The first (and most common) misconception is that a breaker trips when its nameplate rating is exceeded.
One fire text has stated (incorrectly) that a circuit breaker will trip in several minutes with a small increase in current over its rating[1].
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—far different from what the cited text implies.
These two trip points (135 percent and 200 percent) are defined in NEMA Standard AB-1, MCCBs and Molded Case Switches[2].
TABLE 1 lists the 200 percent allowable trip times for different size (amperage) circuit breakers.
MCCBs have characteristic ‘curves’ published by their respective manufacturers.
A sample of such a curve appears in FIGURE 1.
Knowing the amount of current flow, as a multiple of the breaker rating, allows one to determine the minimum and maximum trip times.
Usually, the allowed times (per the manufacturer’s curves) are shorter than what the NEMA spec allows.
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.
One fire text has stated (incorrectly) that a circuit breaker will trip in several minutes with a small increase in current over its rating[1].
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—far different from what the cited text implies.
These two trip points (135 percent and 200 percent) are defined in NEMA Standard AB-1, MCCBs and Molded Case Switches[2].
TABLE 1 lists the 200 percent allowable trip times for different size (amperage) circuit breakers.
MCCBs have characteristic ‘curves’ published by their respective manufacturers.
A sample of such a curve appears in FIGURE 1.
Knowing the amount of current flow, as a multiple of the breaker rating, allows one to determine the minimum and maximum trip times.
Usually, the allowed times (per the manufacturer’s curves) are shorter than what the NEMA spec allows.
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.