Frame Size and Trippin G Current

Modern breakers have adjustable or interchangeable trip units, to allow the trip settings to be adjusted to meet the required protective levels.

A "100AT" designates that the circuit breaker should be configured with a 100 Amp trip unit, or perhaps a trip unit that can be set at 100 Amps.

Depending on the details of the breakers you are using, a 100 Amp Trip breaker will require a frame size of 100 Amps minimum. So, you might see 100AF/100AT, or 250AF/100AT, for example.

The frame size is the designation of the maximum continuous current the breaker can carry, and also an indication of its physical size. A 100 AF breaker will be smaller than a 400 AF breaker.

Frame sizes are determined by what the manufacturer offers in their breaker catalog. Typical values for molded case circuit breaker (MCCB) frame sizes are 100, 250, 400, 600, and 800.

The search function would have given you access to the dozens of times this has been explained in this forum alone... <sigh>

Frame size is a mechanical spec for a range of breaker sizes, sometimes overlapping. Trip rating is an electrical specification regarding when the breaker will trip, usually on thermal current.

So for example, let's say I need a 125A breaker, which is a short way of saying I want a 125A thermal trip; the thermal current sensor in the breaker will trip at current in any pole that exceeds 125A, the higher the excess the faster it trips (i²t function).

But lets say I have a future need for this to possibly increase to 200A some day. If I bought the 125A breaker as a 125AF / 125AT, when I wanted to change to a 200A breaker, the new breaker will be physically larger than the 125A frame breaker and may not fit. But if I order that 125AT as a 250AF / 125AT, then I'm OK for space in the future.

It's as simple as that.