NEC 2023 · checked by a second reviewer · November 7, 2026
NEC Article 250 Practice Questions (10 Free, With Answers)
Grounding and Bonding: 10 original questions on system grounding, electrodes, bonding jumpers and equipment grounding conductors. Click an answer to see the explanation and the NEC section. No sign-up.
NEC Article 250 · 10 questions0 / 10 answered
A metal pole light in a parking lot has its own ground rod at the base. Why must an equipment grounding conductor still be run with the circuit?
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Correct answer: B. The earth's resistance is far too high to let enough fault current flow to trip the breaker, so the NEC does not accept it as the fault return path. An EGC back to the source is required. Bonding the neutral at the pole is tempting but would put load current on the metal parts and is not permitted on the load side of the service.Reference: NEC 250.4(A)(5); NEC 250.54
A service has 3/0 AWG copper ungrounded conductors. What is the minimum size copper grounding electrode conductor to the metal underground water pipe electrode?
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Correct answer: C. For service conductors of 2/0 or 3/0 copper, the GEC table requires 4 AWG copper. 6 AWG is tempting because it is the most a GEC to a ground rod ever needs to be, but that limit applies only to rod, pipe or plate electrodes, not to a water pipe.Reference: NEC 250.66 and Table 250.66
A 20-A circuit normally uses 12 AWG copper conductors and a 12 AWG EGC. To limit voltage drop, the ungrounded conductors are increased to 10 AWG. What size wire-type EGC is now required? (12 AWG = 6,530 cmil; 10 AWG = 10,380 cmil)
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Correct answer: B. When ungrounded conductors are upsized for reasons such as voltage drop, the wire-type EGC must be increased in proportion to the circular-mil increase. 10,380 / 6,530 is about 1.59, and 6,530 x 1.59 = 10,380 cmil, which is 10 AWG. Keeping 12 AWG (the normal table value for 20 A) is the tempting answer, but the table value is only the starting point.Reference: NEC 250.122(B)
While driving an 8-ft ground rod, the installer hits rock. Which alternative does the NEC allow?
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Correct answer: A. A rod must normally be driven to 8 ft. Where rock is hit, it may be driven at an oblique angle of up to 45 degrees, or, if rock is met even at that angle, laid in a trench at least 30 in. deep. A 12-in. trench is tempting because 12 in. appears elsewhere in burial rules, but it is not deep enough for a rod.Reference: NEC 250.53(A)(4)
To qualify as a concrete-encased electrode, steel reinforcing bar in a footing that is in direct contact with the earth must be at least how long (in one or more pieces connected together)?
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Correct answer: C. A concrete-encased electrode needs at least 20 ft of 1/2-in. or larger rebar (or 4 AWG or larger bare copper) encased in at least 2 in. of concrete. 10 ft is tempting because it is the earth-contact length for a metal water pipe electrode.Reference: NEC 250.52(A)(3)
The intersystem bonding termination installed at a service must have capacity for connecting at least how many intersystem bonding conductors (for systems such as telephone, CATV and antennas)?
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Correct answer: D. The intersystem bonding termination must be accessible and have terminals for at least three intersystem bonding conductors, so communication, CATV and antenna systems can each bond to it. Two is tempting because many homes have only phone and cable TV, but the device must allow for three.Reference: NEC 250.94(A)
A service has four parallel sets of 500-kcmil copper ungrounded conductors. What is the minimum size of the copper main bonding jumper?
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Correct answer: D. The main bonding jumper is sized from Table 250.102(C)(1). Above 1,100 kcmil copper, it must be at least 12 1/2% of the total area: 4 x 500 = 2,000 kcmil, and 2,000 x 0.125 = 250 kcmil. 3/0 is tempting because that is where the GEC table (250.66) stops, but the bonding-jumper table keeps growing with the service size.Reference: NEC 250.28(D)(1); NEC 250.102(C)(1)
A detached shed is supplied by one 20-A, 120-V branch circuit that includes an equipment grounding conductor. Is a grounding electrode required at the shed?
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Correct answer: B. A building supplied by a feeder or branch circuits normally needs a grounding electrode, but there is an exception when only one branch circuit, including a multiwire branch circuit, supplies it and that circuit has an EGC. Bonding the neutral to the frame is the tempting wrong idea: at a separate building fed with an EGC, the neutral stays isolated from equipment grounding.Reference: NEC 250.32(A)
An 8 AWG copper grounding electrode conductor runs along a basement wall where it is not exposed to physical damage. What does the NEC require?
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Correct answer: A. Only GECs of 6 AWG or larger may run along a surface without protection when not exposed to physical damage. Conductors smaller than 6 AWG, like 8 AWG, must always be protected in an approved raceway or cable armor. The 'not exposed to damage' answer is tempting because that exception exists, but it is for 6 AWG and larger.Reference: NEC 250.64(B)
A 4-ft length of listed flexible metal conduit (FMC) with listed fittings feeds a motor. The circuit conductors are protected by a 30-A breaker. Can the FMC serve as the equipment grounding conductor?
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Correct answer: C. Listed FMC can act as the EGC only when all conditions are met: listed fittings, circuit overcurrent protection of 20 A or less, trade size 1 1/4 or smaller, a total flexible length of 6 ft or less in the fault path, and no need for flexibility after installation. The 30-A breaker fails the overcurrent condition, so a wire-type EGC is required. At a motor the FMC is usually there to absorb vibration, which also calls for a wire-type EGC. 'Any FMC under 6 ft' is tempting because the length condition is met, but every condition must be met.Reference: NEC 250.118(A)(5)
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