TN LLE Grounding and Bonding 2 — Questions and Answers
Question 1: Per NEC 250.122, what is the minimum size equipment grounding conductor for a circuit protected by a 60-ampere overcurrent device?
- 14 AWG
- 12 AWG
- 10 AWG (Correct answer)
- 8 AWG
Correct answer: 10 AWG
NEC Table 250.122 requires a minimum 10 AWG copper equipment grounding conductor for a 60-ampere rated overcurrent device.
NEC Table 250.122 provides minimum equipment grounding conductor (EGC) sizes based on the rating of the overcurrent protective device: 15A = 14 AWG, 20A = 12 AWG, 30A = 10 AWG, 40A = 10 AWG, 60A = 10 AWG, 100A = 8 AWG, 200A = 6 AWG. For a 60A OCPD, the minimum copper EGC is 10 AWG. These are minimum sizes — the EGC may need to be larger if the circuit conductors are increased in size for voltage drop or other reasons, per NEC 250.122(B).
Question 2: What is the purpose of the main bonding jumper in a service panel?
- To connect the neutral bus to the panel enclosure
- To bond the grounded conductor (neutral) to the equipment grounding conductor and enclosure at the service (Correct answer)
- To connect the panel to the water pipe
- To provide a path for normal current return
Correct answer: To bond the grounded conductor (neutral) to the equipment grounding conductor and enclosure at the service
The main bonding jumper connects the grounded conductor (neutral), equipment grounding conductors, and the service enclosure together, creating the effective ground-fault current path.
The main bonding jumper (MBJ) per NEC 250.24(B) and 250.28 is the connection between the grounded conductor (neutral), the equipment grounding conductor(s), and the service disconnect enclosure. This connection is made at the service equipment only — never at downstream panels (where neutral and ground must remain separate per 250.24(A)(5)). The MBJ creates the effective ground-fault current path that allows fault current to flow back to the source and trip the overcurrent device. Without it, metal enclosures could become energized during a ground fault with no path to clear the fault.
Question 3: Per NEC 250.64(A), what protection is required for aluminum or copper-clad aluminum grounding electrode conductors?
- No special protection required
- They must not be installed where subject to physical damage or corrosive conditions, and must not be in contact with masonry or earth (Correct answer)
- They must be in conduit at all times
- They require a special coating
Correct answer: They must not be installed where subject to physical damage or corrosive conditions, and must not be in contact with masonry or earth
NEC 250.64(A) prohibits aluminum or copper-clad aluminum GECs from being used where in direct contact with masonry, earth, or where subject to corrosive conditions.
NEC 250.64(A) recognizes that aluminum is susceptible to corrosion from alkaline conditions in concrete/masonry and galvanic corrosion in earth contact. Aluminum GECs must not be terminated within 18 inches of the earth, installed in contact with masonry or earth, or used in corrosive environments. Copper GECs have no such restrictions, which is why copper is preferred for grounding electrode conductors in most installations. When aluminum is used, connections must use listed connectors rated for aluminum, and proper anti-oxidant compound should be applied.
Question 4: What is an intersystem bonding termination per NEC 250.94?
- A connection between two electrical services
- An accessible device for bonding communications systems (cable, telephone, satellite) to the building grounding electrode system (Correct answer)
- A splice between ground rods
- A connection between the water pipe and gas pipe
Correct answer: An accessible device for bonding communications systems (cable, telephone, satellite) to the building grounding electrode system
NEC 250.94 requires an intersystem bonding termination at the service or disconnect that provides accessible terminals for bonding communications systems to the grounding electrode system.
NEC 250.94 requires an intersystem bonding termination (IBT) at each service equipment, disconnect, or meter location. The IBT must have at least three terminals accessible for connection, be listed, and be connected to the service grounding electrode system. This allows communications installers (cable TV, telephone, satellite, broadband) to easily bond their equipment grounding to the building's grounding electrode system. Without this, communication installers may create improper or inadequate ground connections, leading to potential shock hazards and interference issues.
Question 5: Per NEC 250.104(A), what building systems must be bonded to the electrical grounding system?
- Only the cold water pipe
- Metal water piping, metal gas piping, and other metal piping systems that may become energized (Correct answer)
- Only systems containing electrical conductors
- Only systems within 5 feet of electrical equipment
Correct answer: Metal water piping, metal gas piping, and other metal piping systems that may become energized
NEC 250.104(A) requires bonding of interior metal water piping, and 250.104(B) requires bonding of other metal piping systems including gas piping that may become energized.
NEC 250.104 requires bonding of: (A) Metal water piping — must be bonded to the service equipment enclosure, grounded conductor, GEC, or grounding electrode. (B) Other metal piping — including gas piping, that may become energized must be bonded. Gas piping is bonded to the grounding electrode system but never used as a grounding electrode (250.52(B)). (C) Structural metal — steel framing that is interconnected to form the building frame must be bonded. The bonding conductor sizes are specified in Tables 250.66 or 250.122 depending on the application.
Question 6: When are supplemental ground rods required in addition to a single ground rod per NEC 250.53(A)(2)?
- Always — a single rod is never sufficient
- When the single rod does not achieve 25 ohms or less to ground (Correct answer)
- Only in commercial installations
- Only when the soil is sandy
Correct answer: When the single rod does not achieve 25 ohms or less to ground
NEC 250.53(A)(2) requires a supplemental electrode when a single ground rod cannot be shown to have a resistance of 25 ohms or less to ground.
NEC 250.53(A)(2) allows a single ground rod if it can be demonstrated to have a resistance to earth of 25 ohms or less. If this cannot be verified (and testing is not required by the NEC), a supplemental electrode must be installed. The supplemental rod must be at least 6 feet away from the first rod per 250.53(A)(3). In practice, most electricians install two rods rather than testing, since earth resistance testing requires specialized equipment. With two rods properly spaced, no additional resistance testing is required regardless of the actual resistance achieved.
TN LLE Grounding and Bonding 2
Per NEC 250.122, what is the minimum size equipment grounding conductor for a circuit protected by a 60-ampere overcurrent device?