Staying High and Dry

How do you design, protect and locate a dry-type transformer of 600 volts (V) or less so that the installation will comply with the National Electrical Code (NEC)?

Providing overcurrent protection: Table 450.3(B) is used to select the percentage for sizing the primary overcurrent protection device (OCPD) for the transformer. For example, a transformer having a full-load current of 80 amperes (A) can be supplied with 100A conductors selected from Table 310.15(B)(16) based on the terminal ratings per Section 110.14(C)(1)(a). The primary of the transformer and conductors can be protected by a 100A OCPD (80A × 125% = 100A) as permitted in Table 450.3(B) and Section 240.6(A). The transformer secondary conductors are installed according to Section 240.21(C) or Section 240.92(C) or (E).

Guarding: Transformers must be guarded where exposed to physical damage. For safety, the transformer enclosure must completely guard all energized components or warning signs, or visible markings must be provided as specified in the NEC sections 110.27 and 110.34. These requirements are clearly outlined in Section 450.8(A), (B) and (C).

Ventilation: Section 450.9 requires the ventilation to be adequate to dissipate the heat created by the transformer’s windings due to loading conditions. In other words, the transformer must never be installed where heat is so excessive that the loaded transformer cannot pull in cool air and remove its generated heat. Neither can the transformer be installed where walls or other obstructions block its ventilation openings. Note that clearance requirements must be marked on the transformer.

Grounding: According to Section 450.10, if the transformer is a separately derived system, it must be grounded as outlined in Section 250.30(A)(1) through (8). In addition, all the noncurrent-carrying parts specified in Section 450.10 must be grounded and bonded together per Parts V, VI and VII of Article 250.

Accessibility: All transformers must generally be readily accessible to qualified personnel for inspection and maintenance or must comply with Section 450.13(A) or (B). For example, they must be readily accessible for infrared inspections and for component cleaning, repair or replacement.

Section 450.13(A) permits the electrician to hang the transformer from the building steel or mount it on a platform above the grade level. Section 450.13(B) allows the transformer to be hung in hollow spaces where the primary and secondary voltage does not exceed 600 respectively. When making this type of installation, the installer must consider the requirements in Section 300.22(C)(3) pertaining to the type of transformer enclosure permitted in spaces used for the circulation of environmental air.

Disconnecting means required: Section 450.14 requires a lockable disconnecting means to be installed within sight of the transformer. Article 100 defines “within sight” as a distance of 50 feet and visible. Where this lockable disconnecting means is remotely installed, the location must be field-marked on the transformer for fast and easy identification. This requirement does not apply to Class 2 or 3 transformers.

Dry-type transformers installed indoors that are not over 112.5 kilovolt-amperes (kVA): As outlined in 450.21(A), dry-type transformers rated at 112.5 kVA or less installed indoors must be provided with a separation of at least 12 inches from combustible material unless separated from such combustible material by a fire-resistant, heat-­insulated barrier. Note that this requirement does not apply to transformers rated 600V or less, if they are completely enclosed, with or without appropriately ventilating openings per the Ex. to Section 450.21(A).

Dry-type transformers installed indoors that are over 112.5 kVA: According to the provisions in Section 450.21(B), dry-type transformers of more than 112.5 kVA must be installed in a transformer room of fire-resistant construction having a minimum fire rating of one hour (general rule). However, the Ex. 1 to (B) permits a transformer with Class 155 or greater insulation systems and separated from combustible material by a fire-resistant, heat-insulating barrier or by not less than 6 feet horizontally and 12 feet vertically. The Ex. 2 to (B) relieves transformers with Class 155 or higher insulation systems that are completely enclosed except for ventilating openings to comply with this requirement.

Transformers rated over 35,000V and installed indoors: Section 450.21(C) requires transformers over 35,000V installed indoors to be installed in a vault and meet all the requirements of Part III of Article 450.

Conclusion: Transformer primary protection is outlined in Table 450.3(B).

The conductors and OCPDs are sized and selected per Table 310.15(B)(16) and Section 240.6(A).

Secondary conductors are sized and selected by the provisions in Sections 240.21(C) or 240.92(C) or (E).

For installing and locating the transformer, apply the requirements in sections 450.8, 450.9, 450.10, 450.13, 450.14 and 450.21 as applicable.

About the Author

James G. Stallcup

Code Contributor

James G. Stallcup is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.

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