9.4 Transformers (Article 450)

Article 450 Scope and the 2020 Table Renumbering

Article 450 covers transformers of any voltage, but the OCPD table you reach for depends on primary voltage. NEC 2020 renumbered the tables:

• Table 450.3(A) — Overcurrent protection for transformers rated over 1000 V.
• Table 450.3(B) — Overcurrent protection for transformers rated 1000 V or less (this is the table the journeyman exam almost always uses).

In earlier code cycles (pre-2020) the labels were swapped, so some references and old study materials still point to "Table 450.3(A)" for low-voltage work — that is now wrong. Use the table for the voltage class.

Installation — 450.9 and 450.10

450.9 Ventilation and clearance. Transformers must be installed so ventilating openings are not blocked, with clearance from walls and other equipment per the nameplate or listing. Dry-type transformers generally require at least 12 in. of clearance from combustible walls unless the transformer is listed for reduced clearance.

450.10 Accessibility. Live parts and terminals must be accessible for inspection and maintenance. Locking doors or hinged panels that require tools to open are acceptable; sealed compartments are not.

Overcurrent Protection — 450.3 and Table 450.3(B)

Table 450.3(B) (≤1000 V) sets the maximum OCPD as a percentage of the transformer's full-load current (FLA). For 3-phase, I_primary = kVA × 1000 ÷ (V_primary × 1.732). For single-phase, I_primary = kVA × 1000 ÷ V_primary.

Primary-Only Protection

Note 1 permits rounding up to the next standard OCPD size per 240.6(A) when the 125% value lands between standard sizes.

Primary + Secondary Protection

Note 1 does not apply to the 250% primary value — you must round down to a standard size if it falls between sizes.

Worked Example — 75 kVA, 480V → 208Y/120V, 3-Phase Transformer

1. Primary FLA = 75,000 ÷ (480 × 1.732) = 90.3 A.
2. Primary-only OCPD = 90.3 × 1.25 = 112.9 A → next standard size up per 240.6(A) and Note 1 = 125 A.
3. Primary conductor ampacity must be ≥ 90.3 A; #3 AWG copper at 75°C (rated 100 A) is sufficient, but 240.4(B) next-size-up allows a 125 A OCPD to protect #3 AWG since 100 A exceeds the 90.3 A load.
4. If using primary+secondary protection:
   • Primary max = 90.3 × 2.50 = 225.75 A → 225 A (round down; Note 1 does not apply).
   • Secondary FLA = 75,000 ÷ (208 × 1.732) = 208.0 A → 125% = 260 A → next standard up = 300 A.

The primary-only approach is more common for small dry-type installations because it saves the cost of a secondary breaker.

Grounding and Separately Derived Systems

A transformer secondary that has no direct electrical connection to the primary (other than the magnetic coupling) is a separately derived system per Article 100. Such systems must be grounded per 250.30:

1. A system bonding jumper connects the grounded conductor (neutral) to the equipment grounding conductor at the source or first disconnect.
2. A grounding electrode conductor bonds the system to a grounding electrode (building steel, water pipe, or driven rod).
3. The grounded conductor must be brought to the disconnect if there are line-to-neutral loads.

For a 480-208Y/120V transformer, the secondary neutral must be bonded to ground at (and only at) the source or the first disconnecting means — never at both ends, or you create a parallel neutral-ground path.

Transformer Vaults — 450.26

450.26 requires a transformer vault when a dry-type transformer rated over 112.5 kVA is installed indoors and does not meet the clearance/ventilation rules of 450.9, or for liquid-filled transformers indoors per 450.26. Vault construction requirements include:

• Walls, floor, ceiling: minimum 3-hour fire rating (or 1-hour with automatic fire suppression per 450.26(C)).
• Doors: fire-rated, self-closing, opening outward.
• Ventilation: adequate to dissipate heat; louvers must not impair the fire rating.
• Location: as close as practicable to the source of supply.

Liquid-filled transformers additionally require oil containment and consideration of fluid type per 450.22–450.24.

Dry-Type vs. Liquid-Filled

• Dry-type (450.9, 450.26): no liquid; the common commercial/industrial choice. Indoor dry-type over 112.5 kVA without adequate clearance requires a vault.
• Liquid-filled (450.21–450.27): better heat dissipation but flammable fluid concerns; indoors nearly always requires a vault, with the construction depending on fluid classification (less-flammable fluids like R-Temp allow lighter construction).

Common Exam Traps

1. Using Table 450.3(A) for a 480 V transformer — wrong table; use 450.3(B).
2. Applying Note 1 next-size-up to the 250% primary value in primary+secondary protection — not allowed.
3. Forgetting that transformer primary FLA is calculated (kVA ÷ V ÷ 1.732), not pulled from a motor table.
4. Bonding the separately derived neutral at both the transformer and the downstream panel — creates objectionable current on the grounding conductor.
5. Sizing the primary conductor at the OCPD rating instead of at the transformer FLA — the conductor must carry the load (90.3 A in the example), and 240.4(B) handles the round-up, not the load calc.