An open-delta bank can preserve limited three-phase service with only two transformer units, but its usable three-phase capacity is lower than the sum of the two individual nameplates and phase currents are not the same as in a closed-delta bank. It is usually a contingency or special arrangement, so load balance, motor starting, voltage regulation and protection should be checked for the actual bank configuration.
The two-unit capacity is sqrt(3) times one unit
For equal single-phase units in the ideal balanced open-delta arrangement, bank capacity is sqrt(3) times the rating of one unit. Two 50 kVA units therefore provide about 86.6 kVA of three-phase bank capacity, not 100 kVA. Compared with the original closed-delta bank of three such units, 86.6/150 = 57.7% remains. Equivalently, open-delta bank capacity is 86.6% of the sum of the two surviving nameplates. These relationships assume the intended balanced connection and do not authorize an unbalanced or motor-start duty.
Check each unit under the actual power factor
The two transformers do not simply split the total three-phase kW into equal halves under every load power factor. Their individual apparent-power and reactive-power duties follow the open-delta phasor relationships. A bank that passes an aggregate kVA check can still encounter unacceptable voltage regulation or one-unit thermal duty with unbalanced loads. Retain the single-phase load allocation and motor operating cases. Do not reuse the closed-delta current distribution or protection model after removing one unit.
Define whether the arrangement is temporary or permanent
For contingency service, identify the loads retained, allowable duration and the restoration plan in the approved operating documentation. A permanent open-delta design needs its own equipment ratings, protection coordination and voltage-quality assessment rather than borrowing an emergency calculation. Starting a large motor can be more restrictive than the steady balanced load. Have the manufacturer and system designer approve the actual bank configuration and duty; this capacity calculation is not a field reconnection procedure or permission to operate an impaired bank.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEEE C57.105-2019 / Cor 1-2023 — Three-phase transformer connections.