(See 215.2, 430.24, 430.24 Exception No. 1, 620.13, 620.14, 620.61, and Tables 430.22(E) and 620.14.)
Determine the conductor ampacity for a 460-volt 3-phase, 60-Hz-ac feeder supplying a group of six elevators.
The 460-volt-ac drive motor nameplate rating of the largest MG set for one elevator is 40 horsepower, and 52 amperes, and the remaining elevators, each have a 30-horsepower, 40-ampere, ac drive motor rating for their MG sets. In addition to a motor controller, each elevator has a separate motion/operation controller rated 10 amperes continuous to operate microprocessors, relays, power supplies, and the elevator car door operator. The MG sets are rated continuous.
Conductor ampacity is determined as follows:
(a) Per 620.13(D) and 620.61(B)(1) use Table 430.22(E), for intermittent duty (elevators). For intermittent duty using a continuous rated motor, the percentage of nameplate current rating to be used is 140 percent.
(b) For the 30-horsepower ac drive motors, 140% x 40 amps = 56 amperes.
(c) For the 40-horsepower ac drive motors, 140% x 52 amps = 73 amperes.
(d) The total conductor ampacity is the sum of all motor currents (1 motor x 73 amperes) + (5 motors x 56 amperes) = 353 amperes.
(e) Per 620.14 and Table 620.14, the conductor (feeder) ampacity shall be permitted to be reduced by the use of a demand factor. Constant loads are not included. (See 620.14, FPN). For six elevators, the demand factor is 0.79. Therefore feeder diverse ampacity is, 0.79 x 353 amperes = 279 amperes.
(f) Per 430.24 and 215.3, controller continuous current is 125% x 10 amperes = 12.5 amperes.
(g) The total feeder ampacity is the sum of the diverse current and all the controller continuous current. I total = 279A + (6 elevators x 12.5) = 354 amperes.
(h) This ampacity shall be permitted to be used to select the wire size.
See Figure D9.
Figure D9. Generator field control