Electrical Myth

Denny, wrote:
The 80% is related to the continuous load on a breaker, if you load a breaker continuously over 80% of its rated value it will trip from heat not overcurrent.

From the document posted wrote:


I agree that the circuits should be sized so that 80% is the maximum, but the breaker should not trip even if run at near 100% of it's rating, as long as the ambient temps don't go about 40 degrees C.

The latter part of the quote above relates to specific load situations and guidelines. But the breaker itself, doesn't trip if run at 80% capacity for 3 hours if it remains within it's operating temperature. It is possible some loads, such as motors, which might frequently draw higher that nominal currents, could cause the temps to rise, and that would cause the breaker to trip. But that's indirectly due to the over-current and directly due to the resulting rise in temperature. That I believe is why the NEC states that they type of load needs to be considered.

Here is a link to the Trip Time Graph for the same series of breakers (I think). Notice that there is no tripping until you reach the 100% figure of the breaker rating. There is NO TRIPPING indicated in the graph at anything less than a multiple of 1 times the breaker rating.

The upper left corner of the graph does suggest that the breaker might trip after 10,000 seconds (2.7 hours) to 300 seconds (5 minutes) at it's rated load. But that's at the lower end of the tolerance range.

And yes, we're probably splitting hairs here, but the breaker by itself isn't the issue. It's the breaker mounted in an enclosure which doesn't allow it to dissipate heat effectively. Thus it's temperature rises, and the graphs by the OP predict the derating of the breaker, and now the "nominal current" may become an overload, and the trip time graph I posted above has to be revisited with the derated breaker rating.

~Rick