A & E Service Maintenance Co., Inc. Service
Company, Inc.
Electrical Contractors

NEC Article 450 - Transformers 1/7/2004
Step up your knowledge of NEC transformer requirements
Extracted from Mike Holt's Understanding the National Electrical Code textbook for EC&M Magazine

Article 450 begins with the terse statement:"This article covers the installation of all transformers." Then lists eight exceptions. In essence, Article 450 covers power transformers, transformer vaults, and most kinds of lighting transformers (Figure 450-1).

Note: Graphic images are not contained in this newsletter; they will be in the magazine.

Overcurrent Protection

Overcurrent protection alone won't prevent transformer overheating. As [450.3 FPH 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 cannot exceed 250% and the secondary protection cannot exceed 125%. If you have primary protection only, it cannot 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.

See if you can you apply Table 450.3(B), by answering this question. What is the maximum primary protection device rating permitted for a 45 kVA, 3, 480V transformer (Figure 450-3), if it has primary protection only?

Answer: (d) 70A

Step 1. Determine primary current

I = VA/(E X 1.732).
I = 45,000 VA/(480V X 1.732) = 54A.

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

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


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 the name of the manufacturer, rated kVA, primary and secondary voltage, impedance (if 25 kVA 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

For transformers rated 600V or less, install them so they are readily accessible to qualified personnel for inspection and maintenance [450.13]. Two exceptions exist:

Open Installations. You can locate dry-type transformers in the open on walls, columns, or structures (Figure 450-6).
Suspended Ceilings. You can install dry-type transformers (rated not more than 50 kVA) above suspended ceilings or other hollow spaces of buildings not permanently closed in by the structure (Figure 450-7).
Specific Provisions

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

Dry-Type Transformers Installed Indoors. If these are not over 112 kVA, they need a separation at least 12 in. from combustible material unless separated by a fire-resistant, heat-insulated barrier. But, this rule does not apply to transformers rated for 600V (nominal or less) that are completely enclosed except for ventilating openings.

If these are over 112 kVA, you must install them in a room of fire-resistant construction having 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 ft horizontally and 12 ft vertically.
Transformers with Class 155 or higher insulation systems completely enclosed except for ventilating openings.
If these are over 35kV, you must install them in a vault that complies with Part III of Article 450.Dry-Type Transformers Installed Outdoors. To place a dry-type transformer outdoors, you must install it in a weatherproof enclosure. If the transformer exceeds 112.5 kVA, 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 addresses other types of transformers, as well:

Less-Flammable Liquid-Insulated (indoor and outdoor)
Nonflammable Fluid-Insulated
Askarel-Insulated (indoor)
Oil-Insulated (indoor and outdoor)
Article 450: Transformers

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. Studs and wallboard construction is not 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.

Exception: If you protect the transformers with automatic sprinkler, 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. This can save considerable money. As the 450.42 FPN notes, a typical 3-hour fire resistance rating is a construction consisting of 6 in. 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 in.

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.

Vault Ventilation Openings

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

Location. Place ventilation openings as far as possible from doors, windows, and combustible material.
Arrangement. A vault ventilated by natural circulation can have no more than fifty percent of the total opening area near the floor, with the remainder of the opening area in the roof or sidewalls near the roof.
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 under 50 kVA.
Covering. Cover ventilation openings to avoid unsafe conditions.
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
Ducts. Ventilating ducts shall be of fire-resistant material.
More Vault Requirements

Article 450 ends with three more requirements:

Vaults containing more than 100 kVA 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 Article 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. You may need to share relevant articles or ask your insurance company for compliance advice.

Copyright 2004 Mike Holt Enterprises,Inc.