11.1 Transformers — Article 450
Key Takeaways
- Table 450.3(B) permits primary-only protection at 125 percent of primary current where the primary current is 9 amperes or more, with the next higher standard rating permitted.
- Where both primary and secondary protection are provided, Table 450.3(B) permits the primary device up to 250 percent and requires the secondary device at not more than 125 percent of secondary current.
- NEC 450.21(A) requires dry-type transformers rated 112.5 kVA or less installed indoors to have a separation of at least 12 inches from combustible material unless separated by a fire-resistant heat-insulating barrier or completely enclosed except for ventilating openings.
- NEC 450.21(B) requires dry-type transformers over 112.5 kVA installed indoors to be installed in a transformer room of fire-resistant construction, with exceptions for Class 155 or higher insulation systems.
- NEC 450.13 requires transformers to be readily accessible, with exceptions for dry-type transformers 1000 volts or less located in the open on walls, columns, or structures, and for dry-type transformers 1000 volts or less not exceeding 50 kVA in hollow spaces of buildings not permanently closed in by structure.
Three Separate Determinations, Again
Section 7.3 of this guide made the point and it is worth repeating because it is the single most common error on transformer questions:
| Question | Rule |
|---|---|
| Protecting the transformer | Table 450.3(B) |
| Protecting the primary conductors | 240.4, or a feeder tap rule in 240.21(B)(3) |
| Protecting the secondary conductors | 240.21(C) |
Overcurrent Protection — Table 450.3(B)
For transformers 1000 volts, nominal, or less:
Primary-Only Protection
| Primary current | Maximum rating of primary protective device |
|---|---|
| 9 amperes or more | 125 percent — where 125 percent does not correspond to a standard rating from 240.6, the next higher standard rating is permitted |
| Less than 9 amperes | 167 percent |
| Less than 2 amperes | 300 percent |
Primary and Secondary Protection
| Secondary current | Primary device maximum | Secondary device maximum |
|---|---|---|
| 9 amperes or more | 250 percent | 125 percent — next higher standard rating permitted where 125 percent does not correspond to a standard rating |
| Less than 9 amperes | 250 percent | 167 percent |
The logic behind the 250 percent primary allowance is that a properly sized secondary device already protects the transformer against overload; the primary device only needs to survive magnetizing inrush, which for a dry-type transformer can be eight to twelve times full-load current for a fraction of a cycle.
Note 1 to Table 450.3(B) clarifies that where 125 percent of the rated current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
Note 2 permits, where secondary overcurrent protection is required, the secondary protection to consist of not more than six circuit breakers or six sets of fuses grouped in one location, with the sum of the device ratings not exceeding the allowed value of a single overcurrent device.
Worked Example 1 — Primary Only
A 30 kVA, 480 V three-phase to 208Y/120 V transformer. Primary full-load current = 30,000 / (1.732 x 480) = 36.1 A.
Primary current 36.1 A is 9 A or more -> 125 percent
36.1 x 1.25 = 45.1 A
45.1 A is not a standard rating; next higher standard = 50 A
A 50-ampere primary device. Because the secondary is a four-wire wye, 240.21(C)(1) does not permit that device to protect the secondary conductors, so secondary conductor protection is still required under one of the 240.21(C) rules.
Worked Example 2 — Primary and Secondary
A 75 kVA, 480 V three-phase to 208Y/120 V transformer.
Primary FLC = 75,000 / (1.732 x 480) = 90.2 A
Secondary FLC = 75,000 / (1.732 x 208) = 208.2 A
Secondary device: 208.2 x 1.25 = 260.3 A
Not a standard rating; next higher standard = 300 A
Primary device (with secondary protection): 90.2 x 2.50 = 225.5 A
Maximum permitted is 225.5 A, so the largest standard device
at or below that value is 225 A
Note the direction of rounding. Note 1 permits rounding up only for the 125 percent values. The 250 percent primary figure is a ceiling with no round-up allowance, so you take the next standard size at or below it.
Ventilation, Clearances and Accessibility
450.9 Ventilation. The ventilation shall be adequate to dispose of the transformer full-load losses without creating a temperature rise that is in excess of the transformer rating. Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions. The required clearances shall be clearly marked on the transformer.
An Informational Note adds that additional space may be required for equipment grounding and bonding, and refers to 110.26 for working space.
450.11 Marking. (A) Each transformer shall be provided with a nameplate giving the name of the manufacturer; rated kilovolt-amperes; frequency; primary and secondary voltage; impedance of transformers 25 kVA and larger; required clearances for transformers with ventilating openings; and the amount and kind of insulating liquid where used. (B) In addition, the nameplate of each dry-type transformer shall include the temperature class for the insulation system.
450.13 Accessibility. All transformers and transformer vaults shall be readily accessible to qualified personnel for inspection and maintenance, or shall meet one of the following:
- (A) Open Installations. Dry-type transformers 1000 volts, nominal, or less, located in the open on walls, columns, or structures, shall not be required to be readily accessible.
