Article 450

Transformers and Transformer Vaults (Including Secondary Ties)

Part I. General Provisions

Scope. This article covers the installation of all trans‐ formers.Exception No. 1: Current transformers.Exception No. 2: Dry-type transformers that constitute a component part of other apparatus and comply with the requirements for such apparatus.Exception No. 3: Transformers that are an integral part of an X-ray, high-frequency, or electrostatic-coating apparatus.Exception No. 4: Transformers used with Class 2 and Class 3 circuits that comply with Article 725.Exception No. 5: Transformers for sign and outline lighting that comply with Article 600.Exception No. 6: Transformers for electric-discharge lighting that comply with Article 410.Exception No. 7: Transformers used for power-limited fire alarm circuits that comply with Part III of Article 760.Exception No. 8: Transformers used for research, development, or test‐ ing, where effective arrangements are provided to safeguard persons from contacting energized parts.This article covers the installation of transformers dedicated to supplying power to a fire pump installation as modified by Article 695.This article also covers the installation of transformers in hazardous (classified) locations as modified by Articles 501 through 504.
Definition. For the purpose of this article, the following definition shall apply.Transformer. An individual transformer, single- or polyphase, identified by a single nameplate, unless otherwise indicated in this article.
Overcurrent Protection. Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers oper‐ ating as a unit.

Informational Note No. 1: See 240.4, 240.21, 240.100, and

240.101 for overcurrent protection of conductors.

Informational Note No. 2: Nonlinear loads can increase heat in a transformer without operating its overcurrent protective device.

Transformers Over 1000 Volts, Nominal. Overcurrent protection shall be provided in accordance with Table 450.3(A).
Transformers 1000 Volts, Nominal, or Less. Overcurrent protection shall be provided in accordance with Table 450.3(B).Exception: Where the transformer is installed as a motor control circuit transformer in accordance with 430.72(C)(1) through (C)(5).
Voltage (Potential) Transformers. Voltage (potential) transformers installed indoors or enclosed shall be protected with primary fuses.

Informational Note: For protection of instrument circuits including voltage transformers, see 408.52.

Autotransformers 1000 Volts, Nominal, or Less.
1. Overcurrent Protection. Each autotransformer 1000 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conduc‐ tor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. Where this calculation does not correspond to a standard rating of a fuse or nonadjustable circuit breaker and the rated input current is 9 amperes or more, the next higher standard rating described in 240.6 shall be permitted.An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B as shown in Figure 450.4(A).Exception: Where the rated input current of the autotransformer is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.
2. Transformer Field-Connected as an Autotransformer. A transformer field-connected as an autotransformer shall be identified for use at elevated voltage.
Informational Note: For information on permitted uses of auto‐ transformers, see 210.9 and 215.11.
Grounding Autotransformers. Grounding autotrans‐ formers covered in this section are zigzag or T-connected trans‐ formers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3-phase, 4-wire distribution system or providing a neutral point for grounding purposes. Such trans‐ formers shall have a continuous per-phase current rating and a continuous neutral current rating. Zigzag-connected transform‐ ers shall not be installed on the load side of any system ground‐ ing connection, including those made in accordance with 250.24(B), 250.30(A)(1), or 250.32(B), Exception No. 1.Informational Note: The phase current in a grounding auto‐ transformer is one-third the neutral current.
Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current)

Primary Protection over 1000 VoltsSecondary Protection (See Note 2.)Over 1000 Volts 1000 Volts or Less
Location LimitationsTransformer Rated ImpedanceCircuit Breaker (See Note 4.)
Fuse Rating
Circuit Breaker (See Note 4.)
Fuse Rating
Circuit Breaker or Fuse Rating
Not more than 6%600%(See Note 1.)300%(See Note 1.)
300%(See Note 1.)250%(See Note 1.)
125%(See Note 1.)Any locationMore than 6% and400%300%
250%225%
125%
not more than 10%(See Note 1.)(See Note 1.)
(See Note 1.)(See Note 1.)
(See Note 1.)Supervised locations onlyAny300%(See Note 1.)250%(See Note 1.)
Not requiredNot required
Not required(See Note 3.)Not more than 6%600%300%300%(See Note 5.)250%(See Note 5.)250%(See Note 5.)
More than 6% and not morethan 10%400%300%250%(See Note 5.)225%(See Note 5.)250%(See Note 5.)Notes:
1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the following shall be permitted:
  1. The next higher standard rating or setting for fuses and circuit breakers 1000 volts and below, or
  2. The next higher commercially available rating or setting for fuses and circuit breakers above 1000 volts.
2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses.
3. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation.
4. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers.
5. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.
  Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less (as a Percentage of Transformer-Rated Current)
  
