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It's commonly accepted that the continuous load of a circuit should usually not be more than 80% of the rating of the lowest ampacity link in that circuit.

Do breakers ever 'enforce' this de-rating by being designed to intentionally trip at a load lower than the number printed on the handle?

For instance, if I put exactly 20 amps of electric heat on a 20 amp breaker, using conduit and all quality components, should I expect that to trip at some point, or not?

19 amps I would expect to trip never. 21 amps I would expect to trip 'eventually'. 10000 amps (dead short) should trip more or less instantly. Correct me if I'm wrong.

Do branch circuit breakers work the same as mains in this regard?

For that matter, do any breaker's trip curve begin before it's displayed rating? Are such breakers in common use? common trip curves

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  • The curves combine both functions of the breaker thermal and magnetic. Slight adjustments can shift the curves but these are sealed at the factory for the specific value on the breaker. You could say the curves are in use below the value as heat takes time to build. On the large frame industrial breakers I test the values are quite a bit different on the 15-20 amp. – Ed Beal May 17 '20 at 2:23
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First, I don't know about those charts. (actually I do know; I browsed them on Friday as part of other research; but you need to look at context and notice they are and NOT for breakers in your home.)

The only trip curve that matters to you is the one published in the catalog for the breakers that are in your own panel.

It's possible to get breakers with trip curves like that, but they don't fit your HOMeline panel lol, and they don't cost 5 dollars! They do, however, let you deep-dive into the arcane subject of breaker selective coordination, i.e. the fine art of reliably and reproducibly having the local breaker at the hydraulic press trip before the feed breaker back at the main panel 3 floors down.

For instance, if I put exactly 20 amps of electric heat on a 20 amp breaker, using conduit and all quality components, should I expect that to trip at some point, or not?

They don't guarantee it won't. In fact, since the thermal trip mechanism will be in play, it will matter greatly whether the Arizona sun is blasting on your outdoor panel vs whether it's buried in a snowdrift.

19 amps I would expect to trip never. 21 amps I would expect to trip 'eventually'. 10000 amps (dead short) should trip more or less instantly. Correct me if I'm wrong.

I wouldn't expect that kind of precision for $5.

Do branch circuit breakers work the same as mains in this regard?

My view is that, if anything, main breakers are more tolerant (to the vexation of anyone trying to use a main-breaker subpanel as a disconnect with hopes that the local breaker trips first)... However, they are cut from the same basic stock. In fact, when a "main breaker" is actually a backfed breaker, they are the same basic stock.

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It is next to impossible to mass produce standard non-electronic breakers which trip exactly with a load of 101 or 102%. The reason is the very reliable, but tolerance-affected bimetal unit which is heated by the current. Already the air temperature and location (Alaska, Death Valley, closed non-vented board, other heat producing equipment nearby etc.) may shift that trigger point. Luckily the temperature mimic matches the maximal load of the protected wires. The higher the temperature, the less current can be safely pushed through wires.

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  • A while back I ran across a pretty cool video on how circuit breakers work. A link follows. While I grimace at the techniques used (not nearly enough emphasis on safety), the video quality with the slo-mo is very good. Not sure it covers how all breakers work. This is just for entertainment value, your mileage may vary! interestingengineering.com/video/… – George Anderson May 17 '20 at 12:49
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    Ok. I'm greedy if i expect a 2% tolerance without paying thousands. What about 5%? 10? 50 percent? Surely your not saying i can expect nothing because i didn't become destitute from the purchase.... – Billy C. May 17 '20 at 16:12
  • Your diagrams show sharp edges of the relevant areas which are defined by current and time. But those are only average data. For the specific tolerances, the manufacturers' data should help. – xeeka May 17 '20 at 16:50

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