Transformer selection for apartment, residence, industry

The transformer for any building is chosen on its maximum demand, found from connected load reduced by utilization/demand factors and diversity (simultaneity) factors, plus a margin for growth, then rounded up to a standard rating.

Apartment building

• Find the load per flat (lights, sockets, kitchen, water heater, AC) e.g. 6–10 kVA.
• Apply a simultaneity factor that falls with the number of flats (e.g. 1.0 for 2–4 flats, about 0.78 for 5–9, 0.53 for 15–19, 0.4 for 50 or more flats).
• Add common services: lifts, water pumps, corridor lighting, fire pumps (with their own Ku and Ks).
• Add 20–25 % margin; usually one transformer (or two for reliability) with standby generator for lifts and pumps.

Residential building (individual house / colony)

• Small loads, high diversity; demand per house ≈ 2–5 kVA after diversity.
• Colony transformer = number of houses × after-diversity demand + street lighting; usually pole-mounted 50–250 kVA.

Industry

• Mostly motor loads: kVA = kW/(η·pf), Ku ≈ 0.75, plus lighting and heating (Ku = 1).
• Apply Ks per SDB and MDB depending on number of circuits and shift pattern.
• Check starting of the largest motor (voltage dip) and harmonics from drives.
• Consider separate or duplicate transformers for critical processes, and future expansion.
• Choose next standard rating (e.g. 250, 315, 400, 500, 630 kVA) so normal loading is about 60–80 %.