These are used to distribute power to domestic or industrial premises. They may be single-phase or three-phase, pole-mounted or ground-mounted, and they have ratings ranging from 16 kVA up to 2500 kVA.

The windings and core are immersed in mineral oil, with natural cooling, and there are two windings per phase. The primary (high-voltage) winding has a highest voltage ranging from 3.6 kV to 36 kV; the secondary (low-voltage) winding voltage does not exceed 1.1 kV. The high-voltage winding is usually provided with off-circuit tappings of ± 2.5 per cent, or + 2 × 2.5 per cent, – 3 × 2.5 per cent.

Losses are assigned from lists, for instance from BS 7281-1, or by using a loss–capitalization formula.We use amorphous alloy and silicon steel sheet iron core to reduce loss.