Continuous current rating of a circuit breaker

[mainmanmerc]

What determines the continuous current rating of a breaker? My answer is size of the frame but I was told that is incorrect. Can someone explain?

[madMAX]

Frame size does not matter because you can have 1600AF/800AT for example. Think of the main components of the LVCB protection, which pieces are interchangeable?

[GavinK5]

Any one of these could be the answer > current transformers > trip unit > rating plug > protective settings

[cj65shelby]

When we refer to the rating as continuous, Is it 3 hours or more, just like a motor?

[GavinK5]

When we refer to the rating as continuous, Is it 3 hours or more, just like a motor?

Yes, the 3 hour standard comes from NEC. Here is some related info:

continuous current rating — (handle rating) (ampere rating) The designated RMS alternating or direct current in amperes which a device or assembly will carry continuously in free air without tripping or exceeding temperature limits.

The 1996 NEC recognizes that overcurrent protective devices will be affected by heat in the system. As such, it defines the concept of continuous loads and the 80% rule to try and offset the effects of heat in the system when sizing a CB.

Continuous loads. To better understand the sizing aspects of a CB, you must first clearly understand the concept of continuous loads. In Art. 100, the NEC defines a continuous load as "a load where the maximum current is expected to continue for three hours or more." It's critical for you to understand that this is a load at its maximum current uninterrupted for at least three hours. Office lighting typically meets this qualification.

http://testguy.net/content/150-Circu...current_rating
http://ecmweb.com/basics/sizing-circuit-breaker