Start with 310.15 Ampacities of Conductors. Section (B) tells us to use Table 310.16 for standard building wire with not more than 3 current-carrying conductors in a raceway. 310.4 covers Conductors in Parallel. Note the last sentence, which tells us to derate for more than 3 current-carrying conductors in a conduit.
Although the NEC never says so, we always simply added the allowed ampacity of the paralleled conductors to determine the total allowed ampacity of the paralleled assembly.
In other words, if I paralleled 3, 250 kcmil in separate conduits, 255 + 255 + 255 = 765 amps.
Reading the commentary, however, I see that conductors connected in parallel are considered a single conductor with a total cross-sectional area of all the conductors in parallel. Would that mean that my 3, 250 kcmil conductors in parallel should have their ampacity determined as if they were a single 750 kcmil conductor?, with an ampacity of only 475 amperes? Doesn't make a lot of sense when run in separate conduits.
Why is the allowed ampacity of a 750 kcmil conductor only 475 amps? Skin effect? I squared heating from all that current?
If I ran all my paralleled conductors in one conduit, I would need to derate to 70%. 70% of 765 is 535.5 amps. Must be a combination of the two. Or maybe something I am missing. Is the surface area of smaller conductors greater than that of one large conductor, and can therefore dissapate heat better? Anyone?