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Transformer Installation Guide

Dec. 15, 2021


The successful operation of a transformer depends on correct installation as well as correct design and manufacture. This article covers installation procedures specific to dry-type transformers and those associated with fully assembled liquid-filled units.


Each transformer manufacturer produces different equipment from competing manufacturers. Furthermore, each manufacturer has its own instructions for installing and testing transformers. Such instructions should be followed carefully to ensure the adequate safety of personnel and equipment. Furthermore, it is important to follow current NEC practices and applicable local regulations.


The purpose of this document is to provide general installation guidelines for putting transformers into service. It is not meant to replace the manufacturer's instructions.

Dry Type Transformer

Dry Type Transformer     

Dry-type transformers

Transformer vents are designed to meet NEMA and ANSI standards which require that 1/2-in. diameter rods cannot be inserted through the openings. For dry-type transformers, the core/coil assembly (without housing) usually has mounting and terminal dimensions suitable for the requirements of your enclosure. The enclosure should provide protection for the coils and have sufficient clearance and adequate ventilation openings. The manufacturer should always be consulted to determine these requirements.


The minimum electrical clearances in the NEC Sec for the installation of lug and cable must be observed. Vented dry-type transformers can be designed for installation indoors or outdoors. They will operate successfully in conditions of high humidity, but in such cases, precautions may need to be taken to keep them dry if they are switched off for a period of time.


Measures should be taken during operation to arrange for frequent inspections to ensure that the surfaces of the live parts of the dry-type transformer are free from contamination. Caution: Ensure that the equipment is disconnected from power first. Excessive contamination can cause problems and may even lead to transformer failure. For locations with harsh atmospheric conditions, it is best to use liquid-filled transformers. However, if the dry type transformer is still preferred, a fully enclosed, unvented unit; sealed coil unit; or cast coil transformer should be considered.

H-Class Dry Type Transformer

H-Class Dry Type Transformer     

Locations with dripping water should always be avoided. If this is not possible, suitable protection should be provided to prevent water from entering the transformer housing. Precautions should be taken to prevent accidental ingress of water, e.g. water that may be obtained from an open window, water or steam line break, or the use of nearby water.


Adequate ventilation is a must. Adequate ventilation is essential for the proper cooling of dry-type transformers. For these types of equipment, the use of clean, dry air is desirable. It is strongly recommended that you take steps to install ventilated dry-type transformers in areas where there are no unusual dust or chemical fumes. If such conditions exist, wet units or other forms of dry units should be used. Filtered air may reduce maintenance if contamination is a problem in that location.


When the transformer is installed in a vault or other restricted space, you should provide sufficient ventilation to keep the air temperature within the established limits when measured near the transformer inlet. For dry-type transformers, this typically requires approximately 100 cubic feet of air per kW of transformer loss. The required ventilation opening area depends on the height of the vault, the location of the opening and the maximum load carried by the transformer. For self-cooling transformers, the required ventilation openings should be at least 1 square foot per 100 kVA of rated transformer capacity, after deducting the area occupied by screens, grilles, or louvres. This is approximately equivalent to the requirements of the NEC Sec.

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