Select Voltage Class — Transformer Sizing Guide

Voltage class selection determines the transformer's primary and secondary winding ratings and directly impacts insulation design, physical size, and cost.

Primary Voltage (HV Side)

The primary voltage is determined by the utility's supply network:

• 11 kV — Most common distribution voltage in the Middle East and India
• 22 kV — Used by some Gulf utilities (e.g., SEWA, FEWA)
• 33 kV — Common sub-transmission voltage globally
• 66 kV — Used in regional transmission networks
• 132 kV — Primary transmission voltage in many countries
• 220–400 kV — Extra-high voltage transmission (power transformers only)

Secondary Voltage (LV Side)

• 400V (3-phase, 50 Hz) — Standard LV distribution in Middle East, Europe, Asia
• 480V (3-phase, 60 Hz) — Standard in North America
• 11 kV or 33 kV — For MV distribution or step-up applications

Tap Changer Considerations

• OCTC (Off-Circuit Tap Changer): Adjusts ratio when transformer is de-energised. Standard on distribution transformers. Typical range: ±2.5% or ±5% in 2 or 4 steps.
• OLTC (On-Load Tap Changer): Adjusts ratio while transformer is energised and under load. Required for power transformers and large distribution units where voltage regulation is critical. Typical range: ±10% in 16–32 steps.

Insulation Coordination

Higher voltage classes require higher Basic Insulation Level (BIL):

• 11 kV → BIL 75 kV
• 33 kV → BIL 170 kV
• 132 kV → BIL 550 kV
• 400 kV → BIL 1425 kV

Higher BIL means larger clearances, more insulation material, and consequently larger and more expensive transformers. Correct voltage class selection optimises cost without compromising safety.