Grounding and Bonding Practice Questions
Here are 30 original, unofficial grounding and bonding practice questions on NEC Article 250, with the answer, an explanation, and why each wrong choice is wrong. These are U.S. study exercises referenced to the 2023 NEC, not a full exam or instructions for doing electrical work.
Concepts and the fault path
Question 1 of 30
GB-01 · Concepts and the fault path
Under the NEC's performance requirements, what is the main job of connecting a grounded electrical system to the earth?
- A. Opening the breaker quickly when a hot conductor faults to a metal enclosure
- B. Limiting voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines, and stabilizing the voltage to earth during normal operation
- C. Carrying the neutral's normal unbalanced current
- D. Replacing the need for an equipment grounding conductor
Show answer and explanation — Question 1
Answer: B. Limiting voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines, and stabilizing the voltage to earth during normal operation
That's system grounding. The earth connection limits voltage imposed by lightning, surges, and accidental contact with higher-voltage lines, and stabilizes the system's voltage relative to earth. It is not the low-impedance return path relied on to trip the breaker.
Why not the others? A is the job of an effective ground-fault current path established by grounding and bonding: a low-impedance path back to the source, not through the earth (see GB-02). C describes normal neutral current, not the purpose of connecting the system to earth. D is wrong because an earth connection does not replace the required equipment grounding path.
Reference: 2023 NEC 250.4(A)(1), (A)(3), and (A)(5). NFPA 70, 2023 NEC — free-access reader
Jump to: Grounding electrodes · Grounding electrode conductors · Service bonding and bonding jumpers · Equipment grounding conductors · Feeders, separate buildings, piping, and generators · Answer key · Which table sizes it?
Question 2 of 30
GB-02 · Concepts and the fault path
A 120-volt circuit feeds a metal pole light. No equipment grounding conductor was run; the pole is connected only to its own ground rod. The hot conductor faults to the pole. Assume the only return path is through the earth with a total resistance of 25 ohms, and ignore all other impedance. There is no other load current, and the 20-ampere thermal-magnetic breaker has no GFCI or separate ground-fault trip function. About how much current flows, and will that current alone operate the breaker's overcurrent trip?
- A. 4.8 A, and that current alone does not operate the overcurrent trip
- B. 20 A, and the breaker trips at its rating
- C. 3,000 A, and the breaker trips instantly
- D. 0 A, because the rod blocks fault current
Show answer and explanation — Question 2
Answer: A. 4.8 A, and that current alone does not operate the overcurrent trip
I = E ÷ R = 120 ÷ 25 = 4.8 A. That's far below 20 A, so this current alone does not operate the overcurrent trip, and the faulted pole remains a shock hazard. This is why the NEC says the earth is not an effective ground-fault current path. The lesson is to provide the required effective grounding and bonding path back to the source, not to rely on a ground rod.
Why not the others? B assumes fault current somehow equals the breaker rating. C multiplies (120 × 25) instead of dividing. D is wrong because some current does flow; it's just too little to trip the breaker.
Reference: 2023 NEC 250.4(A)(5); Ohm's law with the supplied values. NFPA 70, 2023 NEC — free-access reader · OpenStax, Ohm's law
Question 3 of 30
GB-03 · Concepts and the fault path
Which conductor connects the grounding electrode to the equipment grounding conductor, the grounded conductor, or both, at the service equipment?
- A. Equipment grounding conductor
- B. Main bonding jumper
- C. Grounding electrode conductor
- D. Supply-side bonding jumper
Show answer and explanation — Question 3
Answer: C. Grounding electrode conductor
The grounding electrode conductor (GEC) is the link to the electrodes: ground rods, water pipe, concrete-encased electrode, and so on.
Why not the others? A, the equipment grounding conductor (EGC), connects equipment metal back to the source so faults clear. B connects the neutral to the equipment grounding system at the service. D bonds metal parts on the supply side of the service or a separately derived system's disconnect.
Reference: 2023 NEC 250.24(E); OSHA 29 CFR 1910.399, “Grounding electrode conductor.” The OSHA definition supports the conductor’s role; it is not a quotation from NEC Article 100. NFPA 70, 2023 NEC — free-access reader · OSHA electrical definitions
Question 4 of 30
GB-04 · Concepts and the fault path
A 480-volt to 208Y/120-volt dry-type transformer inside a building has electrically isolated primary and secondary windings and supplies a panelboard. At the first system disconnecting means, the connection from the secondary neutral to the supply-side bonding jumper, disconnect enclosure, and equipment grounding conductors is called the:
- A. Main bonding jumper
- B. Equipment bonding jumper
- C. Grounding electrode conductor
- D. System bonding jumper
Show answer and explanation — Question 4
Answer: D. System bonding jumper
A transformer like this creates a separately derived system. Its neutral-to-ground bond is the system bonding jumper. For the ordinary grounded system described, 250.30(A)(1) permits the connection at a single point from the source to the first system disconnecting means or overcurrent device; here it is at that first disconnect.
Why not the others? A is the same job, but only at the service. B connects portions of the equipment grounding path to each other. C runs to the electrode, not between the neutral and the equipment grounding conductor.
Reference: 2023 NEC 250.30(A)(1), including (A)(1)(b); OSHA 29 CFR 1910.399, “Separately derived system.” NFPA 70, 2023 NEC — free-access reader · OSHA electrical definitions
Question 5 of 30
GB-05 · Concepts and the fault path
In a new feeder-supplied panelboard on the load side of the service disconnect, in the same building as the service, the installer leaves the green bonding screw in place, connecting the neutral bar to the enclosure. What's wrong with that?
