Breaker ratings, FL AMPS vs Trip AMPS

[The Brockerts]

I've got a 15 AMP Breaker. Full Load (FL) is 15A as expected. The Trip AMPS is 20.25. So this breaker is not going to trip until it hits 20+ amp? My 30 amp breaker has a trip amp at 40. That's 30% over the ratings?

So if I trip a breaker then I'm more that just over the rating. What gives?

The Brockerts

 

[twistedtree]

Is this something you are testing to get those numbers?


Breaker trip points are actually pretty complicated with the trip happening after some combination of current and time. This is specified as a curve plotting the current level and time before the breaker will trip. The more over current the load is, the quicker it will trip.


Is there a particular problem you are trying to solve, or just curious about it?

 

[The Brockerts]

Just curious about it, I was making sure everything was to code, i.e. 10ga wire on 30 amp breakers, connected to device rated at 30amp. After just catching a retrofit that was fire hazard I'm checking each thing I do and just noticed those numbers.

Thanks for the explanation

The Brockerts

 

[FF]

Many AC motors will pull 2 or 3 times the running amps to start.

Its called a locker rotor start. LRS

The breakers are set to allow this short burst of high amp draw .

 

[diver dave]

look here for a typical marine rated trip curve:
Note the grey area where there is uncertainty. Also, note that wires also have a "heating curve" that is not instantaneous.
It is also possible to specify a faster breaker, for electronic device protection, for instance; vs an airconditioner compressor load, pretty much two extreme examples of starting current loads.

ps: if you are seriously designing a power system, with several breakers from source to load, consider "overload coordination". This demands that the breaker or fuse nearest the load should be the one to trip during that load's overcurrent event, and not one that is upstream. If that is not designed correctly, you will have upstream breakers trip, with resulting wider effects, usually unwanted.

https://www.bluesea.com/products/7254/C-Series_White_Toggle_Circuit_Breaker_-_Double_Pole_60_Amp