Conductors are permitted to operate at 100% loading for continuous loads, unless protection requirements force over sizing.
The issue is that _breakers_ are not in general permitted to operate at 100% loading for continuous loads. So for continuous loads you must increase the size of the breaker to at least 125% of the continuous load.
But if you have oversized the breaker to 125% of the load, it will now not protect a conductor that is sized to 100% of the load. For the conductor to be properly protected by this breaker, it is _also_ oversized to 125%.
You will note that the requirements for over sizing the conductors to 125% have an exception: when used with a breaker that is rated for 100% operation. If you use a breaker rated for 100% operation, then you can use conductors right up to their ampacity, even on continuous loads.
-Jon
