Which breaker should pop first


Old 02-20-14, 11:41 PM
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Which breaker should pop first

I have a Square D QO main 200amp with QO 100amp subpanel (with 100amp breaker at the sub and at the main) and a 20amp 240v off the subpanel. A short occurred on the 240v outlet and the main 200amp breaker popped. Why did the 20amp and /or or 100 breakers (either in the Subpanel and / or off the main panel) did not pop first?
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Old 02-21-14, 07:40 AM
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I cannot explain it, but it does occasionally happen that way. If it makes you feel any better, I have seen a flourescent light fixture with a pinched wire from original installation on a 20 amp 277 volt circuit take out a 2,000 amp main.
Old 02-21-14, 06:23 PM
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Some breakers will sustain an overload for a longer time before tripping. This is sometimes intentional; motorized appliances, tools, etc. draws more current when starting up or cycling on. In some cases a startup current greater than the circuit and breaker rating is allowed, and for the few seconds it takes for the equipment to get started, the heat generated in the wires is not enough to be hazardous..

If you have a dead short, hundreds or even thousands of amperes could flow for an instant or even a few seconds before a breaker trips. Most breakers have a temperature sensitive element that causes the trip to occur.

In your case it is possible that the main 200 amp breaker was faster acting.

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