ECN Forum Forums General Topics of the Trade General Discussion Area Conductor ratings

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 can derate from the 90c column. That puts you in 250MCM for 4 @ 225a @ 80% (290 x .8 = 232)
If you used the round up rule that could be 4/0 since there is no standard breaker between 208a and 225.

Thanks, sounds like 4/0 is the cable.

OK,
What is the length of run?
Voltage drop could come into this as well.
Just a thought.

Approx. 60' in length. Back to the 310.16 table, I thought you had to use the 75C table. it's tempting to go to the 90c coloumn to use a smaller wire. 75c is what all the treminals are rated for?

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.