NEC Code Practice Test:
40 Free Questions by Article

Per NEC 90.1(A), the Code's purpose is the practical safeguarding of persons and property from hazards arising from the use of electricity. It is not a design specification guide.

NEC 90.2(B) excludes installations of electric utilities used for the generation, transmission, and distribution of electricity — these are regulated separately by NERC and FERC.

NEC 90.4 assigns the authority having jurisdiction (AHJ) — typically the building inspector or local electrical inspector — the responsibility for interpreting NEC requirements and approving installations.

The NEC is published by the NFPA as NFPA 70. It is updated on a 3-year revision cycle. OSHA references the NEC but does not publish it.

Per NEC 90.5(C), Informational Notes (formerly Fine Print Notes) are not enforceable requirements — they provide explanatory material and references only. Mandatory rules use "shall"; permissive rules use "shall be permitted."

Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. "Conditions of use" includes ambient temperature, bundling, and installation method.

A branch circuit begins at the final OCPD (breaker/fuse) and ends at the outlet(s). Conductors feeding a subpanel are feeders; conductors from the utility are service conductors.

3 hours is the NEC threshold for a continuous load. This matters because overcurrent devices and conductors feeding continuous loads must be sized at 125% of the continuous load current.

Feeders run from the main service equipment (or a separately derived system) to a subpanel or the last OCPD before the branch circuits begin. They are NOT branch circuits and NOT service conductors.

The key qualifier is "complete and independent" living facilities — including cooking. A hotel room is not a dwelling unit because it typically lacks cooking facilities. This definition triggers Article 210, 220, and 230 residential requirements.

Per NEC Table 110.26(A)(1), for voltages 0–150V to ground (Condition 1), the minimum working space depth is 3 feet. Condition 1 applies when there are no live or grounded parts on the opposite side of the working space.

NEC 110.26(A)(2): working space width = the greater of 30 inches OR the width of the equipment. A 42-inch-wide switchboard needs 42 inches of clear width; a 24-inch panel still needs 30 inches minimum.

NEC 110.26(A)(3) requires a minimum headroom of 6½ feet in front of service equipment, switchboards, panelboards, and motor control centers — or the height of the equipment if taller.

NEC 110.3(B) is one of the most cited code sections — it requires that listed/labeled equipment be used per its listing instructions. This is why you can't use a 60°C device with 90°C conductors at full 90°C ampacity.

NEC 110.9: the interrupting rating must be at least equal to the available fault current (short-circuit current) at the device's terminals. A breaker rated 10kAIC at a 42kAIC fault will explode — a serious safety hazard.

NEC 210.8(A) lists specific GFCI locations: bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchen countertops, laundry areas, boathouses, and others — but standard bedroom wall outlets are NOT in this list.

NEC 210.11(C)(1) requires a minimum of two 20A small-appliance branch circuits to serve kitchen countertop receptacles. These circuits may also serve the dining room and pantry, but not appliances with dedicated circuits.

NEC 210.52(A): receptacles must be placed so no point on a wall is more than 6 feet from a receptacle, measured horizontally. This means receptacles must be spaced no more than 12 feet apart (6 feet on each side).

NEC 210.52(C)(3): an island countertop requires at least one receptacle if it has a long dimension ≥ 24 inches AND a short dimension ≥ 12 inches. This is the 2023 NEC — earlier editions used 12 sq ft area calculations.

NEC 210.19(A)(1): conductors must be sized at 125% of continuous load current. This same 125% rule applies to OCPD sizing (210.20) and feeder sizing (215.2). It accounts for heat buildup over 3+ hours of operation.

NEC Table 250.66 sizes the GEC based on the largest service entrance conductor. For 2/0 AWG copper service conductors → 4 AWG copper GEC. For 350 kcmil → 2 AWG copper GEC.

