TN LLE Grounding and Bonding — Questions and Answers
Question 1: Per NEC 250.30, what grounding requirements apply to a separately derived system such as a transformer?
- No grounding is required if the primary is grounded
- A system bonding jumper and grounding electrode conductor must be installed at the source (Correct answer)
- Only an equipment ground is needed
- The system can share the service grounding electrode only
Correct answer: A system bonding jumper and grounding electrode conductor must be installed at the source
NEC 250.30(A) requires separately derived systems to have a system bonding jumper and a grounding electrode conductor connected to a grounding electrode at or near the source.
NEC 250.30(A) treats separately derived systems (transformers with no direct electrical connection between primary and secondary) similarly to service equipment for grounding purposes. Requirements include: (1) A system bonding jumper connecting the grounded conductor to the equipment ground and enclosure; (2) A grounding electrode conductor connected to a grounding electrode; (3) The grounding electrode must be as near as practicable to the source. The grounding electrode conductor is sized per Table 250.66 based on the derived phase conductor size. The nearest structural metal or water pipe electrode is preferred.
Question 2: What is the difference between 'grounding' and 'bonding' as defined by the NEC?
- They mean the same thing
- Grounding connects to the earth; bonding connects conductive parts together to ensure electrical continuity (Correct answer)
- Bonding is for lightning protection; grounding is for fault protection
- Grounding is for AC systems; bonding is for DC systems
Correct answer: Grounding connects to the earth; bonding connects conductive parts together to ensure electrical continuity
The NEC defines grounding as connecting to the earth, while bonding is connecting conductive parts together to establish electrical continuity and conductivity.
The NEC Article 100 definitions distinguish these terms: Grounding (earthing) — connecting to ground or a conductive body that extends the ground connection. Its purposes include voltage stabilization and lightning/surge dissipation. Bonding — connecting metal parts to establish electrical continuity and conductivity. Its primary purpose is to create a low-impedance fault-current path back to the source so overcurrent devices operate properly. A common misconception is that grounding clears faults — it does not. Bonding creates the fault-current path; grounding stabilizes voltage. Both are essential but serve different safety functions.
Question 3: Per NEC 250.118, which of the following is recognized as an equipment grounding conductor?
- PVC conduit
- A copper conductor, EMT, IMC, RMC, or listed flexible metal conduit under specific conditions (Correct answer)
- NM cable outer sheath
- Duct tape wrapped around a conductor
Correct answer: A copper conductor, EMT, IMC, RMC, or listed flexible metal conduit under specific conditions
NEC 250.118 lists approved equipment grounding conductors including copper/aluminum wire, EMT, IMC, RMC, and certain listed flexible conduits and cable assemblies.
NEC 250.118 recognizes numerous types of equipment grounding conductors: (1) Copper, aluminum, or copper-clad aluminum conductors; (2) Rigid metal conduit (RMC); (3) Intermediate metal conduit (IMC); (4) Electrical metallic tubing (EMT); (5) Listed FMC and LFMC in specific lengths with specific fittings; (6) Armor of Type AC cable; (7) The copper sheath of MI cable; and others. PVC conduit is not conductive and cannot serve as an EGC — a separate EGC must be installed inside PVC. For NM cable, the bare or green-insulated conductor within the cable serves as the EGC.
Question 4: Per NEC 250.68(A), what is required for grounding electrode conductor connections?
- They must be soldered
- They must be accessible, except for connections made by listed irreversible compression-type connectors, exothermic welding, or buried connections (Correct answer)
- All connections must be visible at all times
- No specific accessibility requirement exists
Correct answer: They must be accessible, except for connections made by listed irreversible compression-type connectors, exothermic welding, or buried connections
NEC 250.68(A) requires GEC connections to be accessible except when made with irreversible compression connectors, exothermic welds, or connections to buried electrodes.
NEC 250.68(A) generally requires grounding electrode conductor connections to be accessible for inspection, testing, and maintenance. However, three exceptions allow concealed connections: (1) Listed irreversible compression-type connectors — these crimp-style connectors cannot be reused and provide a permanent, reliable connection; (2) Exothermic welding (Cadweld) — creates a molecular bond between conductors that is essentially permanent; (3) Connections to buried or concrete-encased electrodes — are inherently inaccessible. These exception methods must be listed and installed per manufacturer's instructions.
Question 5: What is the minimum size copper bonding jumper required for a 200A service per NEC Table 250.66?
- 8 AWG
- 6 AWG
- 4 AWG (Correct answer)
- 2 AWG
Correct answer: 4 AWG
Per NEC Table 250.66, a 200A service with 2/0 AWG copper or 4/0 AWG aluminum service conductors requires a minimum 4 AWG copper grounding electrode conductor, which is also the basis for the bonding jumper.
NEC 250.28(D) sizes the main bonding jumper using Table 250.66 (based on the largest ungrounded service conductor) or 12.5% of the equivalent cross-sectional area for conductors larger than 1,100 kcmil. For a typical 200A residential service with 2/0 AWG copper service conductors, Table 250.66 specifies a minimum 4 AWG copper bonding jumper. For 4/0 AWG aluminum service conductors (also common for 200A), the same 4 AWG copper minimum applies. Many panel manufacturers include a factory-installed main bonding jumper (the green bonding screw) sized for the panel's rating.
Question 6: Per NEC 250.24(A)(5), what is the rule regarding neutral-to-ground connections in subpanels?
- They must be bonded together as in the main panel
- The grounded conductor (neutral) must not be connected to the equipment grounding conductor or enclosure at subpanels (Correct answer)
- An additional bonding jumper is required
- It depends on the panel manufacturer
Correct answer: The grounded conductor (neutral) must not be connected to the equipment grounding conductor or enclosure at subpanels
NEC 250.24(A)(5) prohibits reconnecting the neutral to ground at any point on the load side of the service disconnecting means, including subpanels.
NEC 250.24(A)(5) is a critical rule: the grounded conductor (neutral) shall not be connected to normally non-current-carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the service disconnecting means. In subpanels, the neutral bus must be isolated (floating) from the enclosure, and the equipment grounding bus is bonded to the enclosure. Violating this rule creates parallel paths for neutral current through equipment grounding conductors, which can energize metal enclosures, create stray currents, and interfere with GFCI operation.
Question 7: 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 8: 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 9: 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 10: 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 11: 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 12: 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
Per NEC 250.30, what grounding requirements apply to a separately derived system such as a transformer?
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