- A. Nothing, because every panelboard needs its own neutral-to-ground bond
- B. Normal neutral current can now flow on equipment grounding conductors, raceways, and enclosures, and the NEC prohibits that connection on the load side of the service
- C. It only matters if the panel contains a GFCI breaker
- D. It drops the voltage at every 120-volt load to zero
Show answer and explanation — Question 5
Answer: B. Normal neutral current can now flow on equipment grounding conductors, raceways, and enclosures, and the NEC prohibits that connection on the load side of the service
In this new feeder panel, the neutral must remain isolated from the enclosure and equipment grounding conductors. The service bond already provides the required connection. A second bond here puts everyday neutral current onto metal parts and equipment grounding paths that are not intended to carry it. Separately derived systems and express code exceptions have their own bonding rules; 'one bond anywhere in every installation' is not a safe universal rule.
Why not the others? A incorrectly extends the service-bonding requirement to every panelboard. C and D invent conditions and effects the rule doesn't depend on.
Agency cross-check: Minnesota's 2026 feeder bulletin applies the same rule under 250.142.
Reference: 2023 NEC 250.24(B), 250.6(A), and 250.30(A); Minnesota DLI, grounding and bonding for detached buildings bulletin, discussion of 250.142. NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, detached-buildings bulletin (2026)
Grounding electrodes
Question 6 of 30
GB-06 · Grounding electrodes
How much metal underground water pipe must be in direct contact with the earth for the pipe to qualify as a grounding electrode?
- A. 5 ft
- B. 8 ft
- C. 10 ft
- D. 20 ft
Show answer and explanation — Question 6
Answer: C. 10 ft
At least 10 ft in direct contact with the earth, and electrically continuous to the connection points.
Why not the others? A is the interior-piping connection distance in GB-15. B is the rod contact length. D is the length for a concrete-encased electrode or ground ring.
Agency cross-check: Minnesota's 2026 checklist (item 069, p. 8) states the same 10-ft rule under 250.52.
Reference: 2023 NEC 250.52(A)(1). NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, 2026 residential checklist
Question 7 of 30
GB-07 · Grounding electrodes
A building's only grounding electrode is a qualifying metal underground water pipe. What else does the NEC require?
- A. Nothing more, because 10 ft of buried metal pipe is enough
- B. A bond to the metal gas piping to act as a second electrode
- C. Only a resistance test showing 25 ohms or less
- D. A supplemental electrode
Show answer and explanation — Question 7
Answer: D. A supplemental electrode
A water pipe electrode always has to be supplemented by another electrode. The metal pipe could be replaced with plastic years later; the supplemental electrode provides another earth connection rather than relying only on that buried water pipe.
Why not the others? A ignores the supplement rule. B is out because metal underground gas piping is not permitted as an electrode (GB-11). C is wrong because the 25-ohm exception applies to a single rod, pipe, or plate electrode, not to a water pipe.
Agency cross-check: Minnesota's 2026 checklist (item 070, p. 8) states the same requirement under 250.53.
Reference: 2023 NEC 250.53(D)(2). NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, 2026 residential checklist
Question 8 of 30
GB-08 · Grounding electrodes
Which of these qualifies as a concrete-encased electrode?
- A. A continuous 20-ft length of ½-in. bare steel rebar installed horizontally near the bottom of a footing whose concrete is in direct contact with the earth, with at least 2 in. of concrete around the rebar
- B. 20 ft of bare 6 AWG copper in a footing in direct contact with the earth
- C. 12 ft of ½-in. bare steel rebar in a footing in direct contact with the earth
- D. 20 ft of ½-in. bare steel rebar in a slab poured over a vapor barrier
Show answer and explanation — Question 8
Answer: A. A continuous 20-ft length of ½-in. bare steel rebar installed horizontally near the bottom of a footing whose concrete is in direct contact with the earth, with at least 2 in. of concrete around the rebar
A concrete-encased electrode needs at least 20 ft of either ½-in. or larger conductive rebar or bare copper no smaller than 4 AWG (American Wire Gauge). The metal must be encased by at least 2 in. of concrete in a footing or foundation that's in direct contact with the earth.
Why not the others? B uses copper smaller than the 4 AWG minimum. C is short of 20 ft. D fails because concrete separated from the soil by a vapor barrier, insulation, or film isn't in "direct contact" with the earth.
Reference: 2023 NEC 250.52(A)(3) and its informational note. NFPA 70, 2023 NEC — free-access reader
Question 9 of 30
GB-09 · Grounding electrodes
A ground ring used as a grounding electrode must encircle the building and consist of at least:
- A. 20 ft of bare copper not smaller than 2 AWG, buried at least 30 in.
- B. 20 ft of bare copper not smaller than 4 AWG, buried at least 18 in.
- C. 8 ft of bare copper not smaller than 6 AWG, buried at least 30 in.
- D. 10 ft of bare copper not smaller than 2 AWG, buried at least 24 in.
Show answer and explanation — Question 9
Answer: A. 20 ft of bare copper not smaller than 2 AWG, buried at least 30 in.
At least 20 ft of bare copper no smaller than 2 AWG, installed at least 30 in. below the surface of the earth. The ring must be in direct contact with the earth.
Why not the others? B borrows the 4 AWG size from the concrete-encased electrode. C borrows the 8-ft rod length and the 6 AWG rod-conductor cap. D gets both the length and the depth wrong.
