The standard breaker(80%) depends on the attached conductors to serve as a heat sink to take some of the heat away from the breaker. This is why we must size conductors an extra 25% when connected to a standard breaker - there is more heat expected within the conductor especially near the terminations.
100% rated breakers do not depend on conductor to sink heat developed within the breaker, therefore no conductor deration is necessary.
As stated before, a common breaker will be applied correctly if the cable is sized to carry the computed load. The 80% rated breaker just confuses people as the forget that breakers are size to protect cable. I have sold millions of dollars in MCCBs while working for one of the largest domestic breaker manufactures. I had the privilege to meet an know some of the leading breaker engineers in the world before their retirement. What a brain trust and a big loss when they left. I have disassembled breakers, I have set at a test bench in the warranty return facility and tested breaker, been in manufacturing facilities. Sold mining breakers, high shock navy breakers (you should see the inside of those breakers). I know what makes a breaker current limiting, I have seen slot motors and all sorts of other triks used to speed uop contact separation in order to increase interrupting ratings.
I know the engineers who developed the AFCI I my home became a bata sight for testing them.
Yes, when applied correctly a standard breaker will automatically be applied at not more than 80% of its rating. It is extremely important that one does not get the cart before the horse by zeroing in on the breaker. it's load, then cable, then breaker. If you think about is the breaker rating will have nothing to do with the load but it has everything to do with protecting the cable, the cable of which has been sized to carry the load.
When you do the 125%+100% thing to find you calculated load how lucky will you be to find a cable that would have a rating exactly the ampacity? It's is almost always going to be more. When you select a breaker you base it upon the cable ampacity ignoring your load calculation as you have already taken that into consideration by choosing the correct cable size. When you selact a cable based upon the cable rating, the cable rating is certainly greated than the calculated load and you must be very luck to have a breaker ampacit that isexacltly the cable ampacity rating as is probably isn't and you are allowed to select the next higher breaker rating for rating less than 800A. Nowe, if you follow this procedue will you even be close to this "80% rate breaker thing?"
I find it amusing that there are those who get hung up on this 80% rated breaker thing as if you follow the ampacity rules and cable sizing rules the only way that you can get into deep dodo is by under sizing the breaker.
As I related previously I don't argue with those who believe that a 100% breaker is better the "standard" breaker because they is really no difference in them. If the 100% rated breaker is not applied correctly it will be appied as one would a standard breaker. The pricing is functional based upon the 100% rated breaker pedigree, that is they UL testing that is required and how they are applied. Yes, it has to do with temperature rise based upon 3' of rated cable on the line and load side of the breaker installed in a suitable enclosure. The advantages of using a 100% rated breaker is based upon if the combination of the extra cost as the breaker vs. the possibility of saving money if you can use a smaller cable size. I'm sure you are aware of instead of the 125%+100% rule you are allowed to use the 100%+100% rule to find your ampacity. Now the trick is were dos that ampacity fall on cable rating chart? Are you going to be lucky enough to use a smaller cable which can save you money? If you are lucky enough to drop down a breaker frame size which of course will may get you into a smaller breaker which relates to less expense. And panel space, smaller breakers provide more room in the panel.
The most important thing is to verify that the breaker is being installed per the instruction a that 90degC rated cable is used but the sized is to be based upon the 75degC column.
so you can keep its 100% rating. And, yes, I know the breaker temperature heat rise drill well. In any breaker test the cable does act as a heat sink. As I said, there isn't any real difference between 100% rated breakers and standard breakers (I don?t like to use the term 80% rated breaker). It's all in the testing to assure that there will not be any additional heating of the breaker should a smaller conductor be used. But, have you notice that all the 100% rated breaker that I'm aware of use electronic trip units?
