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My house originally had a single main panel, 150A. Some years ago, I decided to split the basement apartment and the main unit. The original main panel is in the basement but I wanted the main unit to have its own panel so I put a 100A breaker in the basement panel to feed the main unit subpanel.

A strange thing happened to me this morning. I woke up and and the power was out in the main unit of my house, where we live. I first thought the whole block lost power, as a result of some construction work nearby. However, that was not the case.

What turned out to be the case is that the 100A basement breaker had been tripped overnight. I know that breakers usually trip when there is excessive demand to draw amperage. I find this to be an extremely unlikely explanation because of the time of the day. Even if the AC was on (20A blower + 30A outside condenser), which I doubt it was because it was something like 70 degrees outside, it wouldn't come near to 100A. Realistically, what was likely drawing current at the time was the fridge, bedroom ceiling fan, and various super low amperage devices, like the modem etc. This is the first time it's happened in probably 5-6 years.

Could it have been that some external surge coming from the street bumped the breaker? I find it odd because it first went through the basement main panel, the rest of whose breakers went uninterrupted.

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The 100 amp breaker could be faulty.

The wiring between the two panels could be faulty. A break in the wire could cause resistance and draw enough current to trip the breaker. Or a break in the insulation could allow a short.

If a breaker in a subpanel is faulty, and it fails to trip when the circuit it protects draws too much current, then the upstream breaker will trip. In your case, the upstream breaker is the 100 amp breaker in the main panel.

There is also a small possibility that you have the wrong 100 amp breaker for you application. The breaker for your AC might have a trip curve that allows for high current draw briefly, but the 100 amp breaker trip curve does not. Please post the model numbers of those breakers.

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  • This is the first time it's happened in probably 5-6 years.
    – amphibient
    Commented Sep 12, 2018 at 15:37
  • All of the above still applies, even after 5-6 years.
    – longneck
    Commented Sep 12, 2018 at 16:40
  • I understand that, what I am saying it has never happened before.
    – amphibient
    Commented Sep 12, 2018 at 17:27
  • I would be looking for something that shorted out in the evening, things like dryer elements when they fault and go to ground can cause a main to trip before the smaller branch circuit especially if different brands of breakers are used we deal with this in industrial settings where a 50 amp equipment breaker did not trip or the 150 feeder breaker but did a 800 amp breaker. If different brands are used the system is usually not well coordinated but this is what I would be looking for.
    – Ed Beal
    Commented Sep 12, 2018 at 18:55
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This is in addendum to @longneck. I am only posting an answer so I can also post a picture.First a molded case breaker trips for two reasons:

One - Over current this could be a surge, a short to phase to ground or phase to phase or something conductive (like a lint ball) falls across a bus and causes a short then clears during the short (burns up). Since you didn't here a loud pop you may be correct in assuming it's a surge.

Two - Internal heat in the breaker itself. The design of the trip in a molded case breaker is a bi-metal bus where each metal expands at different rates. As more current passes through this bus, it heats up and bends then trips itself once it over expands. If there is too much heat being developed because of a loose fitting, then it will trip regardless of the current. Or sometimes if a breaker trips too often, the bi-metal bus gets week and starts to trip and losses its tripping tolerance values (trips at a lower amperage than rated).

I suspect that there might be a loose fitting causing the breaker to overheat. So check all fitting and make sure all terminations are tight and clean. If this is allowed to continue then it will become easy to find the problem and expensive to repair. enter image description here

Good luck and stay safe.

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