How to read circuit breaker temperature ratings

The simple answer is that the breaker is tripping because two much current is flowing through it.

Here's a list of some possible causes for over-current:

• Faulty wiring may be partly short-circuiting
• Faulty appliance is drawing too much current
• Too many devices on that circuit, in total drawing too much power

I would start by proving to yourself that there is definitely nothing else on that circuit other than the opener and the lights. Turn the breaker off and then go around and see if anything else is off. Outlets in the garage? Chest freezer? Sump pump?

Second, see if you notice a pattern of when the breaker trips. If it trips when you flip the light switch, then maybe the switch is faulty or maybe the lights are the extra current.

If you're positive there is nothing on the circuit, then you're looking at a defect. Finding it may or may not be in your skill set, but you could start checking the voltage at the opener with a multimeter. Then start opening outlet covers to see if there are any lose wires. Check for signs of arcing (which may leave black sooty marks around the outlet terminals.

FYI, the time it takes to trip a breaker depends in part on how much over-current there is. It may take several seconds or even a minute if you're only just barely over the circuit breaker rating.

Your question depends on very many variables.

1) Are the breakers magnetic trip or thermal? Which is which?

2) If you are using thermal breakers, which breaker is exposed to the highest ambient temperature?

3) What is the current profile of your high-current loads? Relatively constant or fluctuating?

In general, it is a crap shoot as to which breaker trips first when two identically-rated breakers are in series. The breaker with the smallest amount of internal thermal heat-sinking will usually trip first but there are no guarantees.

Your best bet is to either change the breaker that you have access to down to 100 Amps or increase the feed breaker to 150 Amps. I'd have to check, but I think that your 3/0 feeder cable is rated for at least 150 Amps. Check with a qualified electrician to make sure.