NEC 2023 · checked by a second reviewer · November 7, 2026
NEC Articles 300 & 310 Practice Questions (10 Free, With Answers)
Wiring Methods and Conductors: 10 original questions on cover depths, protection of cables, conductor ampacity and adjustment. Click an answer to see the explanation and the NEC section. No sign-up.
NM cable runs through holes bored in wood studs. How far must the edge of each hole be from the nearest edge of the stud, unless a steel plate is installed?
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Correct answer: D. Bored holes must be kept at least 1 1/4 in. from the nearest edge of the wood member so screws and nails are unlikely to reach the cable. Where that can't be done, a steel plate at least 1/16 in. thick must protect the cable.Reference: NEC 300.4(A)(1)
EMT is installed under a corrugated metal roof deck. What is the minimum distance from the lowest surface of the roof decking to the top of the raceway?
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Correct answer: A. Cables, raceways and boxes under metal-corrugated roof decking must be kept at least 1 1/2 in. below the lowest surface of the deck, to protect them from roofing screws and fasteners. 1 1/4 in. is tempting because it is the distance for holes bored in wood framing.Reference: NEC 300.4(E)
Direct-buried UF cable comes up out of the ground on the side of a building. Over what range must it be protected by an enclosure or raceway?
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Correct answer: B. Direct-buried conductors or cables emerging from grade must be protected from the minimum cover depth required by 300.5 up to at least 8 ft above finished grade. Protecting only where vehicles can hit it is tempting, but the rule applies to every emerging cable.Reference: NEC 300.5(D)(1)
A 120/240-volt feeder in rigid metal conduit (RMC) is buried in a general location (not under a building, slab, street or driveway). What is the minimum cover?
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Correct answer: C. Rigid metal conduit and IMC need only 6 in. of cover in general locations because the steel raceway is strong. 18 in. is the value for nonmetallic raceways such as PVC, and 24 in. is for direct-buried cables, which is why those answers tempt.Reference: NEC 300.5(A) and Table 300.5(A)
A circuit's ungrounded conductors run in one PVC conduit. Where must the neutral and the equipment grounding conductor of the same circuit be installed?
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Correct answer: D. All conductors of the same circuit, including the grounded conductor and the EGC, must be in the same raceway, cable, trench or cord, with limited exceptions. Keeping them together keeps the impedance low and avoids inductive heating. Routing the EGC separately is the tempting mistake.Reference: NEC 300.3(B)
A device box has a 2 in. x 3 in. opening. How much free conductor must be left at the box for splicing or connecting a device?
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Correct answer: B. Each conductor needs at least 6 in. of free length measured from where it leaves the cable or raceway. When the box opening is less than 8 in. in any dimension, the conductor must also reach at least 3 in. outside the opening. The '3 in. from the sheath' answer mixes up the two measurements.Reference: NEC 300.14
Three 6 AWG copper THHN current-carrying conductors (90°C ampacity of 75 A) run in a raceway through an area where the ambient temperature is 38°C. The 90°C correction factor for that ambient is 0.91. What is the corrected ampacity?
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Correct answer: A. The corrected ampacity is the table ampacity times the correction factor: 75 A x 0.91 = 68.25 A, about 68.3 A. 82.4 A comes from dividing instead of multiplying. Remember the result may still be limited by the termination rating.Reference: NEC 310.15(B)(1); NEC 310.16
Nine current-carrying 12 AWG copper THHN conductors are in one raceway, at 30°C ambient. Their 90°C ampacity is 30 A, and the adjustment factor for 7-9 conductors is 70%. What is the adjusted ampacity of each conductor?
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Correct answer: C. The adjusted ampacity is 30 A x 0.70 = 21 A. 20 A is tempting because that is the most overcurrent protection 12 AWG copper can have, but that is a separate limit applied after the ampacity is found. 24 A would be the 80% factor, which applies to 4-6 conductors.Reference: NEC 310.15(C)(1); NEC 240.4(D)
A 208Y/120-volt, 3-phase, 4-wire circuit in one raceway supplies mostly electronic loads (computers and LED drivers). Not counting the EGC, how many current-carrying conductors must be counted for adjustment factors?
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Correct answer: A. When the major part of the load on a 4-wire wye circuit is nonlinear, harmonic currents flow in the neutral, so the neutral counts as current-carrying: 3 phases + neutral = 4. Three is tempting because a neutral that carries only unbalanced current is not counted. The EGC is never counted, so 5 is wrong.Reference: NEC 310.15(E)
A 200-A, 120/240-volt single-phase service supplies the entire load of a one-family dwelling. What is the minimum ampacity permitted for the service conductors?
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Correct answer: D. For dwelling services of 100-400 A supplying the whole load, the service conductors may have an ampacity of not less than 83% of the service rating: 200 A x 0.83 = 166 A. 200 A is the tempting answer, but the NEC allows the 83% reduction because a dwelling's whole-house load is rarely near the service rating.Reference: NEC 310.12(A)
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