  Primary Protection
  
  Secondary Protection (See Note 2.)
  Protection MethodCurrents of9 Amperes or MoreCurrents Less Than 9 AmperesCurrents Less Than 2 Amperes
  Currents of Currents Less 9 Amperes or More Than 9 AmperesPrimary only protection125% (See Note 1.)167%300%
  Not required Not requiredPrimary and secondary protectionNotes:250% (See Note 3.) 250% (See Note 3.) 250% (See Note 3.) 125% (See Note 1.) 167%
  1. Where 125 percent of this 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.
  2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.
  3. A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current shall be permitted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance.
  •AShunt winding(s)
  1. Three-Phase, 4-Wire System. A grounding autotrans‐ former used to create a 3-phase, 4-wire distribution system from a 3-phase, 3-wire ungrounded system shall conform to 450.5(A)(1) through (A)(4).
    1. Connections. The transformer shall be directly connected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system.
    2. Overcurrent Protection. An overcurrent sensing device shall be provided that will cause the main switch or common- trip overcurrent protection referred to in 450.5(A)(1) to open if the load on the autotransformer reaches or exceeds125 percent of its continuous current per-phase or neutral rating. Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device shall be permitted for the purpose of allowing proper operation of branch or feeder protective devices on the 4-wire system.
    3. Transformer Fault Sensing. A fault-sensing system that causes the opening of a main switch or common-trip overcur‐ rent device for the 3-phase, 4-wire system shall be provided to guard against single-phasing or internal faults.Informational Note: This can be accomplished by the use of two subtractive-connected donut-type current transformers installed to sense and signal when an unbalance occurs in the line current to the autotransformer of 50 percent or more of rated current.
    4. Rating. The autotransformer shall have a continuous neutral-current rating that is not less than the maximum possi‐ ble neutral unbalanced load current of the 4-wire system.
  2. Ground Reference for Fault Protection Devices. A grounding autotransformer used to make available a specified magnitude of ground-fault current for operation of a ground- responsive protective device on a 3-phase, 3-wire ungrounded system shall conform to 450.5(B)(1) and (B)(2).B
    FIGURE 450.4(A) Autotransformer.
    1. Rating. The autotransformer shall have a continuous neutral-current rating not less than the specified ground-fault current.
    2. Overcurrent Protection. Overcurrent protection shall comply with (a) and (b).
      1. Operation and Interrupting Rating. An overcurrent protective device having an interrupting rating in compliance with 110.9 and that will open simultaneously all ungrounded conductors when it operates shall be applied in the grounding autotransformer branch circuit.
      2. Ampere Rating. The overcurrent protection shall be rated or set at a current not exceeding 125 percent of the auto‐ transformer continuous per-phase current rating or 42 percent of the continuous-current rating of any series-connected devi‐ ces in the autotransformer neutral connection. Delayed trip‐ ping for temporary overcurrents to permit the proper operation of ground-responsive tripping devices on the main system shall be permitted but shall not exceed values that would be more than the short-time current rating of the grounding autotransformer or any series connected devices in the neutral connection thereto.
    Exception: For high-impedance grounded systems covered in 250.36, where the maximum ground-fault current is designed to be not more than 10 amperes, and where the grounding autotransformer and the grounding impedance are rated for continuous duty, an overcurrent
    device rated not more than 20 amperes that will simultaneously open all ungrounded conductors shall be permitted to be installed on the line side of the grounding autotransformer.
  3. Ground Reference for Damping Transitory Overvoltages. A grounding autotransformer used to limit transitory overvol‐ tages shall be of suitable rating and connected in accordance with 450.5(A)(1).
Secondary Ties. As used in this article, a secondary tie is a circuit operating at 1000 volts, nominal, or less between phases that connects two power sources or power supply points, such as the secondaries of two transformers. The tie shall be permitted to consist of one or more conductors per phase or neutral. Conductors connecting the secondaries of transform‐ ers in accordance with 450.7 shall not be considered secondary ties.As used in this section, the word transformer means a trans‐ former or a bank of transformers operating as a unit.
