Power density (kW or kVA per rack) has become the real sizing factor. Current ballparks:
- 3–5 kW/rack: classic enterprise IT.
- 6–10 kW/rack: dense virtualization, storage.
- 15 kW: practical ceiling of air cooling in a cold corridor.
- 30–100 kW/rack: high-performance computing and GPU/AI, which require liquid cooling (direct-to-chip or immersion).
Consequence when choosing a rack: it is almost never the height in U that blocks you, but the subscribed kVA and the room’s heat-dissipation capacity. A “42 U” rack sold at 3 kVA will be electrically full well before its 20th server. Always ask the maximum allowed density per rack and the available cooling method.