Define the operating state before comparing transformers
Two transformers feeding a split switchboard are not necessarily a parallel installation. A normally open bus coupler may transfer load only after one incomer opens. A closed-transition transfer or a continuously closed coupler creates a different condition. Describe permitted states and transition sequences on the single-line diagram before asking a supplier to approve compatibility.
For an addition to an existing installation, obtain the original transformer’s nameplate and actual test report. A catalogue impedance or an assumed tap position does not describe the installed unit precisely enough. Include the existing transformer’s condition and any known rating restrictions in the capacity review.
Compare electrical characteristics on a common basis
The Electrical Installation Guide identifies voltage ratio, winding connection, short-circuit impedance and LV connection impedance as relevant parallel-operation conditions. Equal kVA alone cannot establish compatibility.
| Review item | Evidence to exchange | Question for the design review |
|---|---|---|
| Voltage and taps | Ratio results and intended operating tap | Will the no-load secondary voltages agree in the permitted states? |
| Phase relationship | Vector group, terminal markings and phase sequence | Are the secondary voltages compatible at the coupling point? |
| Impedance | Measured percentage impedance and its rating/reference conditions | How will each unit share the expected load? |
| LV links | Cable or busway lengths, sizes and layout | Does connection impedance materially alter sharing? |
| Fault level | Source data and all permitted incomer/coupler states | Can the switchboard and protective devices withstand the combined contribution? |
Do not compare percentage impedances without confirming the base rating and reference conditions. Where ratings or characteristics differ, obtain an explicit compatibility calculation rather than using a general tolerance copied from another project.
A capacity example that exposes the purchasing risk
Suppose a site proposes adding a new unit beside an existing transformer with the same kVA. Its business case assumes the available capacity simply doubles. If the units do not share current as expected, one can reach its permitted loading before the pair reaches that assumed total. The upgrade then fails its production-capacity objective despite having purchased the intended nameplate total.
Ask the system designer to return a load-sharing table for normal demand, future peak demand and one-unit-out operation. The table should show the contribution and permitted loading of each transformer, the effect of the approved tap settings, and any required load shedding. This is more useful than a sentence saying that paralleling is possible.
Review the switchboard as part of the same order
Higher available fault current affects more than the transformer feeder. Review the busbars, coupler, outgoing protective devices and their coordination under every allowed operating state. A normally open design should have a clearly specified interlocking philosophy so that an unintended parallel state is not treated as an acceptable shortcut.
Keep operational capacity and redundancy separate. Two transformers that together carry the plant may not provide full-load service with one unit unavailable. If continuity depends on shedding nonessential loads, the shedding schedule belongs in the approved operating philosophy.
Information to send for a compatibility proposal
Submit both transformer data sheets, existing test results, a marked single-line diagram, LV connection details, source fault levels, demand schedule and required operating states. Identify who approves protection settings and who witnesses the final compatibility checks.
Start with the Jinxing oil-immersed transformer range when that equipment family fits the project. Request a written compatibility assessment for the proposed pair and its network; a standard product listing is not approval to connect two units in parallel.