1. Tie Circuits. Tie circuits shall be provided with overcur‐ rent protection at each end as required in Parts I, II, and VIII of Article 240.Under the conditions described in 450.6(A)(1) and 450.6(A)(2), the overcurrent protection shall be permitted to be in accordance with 450.6(A)(3).
  1. Loads at Transformer Supply Points Only. Where all loads are connected at the transformer supply points at each end of the tie and overcurrent protection is not provided in accord‐ ance with Parts I, II, and VIII of Article 240, the rated ampacity of the tie shall not be less than 67 percent of the rated secon‐ dary current of the highest rated transformer supplying the secondary tie system.
  2. Loads Connected Between Transformer Supply Points. Where load is connected to the tie at any point between trans‐ former supply points and overcurrent protection is not provi‐ ded in accordance with Parts I, II, and VIII of Article 240, the rated ampacity of the tie shall not be less than 100 percent of the rated secondary current of the highest rated transformer supplying the secondary tie system.Exception: Tie circuits comprised of multiple conductors per phase shall be permitted to be sized and protected in accordance with 450.6(A)(4).
  3. Tie Circuit Protection. Under the conditions described in 450.6(A)(1) and (A)(2), both supply ends of each ungrounded tie conductor shall be equipped with a protective device that opens at a predetermined temperature of the tie conductor under short-circuit conditions. This protection shall consist of one of the following: (1) a fusible link cable connector, termi‐ nal, or lug, commonly known as a limiter, each being of a size corresponding with that of the conductor and of construction and characteristics according to the operating voltage and the type of insulation on the tie conductors or (2) automatic circuit breakers actuated by devices having comparable time– current characteristics.
  4. Interconnection of Phase Conductors Between Trans‐ former Supply Points. Where the tie consists of more than one conductor per phase or neutral, the conductors of each phase or neutral shall comply with one of the following provi‐ sions.
    1. Interconnected. The conductors shall be interconnec‐ ted in order to establish a load supply point, and the protective device specified in 450.6(A)(3) shall be provided in eachungrounded tie conductor at this point on both sides of the interconnection. The means of interconnection shall have an ampacity not less than the load to be served.
    2. Not Interconnected. The loads shall be connected to one or more individual conductors of a paralleled conductor tie without interconnecting the conductors of each phase or neutral and without the protection specified in 450.6(A)(3) at load connection points. Where this is done, the tie conductors of each phase or neutral shall have a combined capacity ampac‐ ity of not less than 133 percent of the rated secondary current of the highest rated transformer supplying the secondary tie system, the total load of such taps shall not exceed the rated secondary current of the highest rated transformer, and the loads shall be equally divided on each phase and on the indi‐ vidual conductors of each phase as far as practicable.
  5. Tie Circuit Control. Where the operating voltage exceeds 150 volts to ground, secondary ties provided with limiters shall have a switch at each end that, when open, de-energizes the associated tie conductors and limiters. The current rating of the switch shall not be less than the rated current ampacity of the conductors connected to the switch. It shall be capable of interrupting its rated current, and it shall be constructed so that it will not open under the magnetic forces resulting from short-circuit current.
2. Overcurrent Protection for Secondary Connections. Where secondary ties are used, an overcurrent device rated or set at not more than 250 percent of the rated secondary current of the transformers shall be provided in the secondary connections of each transformer supplying the tie system. In addition, an automatic circuit breaker actuated by a reverse- current relay set to open the circuit at not more than the rated secondary current of the transformer shall be provided in the secondary connection of each transformer.
3. Grounding. Where the secondary tie system is grounded, each transformer secondary supplying the tie system shall be grounded in accordance with the requirements of 250.30 for separately derived systems.
