How is the current rating of the primary main circuit breaker in an MPZ determined?

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Issue:
MPZ Mini Power Zone Transformers have Primary and Secondary Main Breakers of specific Amperage

Product Line:
LV Transformers

Environment:
Applies to MPZ (Mini Power Zone) Unit Substation Transformers by SquareD/Schneider Electric

Cause:
Primary and Secondary Breakers are sized according to guidelines set up in National Electrical Code (NEC) Section 450-3(b)

Resolution:
The amperage ratings of Mini-Power Zone Transformer primary breakers were chosen with the goal of no nuisance tripping during the inrush of primary current associated with transformer energization. Because of the relatively high inrush associated with smaller size resin filled transformer products, breaker sizes were adjusted higher than the usual 125% of full load level associated with larger transformers, in most cases. The National Electrical Code breaker sizing rules allow up to 250% of transformer full load if a secondary main is sized at 125% of secondary full load. Feeder panels ahead of the Mini-Power Zone Transformer must be sized at least as large as the primary breaker to avoid nuisance tripping as well. Replacement of factory supplied primary breakers with lower amperage rated breakers should be done with the understanding that nuisance tripping on inrush will sometimes occur on energization, during voltage sags, or during momentary primary power interruptions.