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Old 07-29-2014, 08:17 AM   #29
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George Schweikle's Avatar
Join Date: Apr 2004
Posts: 1,791
Good stuff that I used in my pre-retirement career (Underwriters Lab specifies how the breaker must perform, and the National Electric Code specifies how to install/use). You described the "hold-in, trip-out" specification. However, don't forget that residential breakers are intermittent duty, and only rated for continuous duty at 80% of the handle rating. Also, the trip curve test is performed at ambient room temperature, which is almost always exceeded inside the panel therefore occasionally resulting in nuisance tripping.

Originally Posted by joshuajim View Post
...The first (and most common) misconception is that a breaker trips when its nameplate rating is exceeded. A circuit breaker will trip in several minutes with a small increase in current over its rating. Actually, a 20 amp breaker must trip at a sustained current of 27 amperes (135 percent) at less than one hour, and at 40 amperes (200 percent of wire rating) in less than 120 seconds. These two trip points (135 percent and 200 percent) are defined in NEMA Standard AB-1, MCCBs and Molded Case Switches.

A circuit breaker is designed to open (trip) before the energy passing through it creates enough heat in the branch circuit wiring to cause damage to the wiring.

While breakers are labeled in amps as a convenient measurement, they are tripped by watts.

George Schweikle Lexington, KY
1999 Safari TREK 2830, FMCA 190830, Safari International chapter
1995 Safari TREK 2630, 1983 Winnebago Chieftain, 1976 Midas Mini
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