How to choose the right kVA rating for a distribution transformer

The rated power of a distribution transformer is the first number on every enquiry – and the one most often chosen by habit. A rating that is too small overheats and ages fast; a rating that is too large wastes money twice: once at purchase and then every hour of its life in no-load losses. This note walks through the method our engineers use when a customer sends a load list.

1. Start from apparent power, not from kW

Transformers are rated in kVA because they carry current regardless of the power factor. Convert every load to apparent power first: S = P / cos φ. A 400 kW motor load at cos φ = 0.85 is 470 kVA, not 400. Modern loads with inverters, LED lighting and IT equipment run close to unity, but motors, welders and old fluorescent lighting do not. If you only know kW, assume 0.85–0.9 for mixed industrial loads and 0.95 for commercial buildings.

2. Apply a diversity factor

Not everything runs at the same time. The sum of all nameplates is the installed power; the transformer only sees the maximum demand. Typical simultaneity factors used by European designers:

Type of installation	Diversity factor
Office / commercial building	0.6 – 0.8
Light industry, workshops	0.5 – 0.7
Process industry, continuous operation	0.8 – 1.0
Residential estate (per 50 dwellings)	0.25 – 0.4
Photovoltaic plant (inverter output)	1.0 (rated AC output)

3. Add margin for growth – but not too much

A transformer bought today will run for 30 years. A 15–25 % reserve for future load is reasonable; 100 % is not. Remember that a hermetically sealed oil-immersed transformer can carry short cyclic overloads (IEC 60076-7) and that a cast-resin unit with forced-air fans (AF) gains around 40 % of continuous capacity. Reserve is cheaper as an option than as iron.

4. Check the loss balance

Since July 2021 every new transformer in the EU must meet the Ecodesign Tier 2 limits of Regulation (EU) 2019/1783. Losses are therefore known before you buy. The question is which rating gives the lowest total cost over the service life. A transformer loaded at 40–60 % of its rating is usually close to its efficiency optimum; one that idles at 15 % load pays no-load losses for nothing.

5. Worked example

A logistics warehouse: 120 kW lighting and sockets (cos φ 0.95), 180 kW conveyors (0.85), 90 kW cooling (0.88), 22 × 11 kW EV chargers (1.0). Installed apparent power: 126 + 212 + 102 + 242 = 682 kVA. Diversity 0.7 → 477 kVA maximum demand. With 20 % reserve: 573 kVA. The right choice is a 630 kVA unit – for example a PNACHO-O 630 kVA at 21/0.42 kV with no-load losses of 540 W and load losses of 4 600 W.

Standard ratings in the PNACHO range

Oil-immersed PNACHO-O: 25, 40, 63, 100, 160, 250, 400, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA. Cast-resin PNACHO-D: 100 to 2500 kVA. Both series are available for 6.3, 10 and 21 kV networks and with 0.42 kV or 0.8 kV (photovoltaic) low-voltage windings. If you send us the load list, we return the recommended rating together with a quote within 24 hours.