Most indoor transformer installations fail their first inspection for the same three reasons: not enough ventilation, not enough clearance, and a unit that was never meant to sit inside a building. A dry type transformer solves the oil-fire problem, but it still needs the room to be designed around it.
Why dry type is the default indoors
An oil immersed transformer inside a building brings a fire load and a containment requirement. Many building codes and insurance policies push the unit into a separate vault or outdoors altogether. A dry type transformer removes the oil, so it can sit in the electrical room beside the switchgear.
Ventilation is not optional
A dry type unit rejects its losses as heat into the room, and that heat has to leave. As a working rule, the room needs enough air movement to keep the inlet-to-outlet temperature rise within the design figure on the nameplate. Louvres sized by eye are a common cause of overheating.
For a sealed, air-conditioned room the cooling load has to be added to the HVAC design. A unit that adds several kilowatts of heat to a small room will warm it until the winding runs above class.
Clearances and access
Leave the manufacturer’s clearance on every side, and more in front of the inspection covers. Cable boxes, temperature monitors and forced-air fans all need room to be serviced. A unit boxed into a corner cannot be maintained.
Noise inside a building
Dry type units are quieter than oil filled units at the same rating, but they are not silent. In an office or hospital the transformer should not share a wall with a occupied room. Acoustic treatment on the room, not on the transformer, is usually the cheaper fix.
Protection and monitoring
Fit winding temperature monitoring with alarm and trip contacts. On a dry type unit the winding temperature is the leading indicator of trouble, and it is visible without opening the tank. Add class F or H insulation where the room runs warm.
When to go back outdoors
If the load is large, the room is tight and the site allows it, an oil immersed unit outdoors or in a compact substation is often the more economical answer. The choice is a site question, not only an electrical one.
Send the room dimensions, the load and the voltages and we will confirm which construction fits indoors and what clearances the installation needs.