Parallel Operation. Transformers shall be permitted to be operated in parallel and switched as a unit, provided the overcurrent protection for each transformer meets the require‐ ments of 450.3(A) for primary and secondary protective devi‐ ces over 1000 volts, or 450.3(B) for primary and secondary protective devices 1000 volts or less.
Guarding. Transformers shall be guarded as specified in 450.8(A) through (D).
1. Mechanical Protection. Appropriate provisions shall be made to minimize the possibility of damage to transformers from external causes where the transformers are exposed to physical damage.
2. Case or Enclosure. Dry-type transformers shall be provi‐ ded with a noncombustible moisture-resistant case or enclosure that provides protection against the accidental insertion of foreign objects.
3. Exposed Energized Parts. Switches or other equipment operating at 1000 volts, nominal, or less and serving only equip‐ ment within a transformer enclosure shall be permitted to be installed in the transformer enclosure if accessible to qualified persons only. All energized parts shall be guarded in accord‐ ance with 110.27 and 110.34.
4. Voltage Warning. The operating voltage of exposed live parts of transformer installations shall be indicated by signs or visible markings on the equipment or structures.
Ventilation. The ventilation shall dispose of the trans‐ former full-load heat losses without creating a temperature rise that is in excess of the transformer rating.Informational Note No. 1: See ANSI/IEEE C57.12.00-1993, General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, and ANSI/IEEE C57.12.01-1989, General Requirements for Dry-Type Distribution and Power Transformers.Informational Note No. 2: Additional losses may occur in some transformers where nonsinusoidal currents are present, result‐ ing in increased heat in the transformer above its rating. See ANSI/IEEE C57.110-1993, Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents, where transformers are utilized with nonlinear loads.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.
Grounding.
1. Dry-Type Transformer Enclosures. Where separate equip‐ ment grounding conductors and supply-side bonding jumpers are installed, a terminal bar for all grounding and bonding conductor connections shall be secured inside the transformer enclosure. The terminal bar shall be bonded to the enclosure in accordance with 250.12 and shall not be installed on or over any vented portion of the enclosure.Exception: Where a dry-type transformer is equipped with wire-type connections (leads), the grounding and bonding connections shall be permitted to be connected together using any of the methods in 250.8 and shall be bonded to the enclosure if of metal.
2. Other Metal Parts. Where grounded, exposed non– current-carrying metal parts of transformer installations, including fences, guards, and so forth, shall be grounded and bonded under the conditions and in the manner specified for electrical equipment and other exposed metal parts in Parts V, VI, and VII of Article 250.
Marking.
1. General. Each transformer shall be provided with a name‐ plate giving the following information:
  1. Name of manufacturer
  2. Rated kilovolt-amperes
  3. Frequency
  4. Primary and secondary voltage
  5. Impedance of transformers 25 kVA and larger
  6. Required clearances for transformers with ventilating openings
  7. Amount and kind of insulating liquid where used
  8. For dry-type transformers, temperature class for the insu‐ lation system
2. Source Marking. A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer’s instruc‐ tions.
Terminal Wiring Space. The minimum wire-bending space at fixed, 1000-volt and below terminals of transformerline and load connections shall be as required in 312.6. Wiring space for pigtail connections shall conform to Table 314.16(B).
Accessibility. All transformers and transformer vaults shall be readily accessible to qualified personnel for inspection and maintenance or shall meet the requirements of 450.13(A) or 450.13(B).
1. 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.
2. Hollow Space Installations. 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 require‐ ments of 450.9 and separation from combustible materials requirements of 450.21(A). Transformers so installed shall not be required to be readily accessible.
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 disconnect‐ ing means shall be lockable in accordance with 110.25, and its location shall be field marked on the transformer.

