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If I have an 80% rated 100 A circuit breaker does that mean it will trip at 80 A?

An 80% overcurrent device, either circuit breaker of fused switch, will carry 80% of its rating continuously.  Loading above that level will eventually result in the overcurrent device’s activation if maintained.  The higher the loading level the faster the time for activation.  100% rated devices whether fused switches or circuit breakers, will carry 100% of their load continuously.  Note that 100% rated devices must be labeled as such and may have specific construction /installation requirements, such as the use of larger lug pads or the requirement to use 90 degree C rated conductors sized per the 75 degree C table.
 
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