Art. 450 begins with the short and to-the-point statement, “This article covers the installation of all transformers,” and then lists eight exceptions. In essence, Art. 450 covers power transformers, transformer vaults, and most kinds of lighting transformers (Fig. 1). Once you’ve acquired a thorough understanding of the NEC’s transformer requirements, you’ll be that much closer to a safe installation.

Overcurrent protection. Overcurrent protection alone won’t prevent transformer overheating. As (450.3 FPN 2) notes, nonlinear loads can increase heat in a transformer without operating its overcurrent device. Your first step in transformer protection is to select a transformer suitable to the load characteristics.

Your next step is to provide overcurrent protection, per 450.3. When the secondary current is 9A or more, the primary protection can’t exceed 250% and the secondary protection can’t exceed 125%. If you have primary protection only, it can’t exceed 125%. When supplying overcurrent protection for transformers over 600V, use Table 450.3(A). For transformers not over 600V, use Table 450.3(B). Carefully read the notes below each table.

Now let’s test your ability to apply Table 450.3(B) with the following question:

What’s the maximum primary protection device rating permitted for a 45kVA, 3-phase, 480V transformer (Fig. 2) if it has primary protection?

Step 1. Calculate the primary current

I = VA division sign (E x 1.732)

I = 45,000VA division sign (480V x 1.732) = 54A

Step 2. Calculate the primary protection device rating [240.6(A)].

54A x 1.25 = 68A, next size up is 70A [Note 1, of Table 450.3(B)]

Ventilation. Don’t allow transformer ventilating openings to be blocked by walls or other obstructions (450.9). Not all obstructions may be apparent or present at the time of installation. Anticipate what other equipment will go in that location by consulting the property owner and the construction drawings.

Manufacturers must mark each transformer with their name, the rated kVA, primary and secondary voltage, impedance (if 25kVA or larger), and clearances from ventilating openings (450.11). So look on the transformer for the required clearances. As these are manufacturer’s instructions, you must comply with them [110.3(B)].

Transformer accessibility. Install transformers rated 600V or less so they’re readily accessible to qualified personnel for inspection and maintenance (450.13). You have the following two options at your disposal:

  • Open installations [450.13(A)]. You can locate dry-type transformers of any kVA in the open on walls, columns, or structures (Fig. 3).
  • Suspended ceilings [450.13(B)]. You can install dry-type transformers (rated not more than 50kVA) above suspended ceilings or other hollow spaces of buildings not permanently closed in by the structure (Fig. 4).

Specific provisions. Part II of Art. 450 has requirements for specific types of transformers to prevent fire. For example, look at the requirements for dry-type transformers:

If dry-type transformers installed indoors aren’t over 112.5kVA [450.21(A)], they need separated from combustible material by at least 12 inches unless separated by a fire-resistant, heat-insulated barrier. But this rule doesn’t apply to transformers rated for 600V (nominal or less) that are completely enclosed except for ventilating openings [450.21(A) Exception].

If these units are more than 112.5kVA [450.21(B)], you must install them in a room of fire-resistant construction with a minimum fire rating of 1 hour. Two exceptions exist:

  • Transformers with Class 155 or higher insulation systems separated by a fire-resistant heat-insulating barrier or by not less than 6 feet horizontally and 12 feet vertically.
  • Transformers with Class 155 or higher insulation systems completely enclosed except for ventilating openings.

If these are more than 35kV [450.21(C)], you must install them in a vault that complies with Part III of Art. 450.

Dry-type transformers placed outdoors must be installed in a weatherproof enclosure (450.22). If the transformer exceeds 112.5kVA, you can’t locate it within 1 foot of combustible materials of buildings unless the transformer has Class 155 insulation or higher and is completely enclosed except for ventilation openings.

Part II also addresses the following types of transformers:

  • Less-flammable liquid-insulated (indoor and outdoor)
  • Nonflammable fluid-insulated
  • Askarel-insulated (indoor)
  • Oil-insulated (indoor and outdoor)

Transformer vaults. Wherever practicable, locate vaults so they can be ventilated to the outside air without using flues or ducts (450.41). The floors, walls, ceilings, and roofs of vaults must have adequate structural strength, with a minimum fire resistance of 3 hours. A combination of studs and wallboard construction isn’t an acceptable method of meeting this requirement (450.42). Further, each vault doorway must have a tight-fitting door that has a minimum fire resistance rating of 3 hours [450.43(A)].

Exception: If you protect the transformers with automatic sprinklers, water spray, carbon dioxide, or halon, you can use a 1-hour fire resistance rating (for the vault and the door) instead of a 3-hour one to save considerable money. As the 450.42 FPN notes, a typical 3-hour fire resistance rating is a construction that consists of 6 inch-thick reinforced concrete.

A curb that includes a doorsill must surround the vault (interior or exterior). The curb must be tall enough to confine the oil from the largest transformer. In no case can the curb height be less than 4 inches [450.43(B)].

Doors must swing out. You must equip these with panic bars or pressure plates so a person inside the vault can open the door with simple pressure. These doors must also be equipped with locks so the vaults are accessible only to qualified persons [450.34(C)].

Vault ventilation openings. You must provide openings for ventilation, per (A) through (F), where required by 450.9.

(A) Location. Place ventilation openings as far as possible from doors, windows, and combustible material.

(B) Arrangement. A vault ventilated by natural circulation can have no more than 50% of the total opening area near the floor, with the remainder of the opening area in the roof or sidewalls near the roof.

(C) Size. For a vault ventilated by natural circulation, the total opening area shall not be less than 3 square inches per kVA capacity, but in no case can the area be less than 1 square foot for any capacity less than 50kVA.

(D) Covering. Cover ventilation openings to avoid unsafe conditions.

(E) Dampers. Provide all indoor ventilation openings with automatic closing fire dampers that close in response to a vault fire. These must be rated not less than 1.5 hours.

(F) Ducts. Ventilating ducts shall be made of fire-resistant material.

More vault requirements. Art. 450 ends with three more requirements:

  • Vaults containing more than 100kVA transformer capacity must have the floor pitched to drain oil or water, unless conditions make this impracticable (450.46).
  • Only piping or other facilities provided for vault fire protection or cooling shall be permitted in the transformer vault (450.47).
  • Store nothing in a transformer vault (450.48).

This last requirement is one of the most commonly violated rules of Art. 450. If you have facility responsibilities, you must maintain constant vigilance to enforce this rule. Don’t allow people to store fluorescent lamps, janitorial supplies, or food in transformer vaults. While this is common sense to you, it may not make sense to everyone and few people outside this industry understand the consequences of an arc fault.