Part II. Specific Provisions Applicable to Different Types of Transformers

Dry-Type Transformers Installed Indoors.
1. Not Over 1121∕2 kVA. Dry-type transformers installed indoors and rated 1121∕2 kVA or less shall have a separation of at least 300 mm (12 in.) from combustible material unless separa‐ ted from the combustible material by a fire-resistant, heat- insulated barrier.Exception: This rule shall not apply to transformers rated for 1000 volts, nominal, or less that are completely enclosed, except for ventilating openings.
2. Over 1121∕2 kVA. Individual dry-type transformers of more than 1121∕2 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 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.Informational Note: See ANSI/ASTM E119-15, Method for Fire Tests of Building Construction and Materials.
3. Over 35,000 Volts. Dry-type transformers rated over 35,000 volts shall be installed in a vault complying with Part III of this article.
Dry-Type Transformers Installed Outdoors. Dry-type transformers installed outdoors shall have a weatherproof enclosure.Transformers exceeding 1121∕2 kVA shall not be located within 300 mm (12 in.) of combustible materials of buildings
unless the transformer has Class 155 insulation systems or higher and is completely enclosed except for ventilating open‐ ings.
Less-Flammable Liquid-Insulated Transformers. Transformers insulated with listed less-flammable liquids that have a fire point of not less than 300°C shall be permitted to be installed in accordance with 450.23(A) or 450.23(B).
1. Indoor Installations. Indoor installations shall be permit‐ ted in accordance with one of the following:
  1. In Type I or Type II buildings, in areas where all of the following requirements are met:
    1. The transformer is rated 35,000 volts or less.
    2. No combustible materials are stored.
    3. A liquid confinement area is provided.
    4. The installation complies with all the restrictions provided for in the listing of the liquid.
      Informational Note: Such restrictions may include, but arenot limited to: maximum pressure of the tank, use of a pres‐ sure relief valve, appropriate fuse types and proper sizing of overcurrent protection.
    5. With an automatic fire extinguishing system and a liquid confinement area, provided the transformer is rated 35,000 volts or less
    6. In accordance with 450.26
2. Outdoor Installations. Less-flammable liquid-filled trans‐ formers shall be permitted to be installed outdoors, attached to, adjacent to, or on the roof of buildings, where installed in accordance with (1) or (2).
1. For Type I and Type II buildings, the installation shall comply with all the restrictions provided for in the listing of the liquid.Informational Note No. 1: Installations adjacent to combustible material, fire escapes, or door and window openings may require additional safeguards such as those listed in 450.27.
  Informational Note No. 2: Such restrictions may include, but arenot limited to: maximum pressure of the tank, use of a pressure relief valve, appropriate fuse types, and proper sizing of overcur‐ rent protection.
2. In accordance with 450.27.
Informational Note No. 1: As used in this section, Type I and Type II buildings refers to Type I and Type II building construction as defined in NFPA 220-2015, Standard on Types of Building Construc‐ tion. Combustible materials refers to those materials not classified as noncombustible or limited-combustible as defined inNFPA 220-2015, Standard on Types of Building Construction . Informational Note No. 2: See definition of Listed in Article 100.
Nonflammable Fluid-Insulated Transformers. Trans‐ formers insulated with a dielectric fluid identified as nonflam‐ mable shall be permitted to be installed indoors or outdoors. Such transformers installed indoors and rated over 35,000 volts shall be installed in a vault. Such transformers installed indoors shall be furnished with a liquid confinement area and a pressure-relief vent. The transformers shall be furnished with a means for absorbing any gases generated by arcing inside the tank, or the pressure-relief vent shall be connected to a chim‐ ney or flue that will carry such gases to an environmentally safe area.Informational Note: Safety may be increased if fire hazard analy‐ ses are performed for such transformer installations.For the purposes of this section, a nonflammable dielectric fluid is one that does not have a flash point or fire point and is not flammable in air.
Askarel-Insulated Transformers Installed Indoors. Askarel-insulated transformers installed indoors and rated over 25 kVA shall be furnished with a pressure-relief vent. Where installed in a poorly ventilated place, they shall be furnished with a means for absorbing any gases generated by arcing inside the case, or the pressure-relief vent shall be connected to a chimney or flue that carries such gases outside the building. Askarel-insulated transformers rated over 35,000 volts shall be installed in a vault.
Oil-Insulated Transformers Installed Indoors. Oil- insulated transformers installed indoors shall be installed in a vault constructed as specified in Part III of this article.Exception No. 1: Where the total capacity does not exceed 1121∕2 kVA, the vault specified in Part III of this article shall be permitted to be constructed of reinforced concrete that is not less than 100 mm (4 in.) thick.Exception No. 2: Where the nominal voltage does not exceed 1000, a vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting other materials and the total capacity in one location does not exceed 10 kVA in a section of the building classified as combustible or 75 kVA where the surrounding structure is classified as fire-resistant construction.Exception No. 3: Electric furnace transformers that have a total rating not exceeding 75 kVA shall be permitted to be installed without a vault in a building or room of fire-resistant construction, provided suitable arrangements are made to prevent a transformer oil fire from spreading to other combustible material.Exception No. 4: A transformer that has a total rating not exceeding 75 kVA and a supply voltage of 1000 volts or less that is an integral part of charged-particle-accelerating equipment shall be permitted to be installed without a vault in a building or room of noncombustible or fire-resistant construction, provided suitable arrangements are made to prevent a transformer oil fire from spreading to other combustible mate‐ rial.Exception No. 5: Transformers shall be permitted to be installed in a detached building that does not comply with Part III of this article if neither the building nor its contents present a fire hazard to any other building or property, and if the building is used only in supplying elec‐ tric service and the interior is accessible only to qualified persons.Exception No. 6: Oil-insulated transformers shall be permitted to be used without a vault in portable and mobile surface mining equipment (such as electric excavators) if each of the following conditions is met:
1. Provision is made for draining leaking fluid to the ground.
2. Safe egress is provided for personnel.
3. A minimum 6-mm (1∕4-in.) steel barrier is provided for personnel protection.
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.
In cases where the transformer installation presents a fire hazard, one or more of the following safeguards shall be applied according to the degree of hazard involved:
1. Space separations
2. Fire-resistant barriers
3. Automatic fire suppression systems
4. Enclosures that confine the oil of a ruptured transformer tankOil enclosures shall be permitted to consist of fire-resistant dikes, curbed areas or basins, or trenches filled with coarse, crushed stone. Oil enclosures shall be provided with trapped drains where the exposure and the quantity of oil involved are such that removal of oil is important.Informational Note: For additional information on transformers installed on poles or structures or under ground, see ANSI C2-2007, National Electrical Safety Code.
Modification of Transformers. When modifications are made to a transformer in an existing installation that change the type of the transformer with respect to Part II of this article, such transformer shall be marked to show the type of insulating liquid installed, and the modified transformer installation shall comply with the applicable requirements for that type of transformer.

