After reviewing this topic, I am looking for some input. It seems that conductor size for loading falls under one criteria and conductor size for overcurrent protection falls under another.
If I use THW wires (75-deg C) and have a non-continuous connected load of 290A on a 300A breaker, does the next higher breaker rule allow me to use 300mcm (285A) or do I need to use 350MCM (310A) since I have to size the conductors for the actual load? Please assume no use of 90-deg C conductors.
240.4 doesn't say that a 290 amp load requires 290 amp wire.
However, you will find that requirement in 215.2(A)(1) and 210.19(A)(1).
Before you concern yourself with overcurrent protection the first step is selecting a conductor large enough for the load.
210.19(A)(1) tells us the minimum ampacity required for branch circuits:
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
If the conductor were for a feeder we go to 215.2(A)(1) which is worded similar.
If you have a non continuous load of 290 amps a 300kcmil is only rated 285 amps and does not comply with the minimum size required. If the load were 286 amps you still have the same problem. Next size larger is minimum required conductor size.
Now if you have a non continuous load of 285 amps you can use a 300Kcmil conductor and you can protect it at next higher standard sized overcurrent device which is going to be 300 amps.
If you can not get your load to 285 or less you will need a larger conductor - even if you still use a 300 amp overcurrent device.
