100% rated breaker

[neher]

I have a feeder that draws 900A continuously
I specified 1000A 100% rated LSIG breaker.
The submital i am reviewing calls for a 80% rated 1200AF with a 1000AT LSIG trip unit.

Never really thought about it before but is the breaker rating (100% rated VS 80% rated) for the frame or the trip?






When an elephant is in trouble even a frog will kick him.

 

[jraef]

It's pretty difficult to use a 100% rated breaker, most people don't understand all the rules and restrictions. If it was in a switchboard as a feeder, it's unlikely that option was really available to you, so the 1200AF was the best solution.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Kiribanda]

neher,
It is my understanding that 80% de-rating is due to bkr is mounted in an enclosure because the bkr is tested without an enclosure in free air.
Therefore, if you mount it in your enclosure you have to de-rate it by 80% of AT=1000A. Otherwise it must be mounted without an enclosure in free air so that
you can get 100% of AT=1000A.

 

[neher]

so if i have a 1200A 80% rated breaker with the LTPU set to 1 and 900A of continuous load on it then i meet code
if I adjust the LTPU to 1000A now this is a code violation?

sorry English is not good perhaps i am not getting my point across

When an elephant is in trouble even a frog will kick him.

 

[Kiribanda]

neher,
If you use full 1000A while mounting the bkr inside an enclosure then you cannot consider the bkr as 1000A rated
and size your cable connected to the bkr. You should consider that now the bkr is rated for 800A only and size the feeder cable
accordingly. Otherwise forget about a code (I donot know which code you are referring to), it is a basic design violation.

 

[jraef]

ALL breakers are tested at 100% of their rated capacity. But a breaker can only be connected to cables that are selected based on their ampacity at 75 degree C, and the breaker terminals are rated at 75 degrees C. The cable amapcity then must be selected based on 125% of the continuous load, so being that is the cable size you need, and the breaker is there to protect the cables, the inverse of 125% is 80%, hence the so called "80% rule" on sizing the circuit breaker.

You CAN buy a breaker that is "rated" for 100%, but it must be either in free air or in it's own separate enclosure, you CANNOT use a 100% rated breaker in a switchboard or panelboard with other breakers, except, WHEN TESTED AND LISTED BY THE MANUFACTURER, as a Main Breaker. If you DO use a separately enclosed 100% rated breaker, then what and how you CONNECT to that breaker has to follow an entirely separate set of rules and regulations, i.e. wire rated for 90 degrees C but still selected based on the 75 degrees C column, special 90 degree C lugs or bus bar terminations, etc. etc. etc. etc. etc. etc. etc.

 

[TomPERCDD]

Neher,

I feel your pain. Your question is simple and to the point but there are always people on here that love to make things complicated and answer there question and not the one you asked.

Your question is interesting and I have never really though about it that way before. Instead of making something up or telling you that your design is wrong I am just going to tell you this i do not know. I do however I will ask around as I would like to know the answer to this question as well.

Tom-

 

[neher]

Thank you Tom. Yes very simple question.

There are only 3 answers

1. Frame size OR
2. Trip setting OR
3. I don't know


Also as stated above I am sorry if i offend you my English is not good




When an elephant is in trouble even a frog will kick him.

 

[catserveng]

No there is really only one correct answer, and it is "it depends".

First where are you in the world?

What application?

What codes and standards apply?

If you are in the US and the National Electric Code pertains to your site, then maybe this will help,

http://ecmweb.com/basics/sizing-circuit-breaker

A google search may provide additional info if you are somewhere else.

MikeL

 

[neher]

fair point catserveng NEC, US standard, continuous load <600V

I can not explain is any more simply then i did about 5 posts ago so i will cut and paste

" if i have a 1200A 80% rated breaker with the LTPU set to 1 and 900A of continuous load on it then i meet code.
if I adjust the LTPU to 1000A now this is a code violation? "

When an elephant is in trouble even a frog will kick him.

 

[wroggent]

Look at 215.3 and 240.6(C).

If access to the adjusting means is restricted, then the long time pickup may be used as the RATING of the device.

215.3, for non-100% rated devices, requires the device RATING to be 100% of non-continuous plus 125% of continuous load.



So basically, 240.6(C) allows you to use reduced cables sizes for a particular breaker frame size as long as the LTPU is reduced - it does not allow you to set the LTPU equal to the load.

1200A frame, 1000A LTPU, 1000A load = code violation.


It's quite dumb but that's how the code is written.


" if i have a 1200A 80% rated breaker with the LTPU set to 1 and 900A of continuous load on it then i meet code.
if I adjust the LTPU to 1000A now this is a code violation? " Correct.