Reference: 2023 NEC 250.52(A)(4) and 250.53(F). NFPA 70, 2023 NEC — free-access reader
Question 10 of 30
GB-10 · Grounding electrodes
An 8-ft ground rod hits rock after 4 ft. What's the first alternative the NEC allows?
- A. Cut off the exposed 4 ft and leave the rest in the ground
- B. Drive it at an angle not more than 45° from vertical so that 8 ft is still in contact with the soil
- C. Bury it in a trench at least 18 in. deep
- D. Encase the exposed part in 2 in. of concrete
Show answer and explanation — Question 10
Answer: B. Drive it at an angle not more than 45° from vertical so that 8 ft is still in contact with the soil
The order is: drive at an angle up to 45° from vertical first. Only if rock still stops it at that angle can the rod be laid in a trench, and that trench must be at least 30 in. deep. Either way, 8 ft of rod must be in contact with the soil.
Why not the others? A leaves less than the required 8 ft in the soil. C uses the wrong trench depth and skips the angled attempt. D isn't a permitted method.
Reference: 2023 NEC 250.53(A)(4). NFPA 70, 2023 NEC — free-access reader
Question 11 of 30
GB-11 · Grounding electrodes
Which of these is a prohibited grounding-electrode type, even if its dimensions and installation otherwise appear suitable?
- A. A metal in-ground support structure in direct contact with the earth vertically for 10 ft or more
- B. A bare ¼-in.-thick steel plate exposing 2 sq ft of surface to the soil
- C. A metal underground gas piping system
- D. A rod electrode at least 8 ft long that meets the NEC's material and size rules
Show answer and explanation — Question 11
Answer: C. A metal underground gas piping system
Metal underground gas piping is on the NEC's "not permitted" list, along with aluminum and the swimming-pool structures and structural reinforcing steel described in 680.26(B)(1) and (B)(2).
Why not the others? A, B, and D are all permitted electrode types when they meet their size and installation rules.
Reference: 2023 NEC 250.52(A)(2), (A)(5), (A)(7), and (B). NFPA 70, 2023 NEC — free-access reader
Grounding electrode conductors
Question 12 of 30
GB-12 · Grounding electrode conductors
A service has two parallel sets of 250-kcmil copper conductors per phase, supplied directly from the utility. The grounding electrode conductor runs to a qualifying metal underground water pipe electrode. What is the minimum copper GEC size?
- A. 2 AWG
- B. 4 AWG
- C. 2/0 AWG
- D. 1/0 AWG
Show answer and explanation — Question 12
Answer: D. 1/0 AWG
For parallel sets, Table 250.66 uses the equivalent area: 2 × 250 = 500 kcmil. That falls in the "over 350 through 600 kcmil" copper row, which gives 1/0 copper. No electrode cap applies, because a water pipe isn't a rod, a concrete-encased electrode, or a ground ring.
Why not the others? A reads only one 250-kcmil set, landing in the "over 3/0 through 350" row. B is the 2/0–3/0 row. C is the next row up (over 600 through 1100).
Reference: 2023 NEC Table 250.66 and its note on parallel sets. NFPA 70, 2023 NEC — free-access reader
Question 13 of 30
GB-13 · Grounding electrode conductors
A service is supplied by 600-kcmil copper conductors. A GEC runs only to a concrete-encased electrode and doesn't extend to any other electrode. What's the largest copper size the NEC requires for that conductor?
- A. 1/0 AWG
- B. 6 AWG
- C. 4 AWG
- D. 2 AWG
Show answer and explanation — Question 13
Answer: C. 4 AWG
The part of the GEC that's the sole connection to a concrete-encased electrode never has to be larger than 4 AWG copper, whatever the service size.
Why not the others? A is the plain Table 250.66 value, which the concrete-encased cap overrides. B is the cap for rod, pipe, and plate electrodes. D is the minimum size of a ground-ring conductor, which isn't relevant here.
Reference: 2023 NEC 250.66(B). NFPA 70, 2023 NEC — free-access reader
Question 14 of 30
GB-14 · Grounding electrode conductors
An 800-kcmil copper service has a ground ring made of 2 AWG bare copper. Table 250.66 alone would call for 2/0. What's the most the NEC requires for the GEC portion that connects only to the ring?
- A. 2/0 AWG
- B. 2 AWG
- C. 4 AWG
- D. 3/0 AWG
Show answer and explanation — Question 14
Answer: B. 2 AWG
A GEC that connects only to a ground ring never has to be larger than the conductor the ring is made of. Here the ring is 2 AWG, so the largest required GEC size is 2 AWG; this is not a prohibition on a larger conductor.
Why not the others? A applies the table without the ring cap. C is the concrete-encased cap. D is the table's largest copper value.
Reference: 2023 NEC 250.66(C). NFPA 70, 2023 NEC — free-access reader
Question 15 of 30
GB-15 · Grounding electrode conductors
In a house, interior metal water piping is used to extend the grounding electrode conductor connection to the underground metal water pipe electrode. Measured along the piping, the connection must be within what distance of where the pipe enters the building?
- A. 5 ft
- B. 6 ft
- C. 10 ft
- D. Anywhere on electrically continuous metal piping
Show answer and explanation — Question 15
Answer: A. 5 ft
Within 5 ft of the point of entrance, measured along the pipe. Farther in, a plastic fitting or a pipe repair could break the path without anyone noticing.
Why not the others? B is the rod-spacing number. C is the water pipe's earth-contact length. D ignores the 5-ft limit for this house. A separate exception permits more distant connections in certain industrial, commercial, and institutional buildings only when its maintenance, supervision, and exposed-piping conditions are met.
