Industrial installations take power at a distribution voltage and transform it to the level used by motors, drives and control equipment. A single-line diagram summarises the transformers, protective devices, busbars and feeders, and it is the reference document for any work on the installation.
Transformer rated current
Problem. A three-phase transformer is rated 100 kVA with a 400 V secondary. What is its rated secondary line current?
Solution. I = 100 000 ÷ (√3 × 400) = 100 000 ÷ 692.8 = 144.3 A.
Three-phase distribution
Three-phase distribution delivers the same total power with less conductor material and a steadier power flow than several single-phase circuits. It also provides a natural supply for three-phase motors, which are the dominant industrial load.
- The kVA rating fixes the current for a given voltage.
- The single-line diagram identifies sources, protections, feeders and loads.
- Distribution voltage levels and earthing arrangements are site- and country-specific.
- Balance single-phase loads across the phases where the design allows it.
- S = √3 V_L I_L for a balanced three-phase system.
- Rated current follows from kVA and line voltage.
- The single-line diagram is the authority for the installation.
- Three-phase distribution suits large motors and balanced loads.
Worked example
A 250 kVA transformer with a 400 V secondary has a rated line current of 250 000 ÷ (√3 × 400) = 360.8 A.