frame size?

[treyp3]

What is meant by frame size for an instantaneous trip circuit breaker, and where does the 3 times 5 times 10 times for low medium high come from. Anyone know where I can read up on this. What the heck is frame size?

 

[augie47]

I'll let a more qualified forum member address your specific question but I will note one NEC requirement that has been overlooked by some folks causing them a problem:
430.52
(3) Instantaneous Trip Circuit Breaker. An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and the setting is adjusted to no more than the value specified in Table 430.52.

 

[kingpb]

The frame size is the designation of the maximum continuous current the breaker can carry, and also an indication of its physical size. Breakers listed as frame sizes have adjustable trip settings that will allow you to select a trip setting up to the frame rating.

example, 200AF/150AT

This would mean the breaker frame is acceptable for circuits up to 200A, but the trip setting has been set to trip at 150A.

 

[zog]

To add to Kings comment, the 3x, 5x, 10x are multipliers of the frame size for your INST trip set point. So a 200AF breaker for example will trip instantly at 600A, 1000A, and 2000A depending on the INST setting. This setting should be determined by a coordination study and is mostly dependant on the type of loads served.

Don't think of the physical size of the breaker when you talk about frame size though, some higher frame size breakers can be the same physical size or smaller than smaller frame sizes.

 

[Jraef]

A Molded Case Circuit Breaker (MCCB) consists of 2 basic parts: the switch and the trip unit. "Frame Size" refers to the switch part. So if you have a 250AF, that means the switching element is rated for 250A. But on that switch element you can attach a trip unit that is 100A, 125A, 150A, 175A, 200A, 225A, and 250A. If on the other hand you have a 125A frame, it may have trip units that range from 15A to 125A. The "rating" of the breaker is generally considered the trip unit rating, but sometimes an engineer will specify a frame size because there is usually overlap, as indicated above, and he wants to make sure that if some day in the future that needs to be a larger trip unit, it will fit.

So for example take the term "125A breaker". That could be a 125AF/125AT or a 250AF/125AT. Both are legitimately "125A breakers", but if they are in a panelboard, the "strap kit", the bus connectors that attach the breaker line side to the bus bar, is totally different and the width of the breaker is different. So if at a later date it was determined that you want a 150A breaker in there (assuming a new set of conductors of course) and you had a 125AF/125AT breaker in there, you might have to rearrange the entire panelboard to make room for a 250AF/150AT breaker. If you had a 250AF/125AT to begin with, no big deal.

 

[templdl]

This is what the Eaton/C-H frame ratings are.

G-Frame (15?100 Amperes)
F-Frame (10?225 Amperes)
J-Frame (70?250 Amperes)
K-Frame (70?400 Amperes)
L-Frame (125?600 Amperes)
M-Frame (300?800 Amperes)
N-Frame (400?1200 Amperes)
R-Frame (800?2500 Amperes)

You will see that the ampere ratings of each frame overlap. When a 100a breaker is required the most economical would be the 'G' frame. So it possible to specify a 400a 'K' frame with a 100a trip. There are various reasons to specify a larger frame which would take too long to explain.

Now, it is extremely important to apply MPCs within their continuous current ratings because they do not have overcurrent protection. The 'G' and 'F' frame C-H breakers have multiple magnetic trip ranges dependent upon how they are applied. You will note that even though there may be multiple magnetic trip ranges within those families that they will be marked with an ampacity usually on the end of the toggle. These breakers are built with a solenoid would with wire unique to the magnetic trip range. It is that wire size that is susceptible to being fried should the rating of the MCP be exceeded. Overloading these MCPs will smoke them. Quite often a testing agency will attempt to check the calibration of an MCP by ramping the current to the point of trip and in the process they will be exceeding the ampacity capability.

The larger frame breakers have a less complicated magnetic trip element and you may find various magnetic trip ranges available with a frame size but all will have the same ampacity rating.

 

[Jraef]

...

Now, it is extremely important to apply MPCs within their continuous current ratings because they do not have overcurrent protection.

I think it's important to note what augie47 pointed out: you cannot "apply" an MCP breaker unless you are an electrical equipment manufacturer building factory tested combination starter assemblies. The only thing that can be done with MCPs in the field is to directly replace units with exact model numbers, or factory tested and approved substitutes. There should be virtually no discussion of the field application of MCPs, it is not allowed, by UL or per the NEC.

 

[templdl]

You are correct Jraef, MCPs are reverse UR components listed devices used in combination starters. It should be understood that an MCP has no business as a branch circuit protective device.

I'll go a step farther and point out that each family of starters such as 00, 0, 1, 2, etc are available with a varienty of MCPs depending upon the size of the motor being controled. A size 00 is available with a 3, 7, and 15a MCP but has the same NEMA size 00 starter. A size 1 is available with a 3, 7,15, 30, and 50a MCP.
For example if you specified a #1 combination starter for a 1hp 460V motor it would come with a 3a mcp. However, that same #1 starter can be rated up to 10hp by using a different MCP but the same starter.

1HP-MCP 3A
3HP-MCP 7A
5HP-MCP 15A
10hp-MCP 30A

It would be interesting to find out if one would be able to change the original motor out with a 5HP motor and install a 15A MCP. If you didn't replace the MCP the 5HP motor would fry the 3A MCP.
It is the 150A frame MCP (3a-150a) that can be destroyed should it's ampacity rating be exceeded.