Reference: 2023 NEC 250.68(C)(1) and its exception. NFPA 70, 2023 NEC — free-access reader
Question 16 of 30
GB-16 · Grounding electrode conductors
A grounding electrode conductor is run inside a steel (ferrous) rigid metal conduit for physical protection. What does the NEC require?
- A. Nothing extra, because steel conduit is the best protection
- B. Bond the conduit to the GEC at the service end only
- C. Replace the conduit, because a GEC may never be in metal conduit
- D. Bond the conduit at both ends to the GEC or grounding electrode, as applicable
Show answer and explanation — Question 16
Answer: D. Bond the conduit at both ends to the GEC or grounding electrode, as applicable
A ferrous raceway around a GEC must be electrically continuous and bonded at each end to the GEC or grounding electrode, as applicable. This makes the raceway part of the parallel conductive path rather than an unbonded sleeve.
Why not the others? A skips the required bonding. B bonds only one end. C is wrong because rigid metal conduit is a permitted way to protect a GEC exposed to damage.
Agency cross-check: Minnesota's 2026 checklist (item 073, p. 9) cites 250.64 for GEC continuity and protection.
Reference: 2023 NEC 250.64(B) and (E)(1)–(3). NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, 2026 residential checklist
Service bonding and bonding jumpers
Question 17 of 30
GB-17 · Service bonding and bonding jumpers
A service drawing specifies one 900-kcmil copper conductor per phase and a wire-type copper main bonding jumper. A reviewer notices that the jumper schedule was selected from the equipment-grounding-conductor table using the main breaker's rating. Which correction uses the right input and gives the minimum jumper size?
- A. Keep Table 250.122 and use the main breaker's ampere rating
- B. Use Table 250.102(C)(1), but select the over-350-through-600-kcmil row: 1/0 AWG copper
- C. Use Table 250.102(C)(1) and the 900-kcmil phase-conductor size: 2/0 AWG copper
- D. Apply the 12.5% method because 900 kcmil is above 600 kcmil
Show answer and explanation — Question 17
Answer: C. Use Table 250.102(C)(1) and the 900-kcmil phase-conductor size: 2/0 AWG copper
Main bonding jumpers are sized from Table 250.102(C)(1) using the largest ungrounded conductor. The drawing has one 900-kcmil copper conductor per phase, so the 'over 600 through 1100' copper row gives 2/0 copper. The breaker's ampere rating is not the input for this main bonding jumper.
Why not the others? A keeps the wrong table and input. B selects the row below the actual 900-kcmil conductor size. D applies the percentage method before its over-1,100-kcmil copper threshold is reached.
Reference: 2023 NEC 250.28(D)(1); Table 250.102(C)(1) and Note 1. NFPA 70, 2023 NEC — free-access reader
Question 18 of 30
GB-18 · Service bonding and bonding jumpers
A 208Y/120-volt service has 3/0 copper ungrounded conductors in one raceway. The calculated neutral load is only 40 A. Looking specifically at the grounding-and-bonding minimum in 250.24(D)(1), what is the smallest copper grounded (neutral) service conductor?
- A. 8 AWG
- B. 6 AWG
- C. 3/0 AWG
- D. 4 AWG
Show answer and explanation — Question 18
Answer: D. 4 AWG
The service neutral is also the fault-current return path to the utility, so it has a floor set by Table 250.102(C)(1) no matter how small the neutral load is. For 3/0 copper ungrounded conductors, that floor is 4 AWG copper. It also has to be large enough for the calculated neutral load; use whichever is larger.
Why not the others? A and B are below the fault-return minimum, regardless of the small calculated neutral load. C is larger than the table minimum; this question asks for the grounding-and-bonding floor, not a separate ampacity calculation.
Reference: 2023 NEC 250.24(D)(1); Table 250.102(C)(1). NFPA 70, 2023 NEC — free-access reader
Question 19 of 30
GB-19 · Service bonding and bonding jumpers
A single service disconnect is supplied by three parallel sets of 600-kcmil copper conductors per phase. What's the minimum size of its wire-type copper main bonding jumper? (For this calculation, 4/0 AWG = 211,600 circular mils.)
- A. 3/0 AWG
- B. 4/0 AWG
- C. 250 kcmil
- D. 350 kcmil
Show answer and explanation — Question 19
Answer: C. 250 kcmil
The equivalent area is 3 × 600 = 1,800 kcmil, which is over the table's 1,100-kcmil copper limit. Above that limit, the bonding jumper must have at least 12.5% of the area: 0.125 × 1,800,000 = 225,000 circular mils. 4/0 (211,600) is too small, so the next standard size, 250 kcmil, is the minimum.
Why not the others? A is Table 250.66's largest copper GEC, but bonding jumpers don't stop at 3/0 above 1,100 kcmil. B is just under 225,000 circular mils. D is larger than required.
Reference: 2023 NEC 250.28(D)(1); Table 250.102(C)(1), Note 1. The 4/0 circular-mil area is supplied calculation data. NFPA 70, 2023 NEC — free-access reader
Question 20 of 30
GB-20 · Service bonding and bonding jumpers
At a new service, which proposal for an intersystem bonding termination (IBT) satisfies the external-location, access, and terminal-capacity provisions of 250.94(A)? Assume the chosen device is securely mounted and bonded at the service equipment by a permitted method and is identified for the connected conductor materials.
