We are planning to run a feeder to a 225 amp 3 phase panel. 208V. The cable is a M/C type. Four conductors plus ground. The question is, I have to follow the 75C column in table 310.16 of the NEC, right? Next, I have to derate to 80% because of 3 or more current carrying conductors. Then the size cable is 300kcmil. I wanted to double check this. The contractor wants to use 4/0 size. If I am correct, it may be changed to conduit instead. Thanks - Good Day
You have to use the 75c (or lug rating) for the basic ampacity but when you derate you can use the 90c column for 90c wire. The thought is the heat will be distributed along the wire where it is bundled, not concentrated at the lug.
As a practical matter the ONLY reason for the 90c table is for the derating calculations. All ordinary lugs and terminals are rated for 75c or 60c.
One is also permitted to use the full rating on branch conductors for the crowding calculation, #10 @ 40A derated 50% = 20A.
#12 @ 30A derated to 70% = 21A ... still above standard derating of 20A for branch circuit #12 conductors.
When, in a 3-phase system, the fourth conductor is carrying only the unbalanced current AND no significant harmonics are present -- one may omit that fourth conductor from the wire count for the purposes of derating a bundle.
Meaning that four-wire feeder cable counts as just three and is NOT derated for bundling.