NEC Article 450 Practice Questions: Transformers

Primary and secondary currents, Table 450.3(B) protection percentages, and vault rules — transformer questions reward a small set of memorized moves.

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16 Article 450 questions in the bankWritten in the PSI computer-based exam formatEvery answer cites the NEC section

What Article 450 Covers on the Exam

Article 450 questions combine a current calculation with a table lookup: compute primary or secondary current from kVA and voltage (I = kVA × 1000 ÷ V for single-phase; divide by √3 × V for three-phase), then apply Table 450.3(B) — for transformers 1,000 V and under, primary-only protection at 125% of primary current, or 250% primary with secondary protection at 125%. Where the percentage lands between standard ratings, the next higher standard size is generally permitted per the table notes.

Tested subtopics from Article 450

Primary and secondary current calculations from kVA and voltage
Overcurrent protection for transformers 1,000 V and less — Table 450.3(B)
Ventilation and accessibility of transformers — 450.9 and 450.13
Dry-type transformers installed indoors: separation and ratings — 450.21
Transformer vault requirements — Part III of Article 450

Drill a different article

16 questions in our bank cite Article 450. Practice the other high-yield articles too:

Art. 110 — General Requirements Art. 210 — Branch Circuits Art. 220 — Load Calculations Art. 230 — Services Art. 240 — Overcurrent Protection Art. 250 — Grounding and Bonding Art. 300 — Wiring Methods Art. 310 — Conductors & Ampacity Art. 314 — Boxes & Box Fill Art. 430 — Motors Art. 680 — Swimming Pools & Spas

How to Study Article 450

Get the current formulas automatic first: single-phase I = kVA × 1,000 ÷ V; three-phase I = kVA × 1,000 ÷ (1.732 × V). A 75 kVA three-phase transformer at 480 V primary draws about 90 A — practice until you can produce that in 15 seconds. Then tab Table 450.3(B) and note where the notes allow the next size up; that footnote is what the exam actually tests.

Then put it under exam conditions: the free diagnostic scores you across every category and shows whether transformers is actually a weak area — or whether your study time belongs somewhere else.

Sample Questions (From Our Real Question Bank)

Here are 4 real questions from our question bank. The free diagnostic quiz has 10 more.

NEC NEC 450.3(B)Transformers

1. Per NEC 450.3(B), what is the maximum overcurrent protection rating for a transformer rated 600 volts or less with currents of 9 amperes or more on the primary side?

A100% of rated primary current
B125% of rated primary current
C150% of rated primary current
D200% of rated primary current

Show explanation

NEC 450.3(B) and Table 450.3(B) require primary-only protection not to exceed 125% of rated primary current for transformers 9 amps or more.

NEC NEC 450.21(B)Transformers

2. Per NEC 450.21(B), individual dry-type transformers rated over 112.5 kVA installed indoors shall be installed in a transformer room of fire-resistant construction having a minimum fire rating of how long?

A30 minutes
B1 hour
C2 hours
DNo fire rating required if separated 12 inches from combustible material

Show explanation

NEC 450.21(B) requires individual dry-type transformers rated over 112.5 kVA to be installed in a transformer room of fire-resistant construction having a minimum fire rating of 1 hour. (Exception No. 1 and No. 2 apply where Class 155 or higher insulation systems are used with appropriate separation or complete enclosure.) NEC 450.9 governs ventilation requirements but does not specify a numerical wall clearance.

NEC NEC 450.3(B), Table 450.3(B)Transformers

3. Per NEC 450.3(B) and Table 450.3(B), transformers rated 1000 volts or less shall have overcurrent protection per that table. Under what condition may a separate primary-side OCPD be omitted?

ANo exception exists; OCPD is always required
BThe OCPD may be omitted if the transformer is rated less than 50 VA
CThe primary OCPD is not required where the primary circuit overcurrent device provides the protection specified in 450.3(B)
DThe OCPD may be located on the secondary side only

Show explanation

NEC 450.3(B) and Table 450.3(B) govern overcurrent protection for transformers 1000 V and less. A separate primary OCPD is not required when the feeder or branch-circuit overcurrent device on the supply side provides protection that does not exceed the values in Table 450.3(B). NEC 450.13(A) is a separate provision addressing accessibility of open-mounted dry-type transformers and does not govern OCPD requirements.

NEC NEC 450.21(A)Transformers

4. Per NEC 450.21(A), dry-type transformers installed indoors and rated 112.5 kVA or less shall have a separation of at least how far from combustible material?

A6 inches
B12 inches
C18 inches
D24 inches

Show explanation

NEC 450.21(A) requires dry-type transformers rated 112.5 kVA or less to be installed with at least 12 inches of separation from combustible materials.

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Frequently Asked Questions

How do I calculate transformer primary current?

For single-phase: primary current = kVA × 1,000 ÷ primary voltage. For three-phase: kVA × 1,000 ÷ (1.732 × line voltage). Example: a 75 kVA three-phase transformer with a 480 V primary draws 75,000 ÷ (1.732 × 480) ≈ 90 A.

How is transformer overcurrent protection sized under 450.3?

For transformers 1,000 V or less, Table 450.3(B) allows primary-only protection at 125% of primary current, or — where the secondary has its own protection at 125% of secondary current — primary protection up to 250%. Where 125% does not correspond to a standard rating, the next higher standard size is permitted per the table note.

Does Article 450 protection replace conductor protection?

No. Table 450.3(B) protects the transformer itself. The primary and secondary conductors still need their own protection per Articles 240 and 240.21(C) — secondary conductor tap rules are a separate exam topic that often appears alongside transformer questions.