- A. A single lug inside the panelboard, behind the dead front
- B. A two-terminal device mounted on the water pipe in the crawl space
- C. A ground clamp on the nearest ground rod
- D. A listed device mounted outside the enclosures, accessible, with terminals for at least three intersystem bonding conductors, that doesn't block opening the enclosure
Show answer and explanation — Question 20
Answer: D. A listed device mounted outside the enclosures, accessible, with terminals for at least three intersystem bonding conductors, that doesn't block opening the enclosure
The IBT gives phone, cable TV, satellite, and similar systems an accessible place to bond to the building's grounding system. It sits outside the enclosures, has room for at least three bonding conductors, and can't interfere with opening the service, meter, or disconnect enclosure.
Why not the others? A is inside an enclosure and has only one terminal. B has too few terminals and is not at the service equipment or metering enclosure. C describes a grounding clamp, not the listed IBT with capacity for at least three intersystem bonding conductors required by this question.
Reference: 2023 NEC 250.94(A). NFPA 70, 2023 NEC — free-access reader
Question 21 of 30
GB-21 · Service bonding and bonding jumpers
An equipment bonding jumper is installed on the outside of a metal raceway, across an expansion fitting. Under the general rule, it must be:
- A. No longer than 6 ft and routed with the raceway
- B. No longer than 3 ft, routed any way
- C. No longer than 10 ft if it's 6 AWG or larger
- D. Inside the raceway, because outside jumpers aren't permitted
Show answer and explanation — Question 21
Answer: A. No longer than 6 ft and routed with the raceway
Bonding jumpers may run inside or outside a raceway. Outside, they're limited to 6 ft and must follow the raceway. A separate exception addresses specified bonding at outside pole locations; it does not change the general rule in this question.
Why not the others? B and C use limits the NEC doesn't set. D is wrong because outside installation is expressly permitted.
Reference: 2023 NEC 250.102(E)(2); 250.98. NFPA 70, 2023 NEC — free-access reader
Equipment grounding conductors
Question 22 of 30
GB-22 · Equipment grounding conductors
A feeder is protected by a 125-ampere breaker. It supplies a panelboard with a 225-ampere bus, and the calculated load is 83 amperes. What minimum copper wire-type equipment grounding conductor size does Table 250.122 specify, before any separately required increase?
- A. 6 AWG
- B. 4 AWG
- C. 8 AWG
- D. 2 AWG
Show answer and explanation — Question 22
Answer: A. 6 AWG
EGCs are sized from Table 250.122 by the rating of the overcurrent device protecting the circuit, not the load and not the panel's bus rating. A 125-A breaker falls in the "not exceeding 200 A" row, which gives 6 AWG copper.
Why not the others? B uses the 225-A bus rating (the 300-A row). C uses the 83-A load (the 100-A row). D adds all three numbers together (433 A, the 500-A row), which no rule does.
Agency cross-check: Minnesota's 2026 feeder bulletin sizes the feeder EGC from the supply-side overcurrent protection using Table 250.122.
Reference: 2023 NEC 250.122(A) and Table 250.122. NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, detached-buildings bulletin (2026)
Question 23 of 30
GB-23 · Equipment grounding conductors
What minimum copper wire-type EGC size does Table 250.122 specify for a circuit protected by a 400-ampere overcurrent device, before any separately required increase?
- A. 1 AWG
- B. 3 AWG
- C. 2 AWG
- D. 1/0 AWG
Show answer and explanation — Question 23
Answer: B. 3 AWG
Table 250.122 gives 3 AWG copper (1 AWG aluminum) for 400 A.
Why not the others? A is the 600-A copper row (and the 400-A aluminum value). C is the 500-A row. D is the 800-A row.
Reference: 2023 NEC Table 250.122. NFPA 70, 2023 NEC — free-access reader
Question 24 of 30
GB-24 · Equipment grounding conductors
For this 40-ampere circuit, 8 AWG copper is the minimum ungrounded-conductor size for the installation, and the starting copper EGC is 10 AWG. Solely to limit voltage drop, the ungrounded conductors are increased to 6 AWG; the increase is not required by temperature correction or ampacity adjustment. Apply the proportional method in 250.122(B), not its qualified-person exception. Using conductor areas of 10,380 cmil (10 AWG), 16,510 cmil (8 AWG), and 26,240 cmil (6 AWG), what's the minimum copper EGC?
- A. 10 AWG
- B. 6 AWG
- C. 12 AWG
- D. 8 AWG
Show answer and explanation — Question 24
Answer: D. 8 AWG
For this voltage-drop increase, the wire-type EGC grows by the same area ratio. Ratio = 26,240 ÷ 16,510 ≈ 1.58934. Keeping the unrounded ratio, the new EGC area is 10,380 × (26,240 ÷ 16,510) ≈ 16,497 cmil. The smallest conductor at least that large is 8 AWG (16,510 cmil). The 2023 rule distinguishes this increase from conductor increases required by 310.15(B) or (C); it also has a qualified-person exception that is outside this exercise.
Why not the others? A skips the proportional increase. B matches the phase conductors, which isn't required. C is smaller than the starting size.
Reference: 2023 NEC 250.122(B). Conductor areas are supplied calculation data; retain the unrounded ratio until the final comparison. NFPA 70, 2023 NEC — free-access reader
Question 25 of 30
GB-25 · Equipment grounding conductors
One EMT raceway holds three circuits protected by 20-A, 40-A, and 100-A breakers. A single wire-type EGC will serve all three. What minimum copper size follows from 250.122(C) and Table 250.122, before any separately required increase?
