Cable size

[AkerRobert]

In CEC (Canadian Electrical Code) book, 8-106(1) says: The size of conductors and switches computed in accordance with this section shall be the minimum used except that, if the next smaller standard size in common use has an ampacity not more than 5% less than this minimum, the smaller size conductor shall be permitted. Does NEC have the similar/same code?

My calculation of the cable size is 551A. The allowable ampacity of 750MCM (90 degree C) is 535A on Table 310.16. Can I use this 750MCM cable or shall I use the next larger cable in common use 1000MCM?

 

[don_resqcapt19]

I don't know about the CEC, but you can't use the 90 degree C rating for this purpose in the NEC. I also would be looking at parallel conductors for this application. You could use parallel 350kcmil and have an ampacity of 560 amps even with an 80% derating for more than 3 current carrying conductors in a raceway. If you run parallel conduits you could use a set of 250 kcmil in each raceway and have 580 amps assuming no more than 3 current carrying conductors in the raceway. With the NEC you would have to use a single 1250 kcmil to get 551 amps as 1000kcmil is only 545 amps. We don't have the 5% rule that the CEC has.
Don

 

[AkerRobert]

Thank you very much for your advice.