I have not had it trip once in many uses, and have grabbed the wires after all the slides are in, and they are not hot, so I know the system is working, and it will not be a fire issue.
Something to think about: in normal operation, the pump motor pulls less than 50 amps, so the breaker has no need to trip, and the wires will not be hot. So feeling wires when things are working right doesn't tell you anything. BUT, if there's a problem causing the pump to draw more than 50 amps, that's when the higher current may overheat the wires and create a fire issue.
The unknown part is how long before you overheat the wires.
Laurent,
It doesn't work like that at all. Here is something for you to puzzle over. Breakers in parallel:
40A + 40A = 80A
40A + 50A = 80A
40A + 60A = 80A
40A + 70A = 80A
40A + 80A = 80A
Maximum amperage is 2 times the lowest breaker capacity of the paired breakers.
In a parallel circuit, total current(It) = sum of branch currents. It = I1 + I2....
OK. In a parallel circuit, total current(It) = sum of branch currents. It = I1 + I2. Since the breakers have the same resistance, the current in each branch equal each other. Therefore, I1 = I2 = I Substitute in the summation equation, It = I + I = 2xI. Since the 40-amp breaker will be the limiting breaker, it will trip first. That means that the total current is 80 amps (It = 2 x I = 2 x 40 = 80), 40 amps in each branch. Total current will then flow through the remaining breaker. Since the remaining breaker is rated for less than the total current, it will also trip. Therefore, the total current is all cases is 80 amps or 2x40. QED.
Willym, true, these thermal trip breakers have different resistances. Lacking knowledge of the resistances, I stated the assumption that the breakers have the same resistance. The resistance will change with temperature, so as the higher current carrying breaker heats up its resistance increases. How close will the higher capacity resistance come to the lower capacity breaker resistance as temperature rises? Don't know. Will they more equally share current? Maybe. If the higher capacity breaker carries more current will it trip before the lower capacity breaker? I just chose the simpler example. Time of parctical use is short, so probably close enough. When the lower capacity breaker trips, the higher capacity breaker will not be far behind and trip.