Part III. Transformer Vaults

Location. Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wher‐ ever such an arrangement is practicable.
Walls, Roofs, and Floors. The walls and roofs of vaults shall be constructed of materials that have approved 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 100 mm (4 in.) thick, but, where the vault is constructed with a vacant space or other stories below it, the floor shall have approved structural strength for the load imposed thereon and a minimum fire resistance of 3 hours. For the purposes of this section, studs and wallboard construction shall not be permitted.Exception: Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.Informational Note No. 1: For additional information, see ANSI/ASTM E119-15, Method for Fire Tests of Building Construction and Materials.Informational Note No. 2: A typical 3-hour construction is 150 mm (6 in.) thick reinforced concrete.
Doorways. Vault doorways shall be protected in accordance with 450.43(A), (B), and (C).

Type of Door. 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. The authority having jurisdiction shall be permitted to require such a door for an exterior wall opening where conditions warrant.Exception: Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted.Informational Note: For additional information, see NFPA 80-2013, Standard for Fire Doors and Other Opening Protectives.
Sills. A door sill or curb that is of an approved height that will confine the oil from the largest transformer within the vault shall be provided, and in no case shall the height be less than 100 mm (4 in.).
Locks. Doors shall be equipped with locks, and doors shall be kept locked, access being allowed only to qualified persons. Personnel doors shall open in the direction of egress and be equipped with listed panic hardware.

Ventilation Openings. Where required by 450.9, open‐ ings for ventilation shall be provided in accordance with 450.45(A) through (F).
1. Location. Ventilation openings shall be located as far as possible from doors, windows, fire escapes, and combustible material.
2. Arrangement. A vault ventilated by natural circulation of air shall be permitted to have roughly half of the total area of openings required for ventilation in one or more openings near the floor and the remainder in one or more openings in the roof or in the sidewalls near the roof, or all of the area required for ventilation shall be permitted in one or more openings in or near the roof.
3. Size. For a vault ventilated by natural circulation of air to an outdoor area, the combined net area of all ventilating open‐ ings, after deducting the area occupied by screens, gratings, or louvers, shall not be less than 1900 mm2 (3 in.2) per kVA of transformer capacity in service, and in no case shall the net area be less than 0.1 m2 (1 ft2) for any capacity under 50 kVA.
4. Covering. Ventilation openings shall be covered with durable gratings, screens, or louvers, according to the treat‐ ment required in order to avoid unsafe conditions.
5. Dampers. All ventilation openings to the indoors shall be provided with automatic closing fire dampers that operate in response to a vault fire. Such dampers shall possess a standard fire rating of not less than 11∕2 hours.Informational Note: See ANSI/UL 555-2011, Standard for Fire Dampers.
6. Ducts. Ventilating ducts shall be constructed of fire- resistant material.
Drainage. Where practicable, vaults containing more than 100 kVA transformer capacity shall be provided with a drain or other means that will carry off any accumulation of oil or water in the vault unless local conditions make this impracti‐ cable. The floor shall be pitched to the drain where provided.
Water Pipes and Accessories. Any pipe or duct system foreign to the electrical installation shall not enter or pass through a transformer vault. Piping or other facilities provided for vault fire protection, or for transformer cooling, shall not be considered foreign to the electrical installation.
Storage in Vaults. Materials shall not be stored in transformer vaults.

455.1 ARTICLE 455 — PHASE CONVERTERS

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