Circuit Breaker Keeps Tripping?

I recently moved into a townhouse and have been having some trouble with the breakers. When I try to use my toaster the breaker for that outlet trips part of the way through the cycle. Just to be transparent I have an electric kettle on the same outlet, but I don't use it and the toaster at the same time.

The breaker is 20A so it should be able to handle just the toaster (which is rated at 1800W max). I'd understand if it tripped as soon as I turned it on, but it does it at seemingly arbitrary points. My first instinct was that the outlet was bad, but it and the wiring look good inside. I also thought about replacing the breaker, but for some reason there is no main installed in the box. The place where it should be is empty and covered with a plate.