Transformers practice questions

Correct: B — In series with each ungrounded input conductor

450.4(A) requires the overcurrent device in series with each ungrounded input conductor. It explicitly forbids placing a device in the shunt winding between Points A and B, and grounded conductors are not overcurrent-protected this way.

Correct: C — At or above 125 percent of the autotransformer's continuous current per-phase or neutral rating

450.5(A)(2) sets the trip threshold at 125 percent of the autotransformer's continuous current per-phase or neutral rating. 100 percent, 150 percent, and 80 percent are all plausible-sounding thresholds borrowed from other continuous-load or conductor-sizing rules, but they are not the value specified here.

Correct: C — 10 kVA

Per 450.26, at 1000 volts or less, a vault is not required if suitable fire-prevention arrangements are made and the total capacity does not exceed 10 kVA where the surrounding construction is combustible. The 75 kVA figure applies only where the surrounding structure is fire-resistant construction, and 112½ kVA is the threshold for using thinner reinforced-concrete vault walls, not a no-vault allowance.

Correct: A — 67% of the rated secondary current of the highest rated transformer

Per 450.6(A)(1), when all loads sit at the transformer supply points and standard overcurrent protection isn't used, tie ampacity must be at least 67% of the highest-rated transformer's secondary current. The 100% figure applies instead when loads tap the tie between supply points (450.6(A)(2)), and 133% applies only to non-interconnected paralleled conductors (450.6(A)(4)(b)).

Correct: D — At least 300 mm (12 in.) separation from the combustible material

Under 450.21(A), transformers rated 112½ kVA or less need only 300 mm (12 in.) separation from combustible material unless a fire-resistant, heat-insulated barrier is used instead. The 1-hour room and the 6 ft/12 ft clearance option belong to 450.21(B) for units over 112½ kVA, and skipping separation entirely only applies to enclosed units rated 1000 V or less.

Correct: B — On the load side of an existing system grounding connection

450.5 specifically prohibits installing a zigzag-connected transformer on the load side of any system grounding connection, including those made under 250.24(C), 250.30(A)(1), or 250.32(B), Exception No. 1. Line-side service location, outdoor weather exposure, and healthcare essential systems are not restrictions this section imposes.

Correct: A — It must have a continuous neutral-current rating not less than the specified ground-fault current

450.5(B)(1) requires the autotransformer's continuous neutral-current rating to be not less than the specified ground-fault current. The 125% figure and the 42% figure both relate to overcurrent protection sizing under 450.5(B)(2)(b), not to the transformer's neutral-current rating itself.

Correct: C — No combustible materials may be stored and a liquid confinement area must be provided

450.23(A)(1) requires that no combustible materials be stored and that a liquid confinement area be provided, in addition to the 35,000-volt cap. The scenario's voltage (34,500 V) is within the limit, and Type I buildings are explicitly permitted — the failure is the stored combustibles and missing confinement area.

Correct: C — A liquid confinement area and a pressure-relief vent

450.24 requires every indoor nonflammable fluid-insulated transformer to have a liquid confinement area and a pressure-relief vent, regardless of voltage. A vault is only required indoors above 35,000 volts, so this 25,000-volt unit doesn't need one. Fire-rated enclosures aren't the 450.24 requirement, and the nonflammable rating doesn't eliminate the confinement/venting requirement.

Correct: D — 450.13(B)

450.13(B) is the specific exception allowing dry-type transformers 1000V or less and not exceeding 50 kVA to be installed in hollow spaces without being readily accessible, provided 450.9 ventilation and 450.21(A) separation are met. 450.13(A) covers open wall/column mounting, not hollow spaces, and 450.9 only addresses ventilation itself, not the accessibility exception.

Correct: A — A terminal bar secured inside the enclosure and bonded to the enclosure per 250.12

450.10(A) requires a terminal bar secured inside the enclosure for all grounding and bonding conductor connections, bonded to the enclosure per 250.12. Mounting it on the vented louvers is specifically prohibited, and landing conductors on core screws or the neutral bus isn't the specified method.

