Transformers operating in parallel must have the same voltage ratio, vector group, phase sequence, and similar impedance and tap settings. kVA ratings should be reasonably matched to share load proportionally. Differences in impedance or ratio cause circulating currents and unequal load sharing, leading to overheating or protection misoperation. Proper phase checks, synchronization, and settings coordination are required before closing parallel breakers. Utilities follow detailed procedures to verify all conditions prior to enabling parallel operation on busbars.