If breakers are sized to protect wire where does that to the premise that breakersw are to be size at not less than 125% of the continuous loads?
To size the wire you must know what the load to be carried is, 125%+100%. The wire must be sized to carry the load. The, the breaker is sized to protect the wire. Up to 800a the breaker it either is sized equal to the ampacity of the wire's rated ampacity or if the wire size happens to not match a standard breaker rating you are allowed to select the next higher rating. As such if this procedure is followed the 125% breaker rule is a non issue.
The only thought that I have was that when I installed my A/C, wiried it with #10, the installation instructions specified a 20a breaker and strangely enough I only had a 15A breaker available at the time which I installed. I used my Fluke 26 with a clamp an sensor to measure the actual load which was actually between 9-10a much to my surprise. I said the heck with changing the breaker and left the 15a breaker in placed which was about 15 years ago and has been operating with out an issue ever since.
As such, this is the only appication where that 125% breaker rule may have been an issue, that is the intentional used of an undersize breaker when protecting wire. The only thing that comes to mind when undersizinfg a breaker is the posibility of nuisance tripping which is an inconventience be should not commonly pose to be a safety issue.
