Placement is one of the first decisions in a specification, because it settles the insulation system, the ingress protection and the enclosure before a single electrical parameter is fixed. Indoors, in a building people use, the low fire load of cast resin usually wins, behind a ventilated enclosure such as IP21. Outdoors, oil-immersed units are the norm, with a high IP rating on the connection compartment, oil containment and corrosion protection matched to the site.
What changes when the transformer moves indoors?
Indoors the dominant risk is fire and smoke, not weather. A cast resin transformer classified F1 to IEC 60076-11 is designed to be self-extinguishing and to emit little smoke, which is why it is the default in offices, hospitals, car parks and other occupied buildings. An oil-immersed unit is not ruled out indoors, but it needs a fire-rated room, a bund sized for the full oil volume and often a suppression system — civil costs that quickly exceed the price difference between the two transformers.
The second change is heat. Outdoors the losses disappear into the air; indoors they end up in the room. A 1,000 kVA unit dissipates roughly 10 kW at full load, and that heat has to leave through ventilation openings or forced air. Undersized ventilation raises the ambient temperature, derates the transformer and shortens the life of the insulation.
How high does the IP rating really need to be?
An IP rating describes protection against solid objects (first digit) and water (second digit). Indoors the requirement is usually about people rather than the elements: IP21 keeps fingers and small objects away from live parts and stops dripping water, which is enough for a plant room or a switchroom accessible to trained staff.
Outdoors the picture reverses. The tank of an oil-immersed transformer is sealed by design, so the rating applies mainly to connection boxes and cabinets — IP65 is common for cable boxes on solar and storage sites, where wind-driven rain and dust are constant. Specifying more than you need is not free: a higher rating restricts ventilation, which pushes you towards larger radiators or forced cooling.
Which enclosure fits the room?
Enclosure choice follows from who can reach the transformer and how the space is ventilated. Each step up in protection costs cooling capacity, so take only the step the location actually demands.
- Open cast resin unit in a locked, restricted-access room — lowest cost and best cooling, but only where access is genuinely controlled.
- Factory-fitted IP21 enclosure with ventilation louvres — touch-safe in accessible spaces while preserving natural air cooling.
- Higher-rated cabinet with forced ventilation or cooling — for dusty, humid or semi-public areas; adds auxiliary power, filters and a maintenance task.
Outdoors: corrosion, containment and noise
Outdoor sites add three constraints that indoor sites do not. Corrosion protection is graded by atmospheric category: coastal and industrial locations need a C4-H or better paint system, and the enquiry should say so rather than rely on a standard finish. Oil containment is mandatory wherever a leak could reach soil or surface water, and the bund is normally sized for the full oil volume plus a rainfall allowance.
Where the site sits near housing, sound power level becomes a planning condition. Lowering it means reducing the flux density, which means a larger core and a larger, more expensive transformer — so it is worth confirming the actual limit before assuming the strictest one. Ambient temperature range, altitude above 1,000 m, pollution and humidity all feed into the rating as well.
Can you get it in — and out again?
Access routes are the constraint discovered too late most often. Check door widths, floor loading, lift capacity and crane positions before the design is frozen, and check them for removal as well as delivery. A basement transformer that cannot be replaced without demolition is a thirty-year problem created in an afternoon.
As a rule of thumb: let fire load and access decide the insulation system first, then set the IP rating from what can reach the unit, and only then optimise cooling. When placement is genuinely marginal — a ventilated basement, an accessible outbuilding — compare the two options on installed cost, including the room, the containment and the ventilation. That comparison, rather than the transformer price alone, usually settles it.
