|NEC Article 450 - Transformers
|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
Note: Graphic images are not contained in this newsletter; they
will be in the magazine.
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, 3Ø, 480V transformer (Figure 450-3), if it has primary
Step 1. Determine primary current
I = VA/(E X 1.732).
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
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
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:
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
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:
Installed Indoors. If these are not over 112½ 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
If these are over 112½ kVA, you must install them in a room of
fire-resistant construction having a minimum fire rating of 1 hour. Two
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
Part II addresses other types of transformers, as
Less-Flammable Liquid-Insulated (indoor and outdoor)
Oil-Insulated (indoor and outdoor)
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
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
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.
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
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
More Vault Requirements
Article 450 ends with three more
Vaults containing more than 100 kVA transformer capacity
must have the floor pitched to drain oil or water, unless conditions make this
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.