Open-Delta (V-V) Transformer Bank Capacity

Two single-phase transformers serving three-phase, common where a third unit failed or a light load does not justify a full bank. Two units do NOT give twice one unit: available three-phase kVA = sqrt(3) x one unit's rating (1.732, not 2), because each carries the load / sqrt(3) and the phase angle caps the combined output at 86.6% of the two installed. Two 25 kVA units serve 1.732 x 25 = 43.3 kVA, each at 21.65 kVA (86.6% of rating). That is only 57.7% of the closed-delta three-unit bank (43.3 vs 75 kVA), so removing one of three drops it to 57.7%, not 66.7%. Reports each unit's loading and flags an overload. A sizing screen; the nameplate kVA and impedance, the load balance and power factor, and the utility govern.

available_3ph_kva = sqrt(3) x transformer_kva_each; per_transformer_kva = required_load_kva / sqrt(3); utilization_pct = 100 x per_transformer_kva / transformer_kva_each. Open delta = 86.6% of the two installed, 57.7% of the closed-delta three-unit bank.

Open-delta (V-V) transformer connection capacity (standard power-system relation), by name; the transformer nameplate kVA and impedance, the load balance and power factor, and the utility govern the real bank.

The open-delta sqrt(3) capacity relation is public power-system theory; the transformer rating and the required load are the design values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: transformer_kva_each, required_load_kva, available_3ph_kva, per_transformer_kva, utilization_pct

• Open-delta capacity available 3-phase kVA = sqrt(3) x one unit; each unit carries load / sqrt(3); bank = 86.6% of two installedpower-system V-V connection theory
• Vs closed delta open delta = 57.7% of the closed-delta three-unit bank; removing one of three drops capacity to 57.7%, not 66.7%power-system V-V connection theory