounding and Bonding 3 — Questions and Answers
Question 1: What is the maximum resistance to ground allowed for a driven ground rod per NEC 250.56 before a second electrode is required?
- 5 ohms
- 25 ohms (Correct answer)
- 50 ohms
- 100 ohms
Correct answer: 25 ohms
NEC 250.56 requires a supplemental electrode if a single driven rod has a resistance to ground exceeding 25 ohms.
Question 2: Intersystem bonding terminations (IBTs) are required to be accessible and located within how many feet of the service entrance or the grounding electrode conductor?
- 3 feet
- 6 feet
- 10 feet
- No distance limit is specified (Correct answer)
Correct answer: No distance limit is specified
NEC 250.94 requires IBTs to be accessible but does not specify a maximum distance from the service; placement is determined by practicality.
Question 3: Which conductor connects the grounded service conductor (neutral) to the grounding electrode system at the service equipment?
- Equipment grounding conductor (EGC)
- Main bonding jumper (MBJ) (Correct answer)
- Grounding electrode conductor (GEC)
- System bonding jumper
Correct answer: Main bonding jumper (MBJ)
The main bonding jumper connects the grounded (neutral) conductor to the equipment grounding conductor at the service equipment per NEC 250.28.
Question 4: When bonding metal water piping in a building, the bonding jumper must be connected to the piping at a point within how many feet of where the water service enters the building?
- 2 feet
- 5 feet (Correct answer)
- 10 feet
- 25 feet
Correct answer: 5 feet
NEC 250.68(C)(1) requires the bonding connection to interior metal water piping within 5 feet of where the pipe enters the building.
Question 5: A grounding electrode conductor for a residential 200A service must be at least what size if using copper?
- #4 AWG (Correct answer)
- #2 AWG
- #1/0 AWG
- #2/0 AWG
Correct answer: #4 AWG
NEC Table 250.66 requires a minimum #4 AWG copper GEC for service conductors of 2/0 AWG or smaller copper (typical for 200A residential).
Question 6: What type of electrode is formed by 20 or more feet of bare copper conductor buried at least 30 inches below grade encircling a building?
- Concrete-encased electrode
- Ground ring (Correct answer)
- Rod and pipe electrode
- Plate electrode
Correct answer: Ground ring
NEC 250.52(A)(4) defines a ground ring as bare copper conductor, at least #2 AWG, buried at least 30 inches deep and encircling the building.
Question 7: Which of the following is NOT permitted to be used as a grounding electrode per NEC 250.52?
- Concrete-encased rebar
- Metal underground water pipe
- Underground gas piping system (Correct answer)
- Ground ring
Correct answer: Underground gas piping system
NEC 250.52(B) explicitly prohibits underground gas piping systems from being used as a grounding electrode.
PC Grounding and Bonding 3
What is the maximum resistance to ground allowed for a driven ground rod per NEC 250.56 before a second electrode is required?