- A. 12 AWG
- B. 8 AWG
- C. 10 AWG
- D. 6 AWG
Show answer and explanation — Question 25
Answer: B. 8 AWG
One EGC can serve several circuits in the same raceway. It's sized for the largest overcurrent device, here 100 A, which gives 8 AWG copper.
Why not the others? A sizes it for the smallest circuit. C sizes it for the 40-A circuit. D adds the breakers together (160 A, which falls in the 200-A row); the rule doesn't add them.
Reference: 2023 NEC 250.122(C). NFPA 70, 2023 NEC — free-access reader
Question 26 of 30
GB-26 · Equipment grounding conductors
An 800-A feeder breaker supplies two parallel sets of 500-kcmil copper conductors, each set in its own raceway with a wire-type EGC. What minimum copper EGC size does Table 250.122 require in each raceway under 250.122(F)(1)(b), before any separately required increase?
- A. 3 AWG
- B. 1/0 AWG
- C. 1 AWG
- D. 2/0 AWG
Show answer and explanation — Question 26
Answer: B. 1/0 AWG
Each raceway's EGC is sized for the full 800-A device, not half of it: 1/0 copper in each raceway. A fault in one raceway can push the full fault current through that raceway's EGC alone.
Why not the others? A sizes each EGC for 400 A, the classic "split it in half" error. C is the 600-A row. D is the 1,000-A row.
Reference: 2023 NEC 250.122(F)(1)(b) and Table 250.122. NFPA 70, 2023 NEC — free-access reader
Feeders, separate buildings, piping, and generators
Question 27 of 30
GB-27 · Feeders, separate buildings, piping, and generators
A detached garage is supplied by a new 120/240-volt feeder that includes an equipment grounding conductor. How are the neutral and the grounding connected at the garage's disconnecting means?
- A. Bond the neutral to the enclosure and to new ground rods, the same as at a service
- B. Skip the grounding electrode, because the feeder has an EGC
- C. Connect the neutral to the ground rods but not to the enclosure
- D. Keep them separate: connect the EGC (not the neutral) to the disconnect enclosure and to the garage's grounding electrode system
Show answer and explanation — Question 27
Answer: D. Keep them separate: connect the EGC (not the neutral) to the disconnect enclosure and to the garage's grounding electrode system
At the new feeder-supplied building described, the EGC does the grounding and bonding. The neutral stays isolated from the enclosure, the EGC, and the electrodes. The garage still needs its own grounding electrode system; the single-branch-circuit exception, including a multiwire branch circuit with an EGC, does not apply to this feeder.
Why not the others? A treats the garage like a service, which puts neutral current on metal parts. B misapplies the single-branch-circuit exception to a feeder. C reconnects this feeder neutral to the electrodes, which 250.32(B)(1) prohibits.
Agency cross-check: Minnesota's 2026 checklist (item 067, p. 8) and 2026 feeder bulletin apply the same 250.32 rule.
Reference: 2023 NEC 250.32(A) and (B)(1). NFPA 70, 2023 NEC — free-access reader · Minnesota DLI, detached-buildings bulletin (2026) · Minnesota DLI, 2026 residential checklist
Question 28 of 30
GB-28 · Feeders, separate buildings, piping, and generators
A gas furnace is supplied by a 15-A branch circuit that includes a 14 AWG copper EGC. Rigid metal gas piping is electrically continuous to the bonded furnace, and this is the circuit likely to energize the piping. Considering the electrical bonding rule in NEC 250.104(B), not an installation of corrugated stainless steel tubing (CSST), which statement is correct?
- A. The gas piping must be used as a grounding electrode
- B. A separate 4 AWG jumper from the gas piping to the service is always required
- C. The furnace circuit's EGC is permitted to serve as the bonding means for the gas piping
- D. Gas piping must never be bonded
Show answer and explanation — Question 28
Answer: C. The furnace circuit's EGC is permitted to serve as the bonding means for the gas piping
Metal piping likely to become energized, including gas piping, must be bonded. The bonding is sized from Table 250.122 based on the circuit likely to energize it, and that circuit's own EGC is permitted to do the job. This answer addresses the NEC rule for the rigid-piping scenario, not a complete fuel-gas-system design or CSST installation.
Why not the others? A is prohibited (GB-11). B applies a service-sized jumper this rule doesn't require. D contradicts the bonding requirement.
Reference: 2023 NEC 250.104(B). NFPA 70, 2023 NEC — free-access reader
Question 29 of 30
GB-29 · Feeders, separate buildings, piping, and generators
A house has 2/0 copper service-entrance conductors. What's the minimum copper bonding jumper for the interior metal water piping system?
- A. 8 AWG
- B. 6 AWG
- C. 4 AWG
- D. 2 AWG
Show answer and explanation — Question 29
Answer: C. 4 AWG
Metal water piping bonding jumpers under this service-supplied-house rule are sized from Table 250.102(C)(1) based on the service conductors. The 2/0–3/0 copper row gives 4 AWG. Under 250.104(A)(1), they are not required to be larger than 3/0 copper. Water-piping bonding at a feeder-supplied separate building has a different sizing rule under 250.104(A)(3).
Why not the others? A is the row for 2 AWG and smaller. B is the Table 250.122 value for a 200-A breaker, which is the wrong table. D is the "over 3/0 through 350" row.
Reference: 2023 NEC 250.104(A)(1); Table 250.102(C)(1). NFPA 70, 2023 NEC — free-access reader
Question 30 of 30
GB-30 · Feeders, separate buildings, piping, and generators
A permanently installed, grounded 120/240-volt standby generator supplies a building through a transfer switch that switches the neutral (grounded conductor) as well as the ungrounded conductors. No other solid circuit-conductor connection remains between the generator and utility systems. How is the generator system treated?
