@Aaron You have to look at the Characteristic Tripping Curve for the breaker. You'll want to look at the Thermal Tripping Characteristics. This documentation should be available online from the manufacturer, if it's not supplied with the breaker.

For example: Schneider Electric QO molded case miniature 20 ampere circuit breakers will trip after ~8 - 35 seconds with twice the rated current.

The higher the current, the faster they should trip. If the current is high enough, the magnetic protection will kick in, and the breaker will trip in less than a cycle (1/60th of a second [0.01667 seconds]).

static.schneider-electric.us/docs/Circuit%20Protection/…

wow

I looked at that curve for the cutler hammer BR breakers I have

and 200% load is 10 - 90 seconds

that seems like quite a long time compared to what I though

10,000 times rating? So like 300,000 amps?

I think it would be on fire

so they have those clamp ammeters that allow you to measure the amperage of a circuit in use

How would they work with a 240V cord?

@Tester101 so those mechanical interlock kits we were discussing a while ago

Apparently the panelboard manufacturers also make them:

eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/…