Same 75 kVA, Very Different Current: What Changes Between Single-Phase and Three-Phase?

“We still need 75 kVA—why does anything else change?”

A buyer says, “The old system was single-phase. The new project may be three-phase, but the transformer is still 75 kVA.”

It is tempting to think the current, cables, switching, and terminations should remain roughly the same because the kVA label did not change. They do not.

Same kVA does not mean same current

For a single-phase screen, apparent power is approximately S = V × I. For a balanced three-phase screen, S = √3 × V × I. Current therefore depends on phase configuration and voltage as well as kVA.

That is why “75 kVA transformer” is not a complete electrical specification.

Why phase configuration belongs in the RFQ

• rated current
• cable and bus sizing
• breaker and protection selection
• low-voltage terminals and connection
• load distribution
• neutral requirements
• compatibility with downstream equipment

One important misconception: a transformer is not automatically a phase converter

Another question often appears: “If I only have single-phase supply, can I buy a three-phase transformer and get three-phase power?” A conventional transformer can change voltage and provide isolation according to its winding arrangement; it does not perform the same job as a phase converter, VFD, or rotary phase converter.

If the real problem is supplying a three-phase load from a single-phase source, the system topology must be solved first.

What TransformerGrid confirms before pricing

• single-phase or three-phase
• primary voltage
• secondary voltage
• frequency
• connection and grounding requirements
• downstream load type

Send the diagram, not only the kVA

If you know the capacity but are unsure about phase, voltage, current, or downstream compatibility, send the single-line diagram, equipment nameplates, or voltage information.

One kVA number can describe very different electrical systems.