Please Clarify the concept of Amp Frame and Amp Trip. 5

[toro45]

Hello;

I am an electrical engineer with an MEP consulting firm. I have done tonnes of single line diagrams however, I am unclear as to what the AF and AT really are.

From what I understand, say I have a 100Amp frame, the size of the circuit breaker that can be used has to be less or equal to 100 Amps. So what I get from that is that the AF is just an enclosure for the actuall circuit breaker.

I would appreciate if you guys could shed some more light on this.

Thanks

 

[waross]

Hi toro54
Manufacturers don't make a seperate case or frame for every ampere rating.
Frame size gives the physical dimensions of the breaker case.
Trips are the actual amps rating of the breaker.
yours

 

[cuky2000]

Frame is the insulation housing also known as molded case typically made of composite resin fabricated with an injection molding process.
Trip unit could be fix or interchangeable. Molded case circuit breakers (MCCB) with the same frame size often are available in wide range of trip amp rating.

 

[jtkirb]

To expand-

You can have a 1600 amp frame breaker with different amp trip settings up to 1600.

The amp trip will not go over the amp frame rating. The amp trip is what you use to size the trip rating of the breaker to protect down stream devices and cables.

JTK

 

[jraef]

It is really only important when there is an overlap, which typically occurs at larger sizes. For instance, Most manufacturers who offer a 100A frame and a 250A frame, do not bother to offer a 100A trip in the 250A frame, so the distinction is essentially meaningless. If you want a 100A trip, you will end up with a 100A frame anyway. The problem now with smaller sizes is that manufacturers are no longer as universal about frame ranges as they once were. Now for instance, some offer 150A, 225A or 500A frames etc. In those smaller ratings then, it may be better to just indicate the rating of the breaker and forget the frame issue.

But many manufacturers may offer a 1600AF/1600AT, and a 2000AF/1600AT, so in that case it becomes important for the design engineer to consider his future requirements. You would do so if you were sizing the cables for a planned future service upgrade. By specifying the larger frame, you make it simple for someone to increase the service later by just changing the trip plug on the breaker. So the practice of listing both the frame and trip settings is just a convention used to maintain consistency.



Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[mc5w]

Frame size refers to the rating of the current carrying parts. In some instances such as arc welders the frame size needs to be at least 200% of the full load current. In the case of some machines such as spot welders and induction furnaces expect the manufacturer of the circuit breaker of busway plug to specfy a frame rating that is 300% of full load current. This is because these types of loads draw peak currents that tend to spot weld contacts turning a circuit breaker or fusible switch into something that isn't.

SquareD does offer a 225 amp 240 volt circuit breaker frame with a 100 amp fixed trip which is rather handy for a 100 amp feeder with a 180 amp or 200 amp neutral.