- A. As a separately derived system, requiring a system bonding jumper and a grounding electrode connection under 250.30
- B. As part of the service, so the generator must not have a neutral-to-ground bond
- C. As an ungrounded system
- D. As a branch circuit of the service panel
Show answer and explanation — Question 30
Answer: A. As a separately derived system, requiring a system bonding jumper and a grounding electrode connection under 250.30
Switching the neutral, with no other solid circuit-conductor connection between the systems, makes this generator a separately derived system. It then needs a system bonding jumper and the grounding electrode connection required by 250.30. 'Separately derived' does not mean an isolated electrode system. Grounding-electrode connections follow 250.30(A)(4) and 250.58; an outdoor source also needs the source-location electrode connection required by 250.30(C).
Why not the others? B ignores the switched neutral and incorrectly treats the generator as service equipment. C and D misclassify the system.
Reference: 2023 NEC 250.30, Informational Note No. 1; 250.30(A)(1), (A)(4), and (C); 250.35(A); 250.58. OSHA 29 CFR 1910.399, “Separately derived system.” NFPA 70, 2023 NEC — free-access reader · OSHA electrical definitions
Check your answers
Count your correct answers out of 30. A blank answer counts as a miss.
| Question | ID | Answer | Topic group |
|---|---|---|---|
| 1 | GB-01 | B | Concepts and the fault path |
| 2 | GB-02 | A | Concepts and the fault path |
| 3 | GB-03 | C | Concepts and the fault path |
| 4 | GB-04 | D | Concepts and the fault path |
| 5 | GB-05 | B | Concepts and the fault path |
| 6 | GB-06 | C | Grounding electrodes |
| 7 | GB-07 | D | Grounding electrodes |
| 8 | GB-08 | A | Grounding electrodes |
| 9 | GB-09 | A | Grounding electrodes |
| 10 | GB-10 | B | Grounding electrodes |
| 11 | GB-11 | C | Grounding electrodes |
| 12 | GB-12 | D | Grounding electrode conductors |
| 13 | GB-13 | C | Grounding electrode conductors |
| 14 | GB-14 | B | Grounding electrode conductors |
| 15 | GB-15 | A | Grounding electrode conductors |
| 16 | GB-16 | D | Grounding electrode conductors |
| 17 | GB-17 | C | Service bonding and bonding jumpers |
| 18 | GB-18 | D | Service bonding and bonding jumpers |
| 19 | GB-19 | C | Service bonding and bonding jumpers |
| 20 | GB-20 | D | Service bonding and bonding jumpers |
| 21 | GB-21 | A | Service bonding and bonding jumpers |
| 22 | GB-22 | A | Equipment grounding conductors |
| 23 | GB-23 | B | Equipment grounding conductors |
| 24 | GB-24 | D | Equipment grounding conductors |
| 25 | GB-25 | B | Equipment grounding conductors |
| 26 | GB-26 | B | Equipment grounding conductors |
| 27 | GB-27 | D | Feeders, separate buildings, piping, and generators |
| 28 | GB-28 | C | Feeders, separate buildings, piping, and generators |
| 29 | GB-29 | C | Feeders, separate buildings, piping, and generators |
| 30 | GB-30 | A | Feeders, separate buildings, piping, and generators |
| Topic group | Questions | Count |
|---|---|---|
| Concepts and the fault path | 1–5 | 5 |
| Grounding electrodes | 6–11 | 6 |
| Grounding electrode conductors | 12–16 | 5 |
| Service bonding and bonding jumpers | 17–21 | 5 |
| Equipment grounding conductors | 22–26 | 5 |
| Feeders, separate buildings, piping, and generators | 27–30 | 4 |
| Total | 30 |
Your score describes how you did on these 30 questions. It isn't an official score, and it can't predict whether you'll pass. With four to six questions per group, read a weak group as "worth another look," not as a measurement. The group sizes reflect how this set was built, not how any exam weights the topic.
Review the questions you missed. For each miss, write down three things before you reread our explanation: which conductor or connection the question was about, the condition that decided it (material, electrode type, breaker rating, parallel sets), and why the answer you picked fails. Then find the rule in your own code book, in the edition your exam uses. Retry the missed questions tomorrow with the explanations closed.
Which table sizes it?
A common sizing mistake is reading the right row in the wrong table. Tables 250.66 and 250.102(C)(1) have identical rows up to 1,100 kcmil copper, which is exactly why they get mixed up. What decides the table is what the conductor does, so name its job first.
