14.3 Transformers (Article 450) for Journeymen

Transformers on the Arkansas JW Exam

Article 450 is not a huge slice of the Prov outline by name, but transformer overcurrent and conductor questions show up inside Motors and Controls, Utilization, and general overcurrent items. Journeyman depth means you can open Table 450.3(B) cold, distinguish primary-only from primary-and-secondary protection, and know when secondary conductors must pick up 240.21(C) tap rules.

Use NEC 2023. Tab 450.3, Tables 450.3(A) and 450.3(B), 450.4 (autotransformers), 450.5 (grounding autotransformers where tested), 450.9, 450.10, 450.11, and 450.13. Keep 240.21(C) in the same mental folder.

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Scope — What Article 450 Does and Does Not Do

Article 450 covers the installation of transformers. It sets overcurrent protection for the transformer itself (primary, and secondary when required), plus installation conditions (ventilation, accessibility, guarding, marking, grounding connections to the transformer).

It does not replace:

• feeder/branch conductor ampacity rules in Article 310,
• panelboard rules in Article 408,
• separately derived system bonding/grounding detail in Article 250 Part X,
• or motor FLC tables in Article 430.

If the stem asks for EGC size on a transformer secondary feeder, you may leave 450 and enter 250.122 after the OCPD is known.

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Overcurrent Protection — 450.3

450.3 points you to the tables:

Both tables distinguish supervised vs other locations and primary only vs primary and secondary protection. Read the stem for voltage class and supervision language before touching percentages.

Table 450.3(B) themes (1000 V and less) — verify every cell in the book

Journeyman stems most often live here. Patterns you must confirm in the published table:

• Primary and secondary protection provided: primary OCPD generally limited to a higher percentage of primary current (commonly up to 250% in the applicable row), while secondary OCPD is generally limited to about 125% of secondary current for currents 9 A or more (with “next higher standard size” permissions where the table allows).
• Primary protection only (where the table permits that strategy): primary OCPD generally limited to about 125% of rated primary current for currents 9 A or more, with higher percentage allowances for smaller primary currents (rows such as less than 9 A and less than 2 A use larger percentage caps — open the exact row).
• Supervised locations can allow different maxima than unsupervised locations for the same transformer — never ignore that column header.
• Potential transformers and other specialty rows exist; do not force a power-transformer answer onto a PT stem.

Required skill: compute rated currents first.

[ I_{primary} = \frac{VA_{rating}}{V_{primary}} \quad (\text{single-phase}) \qquad I_{secondary} = \frac{VA_{rating}}{V_{secondary}} ]

For three-phase:

[ I = \frac{VA}{V_{L-L} \times \sqrt{3}} ]

Then apply the table percentage and select the next standard overcurrent device size only when the table/section expressly permits it.

Worked method example (open-book style)

A 45 kVA, 480 – 208Y/120 V, three-phase transformer:

• Primary current ≈ (45000 / (480 \times 1.732) \approx 54.1,A)
• Secondary current ≈ (45000 / (208 \times 1.732) \approx 125,A)

If the stem requires primary and secondary protection under the ordinary 450.3(B) path, you would evaluate a primary device not exceeding the table’s primary percentage of ~54 A and a secondary device not exceeding the table’s secondary percentage of ~125 A (commonly 125% → about 156 A, then apply next-size permissions if allowed). If the stem instead allows primary-only protection under the conditions of the table, you would use that column’s percentage of primary current — often the stricter ~125% primary path for currents ≥9 A — and you would not invent secondary breakers the table does not require for that condition.

Always let the stem’s “primary only” vs “primary and secondary” language choose the column.

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Secondary Conductors vs Transformer OCP — 240.21(C)

Article 450 protects the transformer. Secondary conductor length and protection are frequently governed by 240.21(C) transformer secondary conductor tap rules (single-phase / multiphase cases, lengths such as 10 ft, 25 ft, and outside-building conditions, ampacity vs secondary protection device, etc.).

Exam discipline:

1. Size/protect the transformer per 450.3.
2. Ask whether secondary conductors extend beyond the transformer terminals before an OCPD.
3. If yes, open 240.21(C) and match the exact tap condition — do not assume secondary conductors may run unlimited distance unprotected merely because 450.3 is satisfied at the transformer.

This two-article handshake is a common Prov-style distractor set: one option cites only 450.3, another only 240.4, and the keyed answer coordinates both.

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Autotransformers — 450.4

450.4 covers autotransformers (including buck-boost applications). Overcurrent protection must be provided for the autotransformer windings as the section requires — often viewed from the input circuit ampere rating with specific percentage limits. Buck-boost questions also require correct connection identification (which winding is common). If the stem says “autotransformer” or “buck-boost,” do not apply ordinary two-winding 450.3(B) rows blindly without reading 450.4.

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Installation Rules JW Candidates Still Miss

Ventilation — 450.9

Transformers must be protected from overheating due to inadequate ventilation. Dust, lint, or ambient conditions that block cooling can violate 450.9. Indoor dry-type units need clearances to combustible materials and ventilation openings as required by listing and Article 450 location rules.

Grounding — 450.10

Transformer grounding connections must be accessible (with stated exceptions). Separately derived system bonding/jumpers and GECs are executed under Article 250, but 450.10 reminds you the grounding connection point on the transformer must remain workable for inspection and maintenance.

Marking — 450.11

Transformers must be marked with manufacturer, rated kVA, primary/secondary voltage, impedance (where required), frequency, and other required data. Exam stems may ask whether a replacement nameplate is required after rewind or whether impedance is needed for available-fault-current work — impedance marking matters for short-circuit studies even when the multiple-choice item looks like a “nameplate trivia” question.

Accessibility — 450.13

Dry-type transformers generally must be readily accessible (with exceptions such as certain hollow-space installations meeting the section’s conditions). Do not bury a dry-type unit in a noncompliant ceiling cavity because it “fits.”

Guarding & working space

Live parts and working space around transformer equipment still answer to Article 110. Large equipment may also trigger dedicated equipment space rules. Transformer questions that mention “can the unit be serviced?” often hide a 110.26 working-space issue alongside 450.13.

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Parallel Transformers & Nonlinear Loads (Awareness)

Where transformers are connected in parallel, they must have matching impedance/voltage characteristics as required so load sharing is correct. Nonlinear loads (VFDs, large electronic supplies) can require K-rated or otherwise suitably designed transformers — if the stem emphasizes harmonics, look beyond a bare kVA match. These appear less often than 450.3(B) percentage items but show up in utilization/equipment stems.

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Coordination With Motor Circuits

A transformer feeding a motor control center or a single large motor still needs:

• transformer OCP per 450.3,
• secondary/feeder conductors per 215 / 310 / 240,
• motor branch rules per 430 at each motor.

Do not protect a motor with only the transformer primary breaker percentages and call it compliant motor branch-circuit protection. Each layer keeps its own article.

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Timed Open-Book Tips for §14.3

• Compute primary and secondary currents before opening the table.
• Choose 450.3(A) vs (B) by voltage; then choose the correct supervision and primary-only vs primary-and-secondary column.
• If secondary conductors leave the transformer, add a 240.21(C) check.
• Autotransformer → 450.4, not a forced two-winding row.
• Installation stems → 450.9 / 450.10 / 450.11 / 450.13 before inventing Article 430 answers.
• Recheck units: kVA vs VA, line-to-line vs line-to-neutral, single-phase vs three-phase formulas.