10.4 Transformers (NEC Article 450)

Why This Matters for the TX Journeyman Exam

Article 450 (Transformers) appears in content outline area 2 (Separately Derived Systems) and area 6 (Equipment & Devices). The exam tests the 450.3(B) overcurrent percentages, dry-type installation rules (450.21), and a primary/secondary OCPD calculation that lands in the Calculations part. The transformer calculation is a high-value item because the arithmetic is predictable and the table values are fixed.

Overcurrent Protection (NEC 450.3)

NEC 450.3 governs transformer OCPD. Table 450.3(B) applies to transformers 1000 V or less. For primary current of 9 A or more (the common exam range):

Where the next standard size up is used, the next standard ampere rating from NEC 240.6(A) is selected — recall that 25 A and 35 A are both standard ratings, so 125 A, 150 A, 200 A, 225 A, 250 A, and 300 A are all valid steps.

For primary current less than 9 A, Table 450.3(B) raises the percentages: primary-only fuse = 167% (next size up), and primary + secondary protection permits a primary fuse up to 250% with a 125% secondary fuse. Circuit-breaker percentages are higher still (up to 300% for the smallest primaries). These smaller-transformer rows appear less often on the exam but are worth a quick read of the table.

For transformers rated over 1000 V, Table 450.3(A) applies — separate percentages and different fuse/breaker columns. The Journeyman exam tests 450.3(B) far more often than 450.3(A).

Transformer Vaults and Secondary Grounding (NEC 450.4, 450.5)

NEC 450.4 requires a transformer vault for indoor installations of oil-insulated transformers above specific kVA/ratings, with construction per 450.42 through 450.47 (walls, doors, ventilation, drainage). Dry-type transformers are typically exempt — see 450.21 below.

NEC 450.5 requires the secondary of a separately derived system to be grounded per NEC 250.30 — bonding the neutral at the transformer or at the first disconnect. (Ch9 covers separately derived system grounding in detail.)

Dry-Type Transformer Installation (NEC 450.21)

NEC 450.21(A): a dry-type transformer rated 112.5 kVA or less may be installed without a vault and may be installed in the open. NEC 450.21(C): a dry-type transformer rated 600 V or less and 112.5 kVA or less is not required to maintain clearances from combustible material, but manufacturer-required clearances for ventilation must be maintained.

Ventilation (NEC 450.9)

NEC 450.9 requires transformer installations to provide ventilation so the ambient temperature around the transformer does not exceed 40°C (or the transformer's marked ambient rating). Louvers, grilles, or forced ventilation must direct air so heated air does not recirculate into the transformer intake.

Accessibility (NEC 450.13, 450.22)

NEC 450.13(A) requires transformer vault doors to be lockable and openable from inside without a key. NEC 450.13(B) permits dry-type transformers 600 V or less and 50 kVA or less to be installed in hollow building spaces (e.g., above a dropped ceiling) if accessible and ventilated.

NEC 450.22 requires transformers to be accessible for inspection and maintenance. "Accessible" does not mean "in sight" — access panels are acceptable if the unit can be reached for service.

Tap Rules for Transformer Secondaries (Brief)

Secondary conductors from a transformer to the first OCPD may follow the tap rules of NEC 240.21(C) — the 10-ft and 25-ft tap rules. Detailed tap-rule calculations are in Ch7; here, note only that transformer secondary taps are governed by 240.21(C) and not by 450.3.

Worked Example — 75 kVA, 480V → 208/120V, Three-Phase Transformer

A common commercial service transformer. All arithmetic uses the three-phase formula I = kVA × 1000 / (V × √3), where √3 = 1.732.

Step 1 — Primary full-load current:

I_pri = 75,000 VA / (480 V × 1.732) = 75,000 / 831.4 = 90.3 A

Step 2 — Primary OCPD (primary-only protection, fuse at 125%):

90.3 A × 1.25 = 112.9 A → next standard size up (NEC 240.6(A)) = 125 A

Step 3 — Secondary full-load current:

I_sec = 75,000 VA / (208 V × 1.732) = 75,000 / 360.3 = 208.2 A

Step 4 — Secondary OCPD (primary + secondary protection, fuse at 125%):

208.2 A × 1.25 = 260.3 A → next standard size up = 300 A

If primary + secondary protection is used, the primary fuse may be sized at 250% instead: 90.3 × 2.50 = 225.8 A → 250 A (next standard), while the secondary fuse remains 300 A.

Verify the arithmetic precisely:

• 90.3 × 1.25 = 112.9 → next standard up = 125 A ✓
• 208.2 × 1.25 = 260.3 → next standard up = 300 A ✓

Both figures match the brief's stated values. The 125 A primary fuse and 300 A secondary fuse are the conventional exam answers for this transformer.