| What you're sizing | Size it from | Table or rule | Watch for |
|---|---|---|---|
| Grounding electrode conductor (to rods, water pipe, concrete-encased electrode, ground ring) | Largest ungrounded service or derived conductor (or the parallel equivalent area) | 250.66 | Maximum required sizes for a portion that does not continue to an electrode requiring a larger conductor: 6 AWG copper to rod, pipe, or plate electrodes; 4 AWG copper to a concrete-encased electrode; the ring's conductor size to a ground ring. These are not maximum permitted sizes. The largest copper value in the table is 3/0. |
| Equipment grounding conductor and load-side equipment bonding jumpers | Rating of the breaker or fuse ahead of it, not the load or the bus rating | 250.122 | The table is a starting minimum. Apply proportional increases when required by 250.122(B), including its stated exceptions. One EGC for several circuits is sized for the largest breaker. For the parallel-raceway arrangement in GB-26, each raceway's wire-type EGC is sized from the full protective-device rating. |
| Main and system bonding jumpers; supply-side bonding jumpers and the service-neutral grounding/bonding minimum | Largest ungrounded conductor or the applicable parallel-conductor area | 250.102(C)(1), as directed by 250.24(D), 250.28(D), and 250.102(C) | Above 1,100 kcmil copper or 1,750 kcmil aluminum, the table's notes invoke the 12.5% method. Individual versus common parallel-raceway supply-side jumpers have different input rules. The service neutral must also carry its calculated load. |
| Metal water piping bonding jumper at a service-supplied building | Service conductors | 250.104(A)(1) and Table 250.102(C)(1) | Not required to be larger than 3/0 copper under this rule. Do not transfer this table to every feeder-supplied building. |
| Metal water piping bonding jumper at a separate building supplied by a feeder or branch circuit | The feeder or branch circuit's overcurrent protection | 250.104(A)(3) → 250.102(D) → 250.122 | Not required to be larger than the largest ungrounded feeder or branch-circuit conductor supplying the building. This is distinct from the service-supplied-house example in GB-29. |
| Other metal piping (gas, air, and so on) likely to become energized | Rating of the circuit likely to energize it | 250.104(B) and Table 250.122 | That circuit's EGC is permitted to serve as the bond. The rigid-gas-piping scenario in GB-28 is not a CSST design example. |
Memory rule for these exercises: GEC → 250.66; wire-type EGC → 250.122; main or system bonding jumper → 250.102(C)(1). Then read the applicable section and exceptions. Whether a breaker happens to be nearby is not enough to classify every conductor.
Before any table, ask which of these jobs the conductor is doing:
| Ask this | If yes, it's the | Practice |
|---|---|---|
| Does it carry normal current as part of the circuit and connect to the system's grounded point? | Grounded conductor (often the neutral) | GB-05, GB-18 |
| Does it connect equipment metal back toward the source as part of the effective fault-current path? | Equipment grounding conductor | GB-22 to GB-27 |
| Does it connect the system to an electrode? | Grounding electrode conductor | GB-03, GB-12 to GB-16 |
| Does it join the neutral to the equipment grounding/bonding system at the service or a permitted separately derived system bonding point? | Main or system bonding jumper | GB-04, GB-17, GB-19, GB-30 |
References: 2023 NEC 250.24(D), 250.28(D), 250.30(A), 250.66, 250.102(C)–(E), 250.104, and 250.122. Sources are linked under each question above.
Grounding vs. bonding in one minute
- System grounding connects the system to the earth. It limits voltage from lightning, surges, and contact with higher-voltage lines, and stabilizes the voltage to earth. Earth alone is not the effective ground-fault current path relied on to operate overcurrent protection (GB-01 and GB-02).
- Bonding establishes electrical continuity between conductive parts. Together with the equipment grounding path, it provides the low-impedance return path to the source needed for protective-device operation.
- Where they meet: The main bonding jumper connects the grounded conductor to the grounding/bonding system at the service. A separately derived system uses a system bonding jumper under its own rules; an ordinary new feeder panel is not another permitted neutral-bonding point (GB-04 and GB-05).
References: 2023 NEC 250.4(A), 250.24(B)–(C), and 250.30(A); OSHA electrical definitions, “Bonding (Bonded).”
Which NEC edition do these questions use?
Use the 2023 NEC for the code references in this set. Use the edition named in your exam's current candidate bulletin, not the year in a practice page's title. An installation-code adoption date is not automatically an exam-transition date: the Minnesota DLI code page lists them separately. The Texas electrician exam page provides a direct route to that state's candidate bulletin and reference-material requirements; it is not a rule for other states.
The agency cross-checks beside selected answers come from Minnesota DLI's 2026 NEC inspection checklist, grounding items 066–073 on pages 8–9, and its 2026 bulletin on feeders to detached buildings. They support the particular points identified, not a claim that every question, exception, and subsection has been verified against the 2026 NEC. This is not a 2026-edition practice set.
The subsection letters in parentheses can shift between editions, so confirm each reference in your own book. Consult NFPA's 2023 free-access reader for the edition used here.
Next practice
For broader topic review, use the journeyman electrician exam prep guide. To stay with this topic, return to Question 1 and retry the questions you missed before revealing the answers.
Sources and independence
By Castleport Test Prep Editorial Team. Last verified: September 29, 2026 — 2023 reference passages and table entries, the linked agency notes, and the supplied-input calculations. We could not open the full NEC text in NFPA's interactive reader. The code checks used an externally hosted reproduction of NFPA 70 (2023); the authority is the NFPA text, not an independent interpretation by that host. Federal definitions and state guidance support the specific supplemental points identified above, not the entire code edition.
This page was prepared with AI assistance under our editorial standards. It has not been reviewed by a licensed electrician.
- NFPA 70, 2023 NEC free-access reader. The exact section or table to consult is named under each question.
- Minnesota DLI, Electrical inspection checklist, 2026 NEC, grounding and bonding items 066–073, pages 8–9. Supplemental agency context, not a complete code text.
- Minnesota DLI, Grounding and bonding for detached buildings (2026 bulletin), one page.
- OSHA, 29 CFR 1910.399 electrical definitions, grounding electrode conductor, bonding, and separately derived system.
- OpenStax, University Physics Volume 2, section 9.4, equation 9.11, for the resistive model in GB-02. Its question and numbers are this set's teaching example, not a field measurement.
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