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How should a M9 breaker be sized to prevent nuisance tripping?<br />

M9 breakers are designed to carry their rated current at an ambient of 25 deg. C. They need to be sized (rerated) for the actual ambient inside their enclosures per tables found in the Class 860 catalog. A little margin wouldn`t hurt if they have a history of nuisance tripping--like sizing for 125% of the load as required by the NEC for branch circuit protectors. Also, the curve (B C D etc.) needs to be chosen properly to prevent tripping on inrush.

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