Table 250.122: EGC size is based on the OCPD rating. For a 20A breaker → 12 AWG copper EGC. For a 60A breaker → 10 AWG copper EGC. The EGC carries fault current, not load current.

NEC 250.52(A)(1): metal underground water pipe must have at least 10 feet in contact with the earth to qualify as a grounding electrode. If used, it must always be supplemented with another electrode per 250.53(D)(2).

The MBJ bonds the neutral (grounded conductor) to the equipment grounding conductor at the main service disconnect — this is the critical system connection point. In subpanels, neutral and ground must be separated (no MBJ).

Per NEC 250.53(D)(2), the metal water pipe electrode must always be supplemented. Acceptable supplements include ground rods (250.52(A)(5)), a ground ring, plate electrode, concrete-encased electrode, or other listed electrode.

NEC Table 300.5: Direct-buried cables under a one- and two-family driveway require 24 inches minimum cover. Under a general yard/landscape area, direct burial is 24 inches; under a concrete slab, it's 0 inches (no cover required under slab).

PVC conduit under a residential driveway requires 18 inches minimum cover per Table 300.5. Rigid metal conduit (RMC) under a driveway only needs 6 inches — the rigid metal provides physical protection.

NEC 300.14: at least 6 inches of free conductor (measured from the point where it emerges from the cable or raceway sheath) must be left at every box. The minimum at the front of the box is 3 inches for device connections.

NEC 300.4(A)(1): when a cable is within 1¼ inch of the face of the framing member, a steel plate at least 1/16 inch thick must be installed to protect the cable from drywall screws and nails.

NEC 300.5(D)(1): conductors rising from underground must be enclosed in a raceway from the point of emergence to 8 feet above finished grade to protect against physical damage.

NEC Table 310.16: 12 AWG THWN copper = 20A at 75°C. However, most residential device terminations are rated 60°C, so you use the 60°C column (15A) for sizing purposes unless the device is listed for 75°C terminations.

Table 310.16 is based on 30°C (86°F) ambient temperature. When ambient exceeds 30°C, you must apply correction factors from the table's temperature correction section, which reduce the allowable ampacity.

4–6 conductors: multiply by 0.80. 7–9 conductors: 0.70. 10–20: 0.50. Note: neutral conductors carrying only unbalanced current are NOT counted as current-carrying for derating purposes.

4 AWG THHN/THWN copper = 85A at 75°C per Table 310.16. At 60°C it's 70A. At 90°C it's 95A — but you can only use the 90°C ampacity if both the conductor and all termination points are rated for 90°C.

NEC 110.14(C)(1): conductors rated 60°C or those used with 60°C terminations must use the 60°C ampacity column, even if the conductor insulation is rated 90°C. This is why 12 AWG THHN is limited to 20A (not 30A) on a 20A circuit with standard devices.

NEC Table 430.250: 10 HP, 3-phase, 230V = 28 amperes. Always use the NEC table value for motor calculations — NOT the motor nameplate FLA, unless specifically instructed otherwise in the code section.

NEC 430.22: motor branch circuit conductors must have ampacity of at least 125% of FLC. For the 10 HP, 230V, 3-phase motor: 28A × 1.25 = 35A minimum. Select the next standard conductor size ≥ 35A.

NEC Table 430.52: inverse-time breakers for polyphase squirrel cage motors = max 250% of FLC. For the 28A motor: 28 × 2.5 = 70A max. If 70A doesn't allow the motor to start, NEC 430.52(C)(1) Exception allows up to 400%.

NEC 430.32(A)(1): for motors with SF ≥ 1.15 or temperature rise ≤ 40°C → max overload = 125% of nameplate FLA. For all other motors → 115% of FLA. Note: overload uses nameplate FLA, not Table 430.250 FLC.

NEC 430.102(B): the motor disconnect must be within sight of the motor AND the driven machinery, OR the disconnect may be out of sight if it is lockable in the open position with a lock that cannot be removed with the switch closed.