- (B) Hollow Spaces. Dry-type transformers 1000 volts, nominal, or less and not exceeding 50 kVA shall be permitted in hollow spaces of buildings not permanently closed in by structure, provided they meet the ventilation requirements of 450.9 and separation from combustible materials of 450.21(A). Transformers so installed shall not be required to be readily accessible.
450.14 Disconnecting Means. Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means shall be lockable, and the location shall be field marked on the transformer.
Dry-Type Transformers Installed Indoors — 450.21
450.21(A) Not Over 112.5 kVA. Dry-type transformers installed indoors and rated 112.5 kVA or less shall have a separation of at least 12 inches (300 mm) from combustible material unless separated from the combustible material by a fire-resistant, heat-insulating barrier, or unless the transformer is rated 600 volts, nominal, or less and is completely enclosed except for ventilating openings.
450.21(B) Over 112.5 kVA. Individual dry-type transformers of more than 112.5 kVA rating shall be installed in a transformer room of fire-resistant construction. Unless specified otherwise in this article, the term fire resistant means a construction having a minimum fire rating of 1 hour.
Exception No. 1: Transformers with Class 155 or higher insulation systems and separated from other combustible material by a fire-resistant, heat-insulating barrier or by not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically.
Exception No. 2: Transformers with Class 155 or higher insulation systems and completely enclosed except for ventilating openings.
450.21(C) Over 35,000 Volts. Dry-type transformers rated over 35,000 volts shall be installed in a vault complying with Part III of Article 450.
Dry-Type Transformers Installed Outdoors — 450.22
Dry-type transformers installed outdoors shall have a weatherproof enclosure. Transformers exceeding 112.5 kVA shall not be located within 12 inches (300 mm) of combustible materials of buildings unless the transformer has Class 155 or higher insulation systems and is completely enclosed except for ventilating openings.
Oil-Insulated Transformers
450.26 Oil-Insulated Transformers Installed Indoors. Oil-insulated transformers installed indoors shall be installed in a vault constructed as specified in Part III. There are exceptions for transformers not exceeding 112.5 kVA where the vault is constructed of reinforced concrete not less than 4 inches thick; for electric furnace transformers not exceeding 75 kVA where installed with approved protection; for transformers used with induction heating equipment; and for transformers rated 35,000 volts or less used exclusively to supply industrial electric furnaces under specified conditions.
450.27 Oil-Insulated Transformers Installed Outdoors. Combustible material, combustible buildings, and parts of buildings, fire escapes, and door and window openings shall be safeguarded from fires originating in oil-insulated transformers installed on roofs, attached to or adjacent to a building or combustible material. Space separations, fire-resistant barriers, automatic fire suppression systems, and enclosures that confine the oil are recognized safeguards.
Vaults — Part III
450.41 Location. Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wherever such an arrangement is practicable.
450.42 Walls, Roofs, and Floors. The walls and roofs of vaults shall be constructed of materials that have adequate structural strength for the conditions with a minimum fire resistance of 3 hours. The floors of vaults in contact with the earth shall be of concrete that is not less than 4 inches (100 mm) thick.
Exception: Where the vault is protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.
450.43 Doorways. (A) Each doorway leading into a vault from the building interior shall be provided with a tight-fitting door that has a minimum fire rating of 3 hours — reducible to 1 hour under the 450.42 exception. (B) Personnel doors shall swing out and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure. (C) Doorways shall be provided with locks, and doors shall be kept locked, with access allowed only to qualified persons.
450.45 Ventilation Openings and 450.46 Drainage complete the vault requirements. 450.47 prohibits foreign systems: any pipe or duct system foreign to the electrical installation shall not enter or pass through a transformer vault.
450.48 Storage in Vaults. Materials shall not be stored in transformer vaults.
Autotransformers and Zig-Zag Transformers
450.4 Autotransformers 1000 Volts, Nominal, or Less. An autotransformer 1000 volts or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. An overcurrent device shall not be installed in series with the shunt winding of the autotransformer.
450.5 Grounding Autotransformers. Grounding autotransformers covered in this section are zig-zag or T-connected transformers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3-phase, 4-wire distribution system or providing a neutral reference for grounding purposes.
Remember from section 9.6: an autotransformer is not a separately derived system, because its windings share a common connection with the supply.
A 45 kVA, 480-volt three-phase transformer has a primary full-load current of 54.1 amperes. Using primary-only protection under Table 450.3(B), what is the maximum primary overcurrent device?
A 150 kVA dry-type transformer with a Class 130 insulation system is installed indoors. What does NEC 450.21(B) require?
Where both primary and secondary overcurrent protection are provided for a transformer of 1000 volts or less with a secondary current of 9 amperes or more, what maximum percentage applies to the primary device under Table 450.3(B)?
A dry-type transformer rated 480 volts and 45 kVA is installed in a hollow space of a building that is not permanently closed in by the structure. What does NEC 450.13(B) permit?