Correct: D — It serves only equipment within that transformer enclosure and is accessible to qualified persons only

450.8(C) permits switches or equipment operating at 1000 V nominal or less inside a transformer enclosure only if they serve equipment within that same enclosure and are accessible to qualified persons only. Serving building-wide circuits, height above grade, and bonding/labeling alone don't satisfy the qualified-persons-only condition in 450.8(C).

Correct: C — Table 450.3(B)

Per 450.3(B), general-purpose transformers rated 1000 volts or less are protected in accordance with Table 450.3(B). The 430.72(C) path and the motor-control exception only apply when the transformer is installed as a motor control circuit transformer, which this one is not. Table 450.3(A) applies to transformers over 1000 volts, not this case.

Correct: D — 70 A

Primary FLA = (45 × 1000) ÷ (480 × √3) = 54.1 A. Being ≥9 A with primary-only protection, Table 450.3(B) allows 125%: 54.1 × 1.25 = 67.7 A, rounded up to the next standard size = 70 A. Distractors skip the 125%, use 167%, or don't round to a standard size.

Correct: B — 3 hours

Per 450.42, vault walls and roofs must have a minimum fire resistance of 3 hours. The 1-hour rating is only allowed as a reduction when the transformer is protected by automatic sprinkler, water spray, CO2, or halon — not the standard baseline.

Correct: C — 3 hours

Per 450.43(A), an interior vault doorway needs a tight-fitting door with a minimum 3-hour fire rating. The 1-hour option only applies where the transformers are protected by automatic sprinkler, water spray, CO2, or halon suppression — not the baseline case described here.

Correct: A — A pressure-relief vent, plus a means to absorb arcing gases or a vent connected to a chimney or flue to outside

450.25 requires a pressure-relief vent for indoor askarel transformers over 25 kVA, and in poorly ventilated locations also requires either a means to absorb gases from arcing or a vent connected to a chimney or flue carrying gases outside. The vault requirement only applies above 35,000 volts, not due to poor ventilation, and askarel's fire resistance doesn't eliminate the venting requirement.

Correct: A — It must be identified for use at the elevated voltage

450.4(B) requires that a transformer field-connected as an autotransformer be identified for use at the elevated voltage that connection creates. Simply changing an overcurrent rating doesn't address the insulation and voltage-stress concern, and derating to the original nameplate voltage would defeat the purpose of the autotransformer connection. Isolation-transformer-only listings don't satisfy this identification requirement.

Correct: A — 450.22

450.22 sets the outdoor dry-type transformer rules, including the 300 mm (12 in.) clearance to combustibles for units over 112 1/2 kVA and the Class 155-or-higher insulation exception; 450.21(A) covers indoor units not over 112 1/2 kVA, and 450.27 governs oil-insulated outdoor transformers, not dry-type.

Correct: A — All restrictions provided in the listing of the liquid

450.23(B)(1) requires that installations on Type I and Type II buildings comply with all restrictions in the liquid's listing, such as maximum tank pressure, pressure relief valves, fuse types, and overcurrent protection sizing. Vault construction rules and dry-type clearance rules belong to other transformer installation methods, and NEC does address Type I/II buildings directly in this section.

Correct: D — Any one or more of: space separation, fire-resistant barriers, automatic fire suppression, or an oil-confining enclosure, matched to the degree of hazard

450.27 lists space separations, fire-resistant barriers, automatic fire suppression systems, and oil-confining enclosures as acceptable safeguards, applied according to the degree of hazard — not a single mandatory method. A sealed tank does not remove the requirement since the rule addresses fire originating from a ruptured tank.

Correct: D — A secondary tie connects two separate power sources or supply points

Per 450.6, a secondary tie is a circuit at 1000 volts or less between phases that connects two power sources or power supply points, such as the secondaries of two transformers. It is not limited to one conductor per phase — one or more conductors per phase or neutral are permitted. The 1000 V figure is a ceiling ('or less'), not a minimum.