Amp Frame / Amp Trip

[edavis2293]

Industrial one-line diagrams illustrate circuit breakers in terms of "Amp Frame / Amp Trip: AF/AT. Any one have a definition of what the terms mean?

Thanks Folks

[kingpb]

There are multiple meanings, but the most common is that the breaker can handle for example a maximum of 400A, but it has a built-in trip setting that allows you to adjust the long-time overcurrent protection down to say 320A, or within some other range. Some devices come with what they called plugs, that allowed the user to change the range by taking one out and replacing with another. Usually there is a limit to the lower setting before you have to change to a smaller frame.

[infinity]

It similar to a fused disconnect. The frame of the CB will allow a certain maximum size, the trip setting is changeable by inserting different fixed sized or adjustable trip devices.

[zog]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

[infinity]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?

[zog]

Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?

The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

[infinity]

The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

Sounds like semantics to me. The OP asked a simple question, the amp frame size pertains to the breaker frame size and the amp trip pertains to the trip setting. Why does it have to be more complicated than that?

[zog]

Sounds like semantics to me. The OP asked a simple question,

And I gave a simple correct answer, it is not semantics at all the CT size is not the same in any way shape or form as the trip setting.

the amp trip pertains to the trip setting.

It most certianly does not.

[jim dungar]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Only if the breaker has CTs.

A Square D LA36350 is a thermal magnetic trip breaker in a 400AF (this is the physical size of the breaker) with a 350A long time trip element.

I have always used:

AF = maximum long time current available in that physical package.
AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
AT = Long time trip setting of the breaker.

[brian john]

[QUOTE

AF = maximum long time current available in that physical package.
AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
AT = Long time trip setting of the breaker.[/QUOTE]

And then hopefully you receive a coordination study to adjust the trip settings
(LSIG).

L=Long Time
S=Short Time
I=Instantaneous
G=Ground Fault