The best-sized transformer will still trip if its room cannot breathe. Ventilation is not a civil-works detail: it is part of the electrical sizing. Here is how it works, and how to get it right.
All the energy lost in a transformer — the no-load losses (magnetic core) and the load losses (windings) — ends up as heat. On a distribution unit, that commonly means several kilowatts dissipated continuously inside the room. This heat must leave as fast as it arrives, otherwise the room temperature rises, the air cooling the unit gets hotter and hotter, and the margin disappears.
The classic, effective scheme relies on the stack effect: hot air rises. So you provide a cool-air inlet low down (at floor level, on the opposite or adjacent side) and a hot-air outlet high up. The height difference between the two creates an airflow that sweeps the unit without a fan. The greater the height difference and the louvre area, the higher the flow.
What makes natural ventilation fail, in practice:
Forced ventilation — fans controlled by a thermostat — becomes necessary when the natural draught is not enough: a cramped or buried room, high dissipated power, high outdoor ambient, or an architectural constraint that rules out large louvres. On the transformer itself, the equivalent is moving from ONAN (natural) to ONAF (forced) cooling: fans on the radiators raise the allowable power without changing the unit. This option is specified at order time.
That is the central message: you do not size a transformer "then" its room. The two go together. The same 1000 kVA unit will not offer the same usable capacity in a well-ventilated room and in an overheated closet. Three levers address this: improve the room (louvres, draught, fans), take power headroom at purchase, or choose a cooling mode to suit (ONAF, or dry-type for some environments). The right trade-off depends on the site.
For a dry-type transformer as for an oil-immersed one, we build the room's real conditions into the sizing, rather than shipping a "theoretical" nameplate that trips at the first summer. If the room is a known weak point, that is exactly the moment to fix it, especially during a replacement.
This article presents general installation principles and is not a calculation note. The exact sizing of openings and airflows is for your design office, according to the power, the type of unit and the applicable regulations (including NF C 13-100 / NF C 13-200 for substations in France).
A transformer dissipates its losses as heat. If the room's hot air cannot escape, the ambient temperature around the unit rises, its cooling margin shrinks, and it may trip or age prematurely. Ventilation determines the power actually available.
Natural ventilation (a low louvre for cool air in, a high louvre for hot air out) is often enough for a correctly sized substation. Forced ventilation is needed when the room is cramped, poorly exposed, or when dissipated power is high.
Yes. If the ambient around the unit stays above the reference ambient, you must either improve ventilation, specify a transformer with headroom, or move to forced cooling (ONAF). The room is part of the sizing, not just the unit.
Ventilation, sizing and summer overheating are three faces of the same subject: substation thermics.
Describe the substation and its conditions: we choose power and cooling accordingly, not on a theoretical nameplate.
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