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Parallel operation of transformers is about how multiple transformers feeding the same system must be connected so they share the load safely. When transformers are wired in parallel, they will share load based on their impedances. The rule is to have matching voltage ratios and the same polarity (phase relationship) and to keep their impedances effectively similar. If the transformers don’t meet these conditions, one can end up carrying most of the load while the other carries less, leading to circulating currents, overheating, and uneven aging. NEC 450.7 sets the requirements so that, in parallel, transformers contribute to the overall system reliably and safely by ensuring proper voltage, phase alignment, and proportional load sharing.

The other topics mentioned—ground fault protection, overcurrent protection of motors, and harmonic distortion limits—are governed by different sections or articles and address different protection, coordination, or power-quality concerns, not the parallel operation rules for transformers.