Sizing Circuit breaker, and I believe my calculation is wrong. Can anyone help me out

Question

I am looking at a 3Amp rated circuit breaker, and the datasheet given below of the derating factor, and trip time in second. The average office temperature is 71F.

Cable Current rating = Rated current of the circuit breaker * derating factor at 73.4 F

                  = 3 * 1 = 3Amp 

So, for circuit breaker, does that mean that it can handle upto 3amp or do I need to do subtract it (original rating - the derating = 3 - 3 = 0)

I don't know why but I think I need to subtract the derating to the current rating but I believe that's wrong.

Answer 1

You don't need to subtract anything.

The derating factor is a simple scalar which you multiply the breaker rating by to compensate for temperature, as described in the image above the table.

So if you would typically want it to trip at nominally 3A, but you are running at say 60*C, then on the graph you would read off 1.24 times rated current rather than 1 times rated current to get the time to trip.

Alternatively, a breaker sized at 3A x 1.24 = 3.72A running at 60*C would give the same trip time as a 3A breaker at room temperature.

Alternatively, a breaker sized at 3A running at 60*C would be equivalent to a breaker with a rating of 3 / 1.24 = 2.4A at room temperature.