Given two three-wire three-phase power circuits, No. 4 cu, in one conduit, Table 310.15(B)(2)(a) requires that the ampacity be limited to 80 percent of that shown in Table 310.16. If the conductors of one of the circuits has 60 C insulation, type TW, the Table 310.16 ampacity is 70 amperes, and the adjustment factor of 80 percent brings the ampacity down to 56 amperes. Per 240.4(B), the overcurrent protection is permitted to be not greater than 60 amperes. The other set of conductors is type THWN with an insulation rating of 75 C and ampacity of 85. Taking the adjustment factor of 80 percent, the ampacity is reduced to 68, with the overcurrent protection at 70 amperes.
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