AMP FRAME & ADJUSTABLE RATINGS

[maxace]

1. What is the difference between amp frame and adjustable ratings?
2. As I look at a one line diagram, a breaker is showing 250AF/150AT, what does AT stand for?
3. How to determine amp frame? For example, in a one line diagram, there are two circuit breaker showing 250A and 400A.

 

[Jraef]

• F = Frame, so the maximum size in that physical package. A 250AF means you can get a range of trip settings, stopping at 250A, probably 100-250A are available in that frame.
• T - Trip setting, so a 150AT = a fixed trip setting of 150A.

So you have a breaker that is a 250A frame but has a 150A trip unit installed. Depending on brand, you might get a 150AF/150AT, but some stop at 125AF, it varies from one mfr to another. You can also probably get a 400AF/150AT if you want. It allows for future increase in trip ratings without having to re-arrange your panel.

Adjustable trips is a separate issue. In North America, standard Thermal-Magnetic (T-M) breakers are not allowed to have Adjustable Trips, unless they are Electronic Trip Breakers where they are "programmable" setting, ostensibly because Joe-Blow wrench turner would not be able to "crank 'er up" when it trips, it would need to be a "qualified electrician" that makes any adjustments, someone who would understand the ramifications.

You can get a special class of T-M circuit breakers specifically for motors, called "Motor Protection Circuit Breakers" that have adjustable thermal trips, because they can take the place of a motor thermal OL relay. But you cannot use those for any other purpose (in North America).

Elsewhere in the world adjustable thermal trips are allowed so most breaker mfrs offer them, but they cannot be UL listed unless the trips are fixed.

 

[Carultch]

1. What is the difference between amp frame and adjustable ratings?
2. As I look at a one line diagram, a breaker is showing 250AF/150AT, what does AT stand for?
3. How to determine amp frame? For example, in a one line diagram, there are two circuit breaker showing 250A and 400A.

Question 1:
A manufacturer isn't going to build a completely unique design for every breaker in section 240.6. Instead, what they do is group several sizes within a range together, and standardize on the design. This way they can have about 10 breaker designs, instead of 40 designs. The largest breaker within each group, is the frame size.

As an example, the 100A frame size might be applicable to the 70A, 80A, 90A, and 100A breaker. In order to reduce the number of unique parts to source, the manufacturer groups the sizes, so they can use the same housing, the same handle, the same busbar connections, the same terminal lugs, and the same inner workings of the breaker, for all breakers within this range. The essential difference is how the trip mechanism is calibrated, and how the handle is labeled to reflect this calibration. For a molded case thermal magnetic circuit breaker (as is common for 400A and less panelboards and load centers), it is factory internal. For an electronic trip breaker, it is usually a separate accessory called the trip plug that can be swapped.

Question 2:
AT stands for the ampere trip rating, while AF stands for Ampere Frame Size. The trip setting is what people think of, when they think of an 80A breaker. The fact that it has a 100A frame isn't relevant to its functionality. The reason you might care about the frame size of the breaker, is that it helps determine the physical size to reserve when planning a panelboard or switchboard.

Question 3:
Experience and correspondence with the manufacturer. That's how one determines the frame size, and its relation to the trip size. Some manufacturers might have a chart that tells you how they group the trip ratings in frame sizes.

 

[paulengr]

Physically most rating plugs are just one or more resistors.

Even with electronic breakers often there is a rating plug that limits the allowable trip range.

And this is just the ultimate (long term) trip. Shirt time, ground fault, instantaneous are all adjustable.