Panelboards and Switchboards practice questions

Correct: A — B phase

408.3(E)(1) specifically designates B phase as the one with the higher voltage to ground on 3-phase, 4-wire, delta-connected systems — this is the 'high leg' or 'wild leg' that must also be marked per 110.15. A and C are the standard phases, and bus position relative to neutral has no bearing on which phase carries the higher voltage.

Correct: D — It has two main circuit breakers or two sets of fuses with a combined rating not greater than the panelboard rating

408.36's exception allows omitting individual (single main) protection only when two main breakers or two fuse sets, in other than service equipment, have a combined rating not exceeding the panelboard's rating. Sizing by connected load, feeder sizing, or an oversized single main don't satisfy this exception.

Correct: C — It must use Table 312.6(B)(2) spacing, since the 225 A exception does not apply above 225 amperes

The reduced Table 312.6(A) exception in 408.55(A) requires both conditions: rated 225 amperes or less AND not over 42 overcurrent devices. This panelboard fails the ampere-rating condition even though it meets the device-count condition, so the default Table 312.6(B)(2) spacing in 408.55(A) applies.

Correct: A — Not less than the available fault current at its location

408.6 requires the SCCR to be not less than the available fault current at the point of installation. Ampere rating and breaker trip rating are different properties from SCCR and don't substitute for a fault-current comparison.

Correct: C — 408.18(C)(3)

408.18(C)(3) specifically requires that load terminals for ungrounded conductors be located so a worker doesn't reach across energized uninsulated bus. 408.18(C)(1) instead governs equipment grounding conductor terminals, and 408.18(C)(2) governs grounded (neutral) circuit conductor terminals — different conductor types than the ungrounded conductors in this scenario.

Correct: D — CAUTION ___ PHASE HAS ___ VOLTS TO GROUND

408.3(F)(1) requires the specific high-leg wording identifying which phase and its voltage to ground. The ungrounded-system and high-impedance wordings belong to different system types under 408.3(F)(2) and 408.3(F)(3), and a generic high-voltage sign doesn't meet the required text.

Correct: A — No, because the description must be specific enough to avoid confusion between circuits

408.4(A)(4) requires enough detail and clarity to avoid confusion between circuits; six identical 'Recept.' entries fail that test regardless of whether the abbreviation is defined. There's no exemption for receptacle circuits, and fixing only the abbreviation issue wouldn't resolve the underlying ambiguity between the six circuits.

Correct: B — 15 amperes

408.52 sets 15 amperes as the standard maximum overcurrent device rating for instrument, pilot light, and potential transformer circuits. The 20 A and 30 A options are common general branch-circuit ratings but don't apply here, and 2 amperes is the threshold below which special enclosed fuses are permitted, not the standard cap.

Correct: D — 408.40

408.40 specifically requires a secured EGC terminal bar when nonmetallic raceway or cable is used or separate EGCs are provided, and addresses bonding of that bar to the cabinet/frame. 408.41 instead covers termination of grounded (neutral) conductors, not equipment grounding conductors.

Correct: A — It is permitted to be maintained

Per 408.9(A), when a replacement panelboard is listed for the specific enclosure by catalog number or dimensional information, it is permitted to maintain its short-circuit current rating — no new study or third-party verification is required.

Correct: C — A main bonding jumper sized in accordance with 250.28(D)

408.3(C) requires a main bonding jumper sized per 250.28(D) to connect the grounded service conductor to the panelboard frame. The isolated ground bus and separate grounding electrode conductor described are not what this section requires, and sizing the supply-side bonding jumper only to the largest ungrounded conductor ignores the 250.28(D) reference.

Correct: C — Isolated from the busbars by a barrier

408.3(A)(2) permits conductors to travel horizontally through vertical sections they don't terminate in only where they are isolated from busbars by a barrier. Ampacity margin, labeling, and routing in a separate raceway are not the mechanism the code specifies.

Correct: A — 408.5

408.5 hosts Table 408.5, which sets the minimum wiring space for conductors entering bus enclosures below the busbars. 408.3(G) instead covers minimum wire-bending space at terminals, a related but distinct clearance requirement.

Correct: A — 408.4(B)

408.4(B) is the Source of Supply rule requiring a permanent, non-handwritten label on feeder-fed switchboards/panelboards (outside one- and two-family dwellings) identifying where the power originates. 408.4(A) instead governs the circuit directory/description inside the panel, and 408.3(F) covers phase/system identification markings, not source-of-supply labeling.

Correct: B — As two overcurrent devices

Per 408.54, a 2-pole circuit breaker or fusible switch is considered two overcurrent devices, not one, regardless of it being a single physical unit. There's no half-count or motor-load exemption in this section.

Correct: B — Table 408.56

408.56 states the distance between uninsulated live parts shall not be less than specified in Table 408.56. Working space clearances under 110.26 cover a different concern (space to work safely around equipment), not spacing between bare parts inside it.

Correct: D — It must be secured with an additional fastener that requires more than a pull to release it

408.36(D) requires an additional fastener beyond the normal plug-in mounting, one that takes more than a pull to release, whenever a plug-in breaker or main lug assembly is backfed to terminate field-installed ungrounded conductors. Replacing it with a bolt-on device, only labeling it, or positioning it at the bottom of the panel are not the rule's requirement.

Correct: A — When available fault current exceeds 10,000 amperes

408.38 requires the panelboard and enclosure combination to be evaluated for the application where available fault current is greater than 10,000 amperes. Dwelling use, outdoor mounting, and overcurrent device count are not the trigger specified in this section.

Correct: A — 408.30

408.30 is the general rule requiring a panelboard's rating to be not less than the minimum feeder capacity from the applicable Article 220 load calculation. 408.36 instead covers overcurrent protection for the panelboard, not the panelboard's own ampere rating.

Correct: D — 408.41

408.41 requires each grounded (neutral) conductor to land on its own individual terminal not shared with another conductor, except for parallel-conductor sets on a terminal identified for multiple conductors. 408.40 covers panelboard grounding (equipment grounding/bonding), not neutral terminal sharing, and 408.54 addresses the maximum number of overcurrent devices, not terminations.