Open-Delta Transformer Calculator
Enter the transformer kVA and the load to see open-delta capacity, loading and how the two units share kW.
Step-by-step
- Enter valid values to begin.
Two identical single-phase transformers assumed, with the same impedance and a balanced three-phase load. kVA loading per transformer for a balanced load is S/√3 and the real-power split follows P1 = (S/√3)cos(30°−φ), P2 = (S/√3)cos(30°+φ). Check the utility’s rules and the manufacturer’s ratings before using an open-delta bank for permanent service.
Open-Delta Transformer: Two Transformers Doing the Work of Three
Core Engineering Principles
A closed delta bank uses three transformers. Remove one and the other two can still deliver three-phase voltage, because the third line-to-line voltage is simply the vector sum of the other two. That’s the open-delta, or V-V, connection. It’s a common way to keep a plant running when one transformer in a bank fails, and a cheap way to serve a small three-phase load from a rural single-phase line with two pots instead of three.
The catch is capacity. Each remaining transformer must carry the full line current, not the phase current it used to carry in the closed delta. Two transformers of S kVA each deliver √3 × S, not 2 × S. A closed bank of three 50 kVA units serves 150 kVA, but two of them in open-delta serve only 86.6 kVA, which is 57.7% of the original. The two transformers are installed at 100 kVA total but are used at only 86.6% of that. And the load isn’t shared equally. At unity power factor one transformer carries 50% of the real power and the other 50%, but at 0.9 PF the split is closer to 64% and 36%, so one runs hotter.
Real power split: P1 = (S/√3) cos(30° − φ), P2 = (S/√3) cos(30° + φ)
NEC & Standard References
IEEE C57.105 is the guide for transformer connections in three-phase systems, with the open-delta and open-wye connections described in detail. NEC 450.7 allows a bank to be treated per its connection, and NEC 450.3 sets protection for each transformer individually, so each unit gets its own primary protection. NEC 230.2 and 215.2 are relevant when you feed a plant from a single-phase utility line. For 4-wire delta, NEC 110.15 requires the high-leg conductor, with 208 V to ground, to be marked orange, and 408.3(E) places that phase on the right side of the panel. Utilities may restrict open-delta service, so check with the supplier first.1. Capacity = 1.732 × 50 = 86.6 kVA (57.7% of a 150 kVA closed bank).
2. Load = 70 / 0.9 = 77.8 kVA; loading = 77.8 / 86.6 = 89.8%.
3. Each transformer carries S / √3 = 77.8 / 1.732 = 44.9 kVA (89.8% of 50).
4. Real power split: φ = 25.8°, P1 = 44.9 × cos(4.2°) = 44.8 kW, P2 = 44.9 × cos(55.8°) = 25.2 kW.
The bank is legal, but at 90% loading with the unbalanced split it’s time to order the third transformer.
- Voltage is not quite balanced. Impedance and the phase relationship cause a small unbalance. Motors on the bank need to be derated per the voltage-unbalance calculator.
- Don’t forget the third unit is not just a spare. Adding it back gives 173% of the capacity.
- Don’t mix sizes casually. Different kVA or impedance units share load in proportion to their impedance, and the smaller one overloads first.
- Neutral availability. A 4-wire open delta supplies single-phase 120 V lighting from only one transformer, the one with the center tap. That one carries both.
- Treat it as a temporary fix. Ferroresonance can occur with an open phase on an underground cable feed, so open and close the primary with the manufacturer’s procedure.