I need to know what is the allowed reduction of parallel conductors to still meet the requirements of the load? If you use parallel conductors does each conductor have to be rated to 80% of full load current? On a service entrace that I am workig on, the existing SEC's are 4/0 aluminum in parallel (RWH 4/0 compact to be exact). It's a 3 phase system and max amps is 208 from the transformer. I know I can use 4/0 copper and just run 3 line conductors and 1 neutral, why didn't they? Cost? It would be easier to pull the 4 coppers instead of the 8 aluminums. What is the deciding factor? Thanks!
What is the calculated load and the overcurrent device rating? Sometimes aluminum is used to save money or it is specified by an engineer when the owner has requested that the engineer look for cost savings. You are right that a single set of copper cables will carry the output of the transformer and should be easier to install than double runs of aluminum. To answer your question about individual conductors needing to meet 80% of the load, I do not believe that has ever been a requirement. You could certainly install parallel aluminum conductors smaller than the 4/0 as long as they combine to meet the requirement for the calculated load and the sizing of the overcurrent device to which they are connected.
I do believe the question was about parallel conductors in the same conduit. If so, then yes, you must derate to 80% when you pull eight conductors (six current carrying) in the same conduit. The last sentence of 310.4 requires this.