Panel Incomer breaker AT rating

[tlp23]

Hello,

I have a panel connected with different types of loads like motors, lighting, sockets etc. The total load current on the panel is 135Amps. By considering the 80% loading of the breaker(135A x 1.25 = 168.75A) i need to select a breaker of size 200A.

Now what is the breaker AT rating i have to specify corresponding to my load current. How to select the breaker AT rating based on IEC/BS standards.

 

[jraef]

"AT" means "Amp Trip", as opposed to "AF" which means "Amp Frame". So in essence you are sort of using AT out of context here as if it is a parameter. It is just a shorthand notation. The notation is used like this: "400AF/200AT", or "225AF/200AT" or even "600AF/200AT". All three of those notations depict a 200A circuit breaker, because the Trip Unit is centered around a 200A thermal trip threshold. The "AF" values are the frame size, meaning in these cases, a 400A frame size with a 200A trip unit, or a 225A frame size with a 200A trip unit, or a 600A frame size with a 200A trip unit.

So bottom line; if you want a 200A circuit breeaker, you can use any frame that will fit in your panel that has a 200A trip unit, which means it will have a notation of XXXAF/200AT. You select the frame size to be as large and you might ever think that circuit could be expanded to. If that is totally unimportant, then size it at the smallest frame size that will have a 200AT. That will vary by supplier, could be a 200AF, 225AF, 250AF, maybe even a 400AF.

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[burntcoil]

200A shall be the ampere trip rating. Ampere Frame Rating (AF) you can choose based on other circuit breaker available in your facility. Same frame rating helps you to interchange the breakers in case you have problem in any of the breaker. With same AF rating, the breaker will fit in same dimensions instead of different trip ratings.

Also suppose you choose 200A AT rating and 400A AF rating, in future if your load grows beyond 200A, you can just change the trip unit and your breaker will be suitable for loads upto 400A, as the frame is rated to carry 400A.