Example No. D8 Motor Circuit Conductors, Overload Protection, and Short-Circuit and Ground-Fault Protection

(See 240.6, 430.6, 430.22, 430.23, 430.24, 430.32, 430.52, and 430.62, Tables 430.52 and 430.250.)

Determine the minimum required conductor ampacity, the motor overload protection, the branch-circuit short-circuit and ground-fault protection, and the feeder protection, for three induction-type motors on a 480-V, 3-phase feeder, as follows:

(a) One 25-hp, 460-V, 3-phase, squirrel-cage motor, nameplate full-load current 32 A, Design B, Service Factor 1.15

(b) Two 30-hp, 460-V, 3-phase, wound-rotor motors, nameplate primary full-load current 38 A, nameplate secondary full-load current 65 A, 40°C rise.

Conductor Ampacity

The full-load current value used to determine the minimum required conductor ampacity is obtained from Table 430.250 [see 430.6(A)] for the squirrel-cage motor and the primary of the wound-rotor motors. To obtain the minimum required conductor ampacity, the full-load current is multiplied by 1.25 [see 430.22 and 430.23(A)].

For the 25-hp motor, 34 A x 1.25 = 42.5 A

For the 30-horsepower motors, 40 A x 1.25 = 50 A

65 A x 1.25 = 81.25 A

Motor Overload Protection

Where protected by a separate overload device, the motors are required to have overload protection rated or set to trip at not more than 125% of the nameplate full-load current [see 430.6(A) and 430.32(A)(1)].

For the 25-hp motor, 32 A x 1.25 = 40 A

For the 30-hp motors, 38 A x 1.25 = 47.5 A

Where the separate overload device is an overload relay (not a fuse or circuit breaker), and the overload device selected at 125% is not sufficient to start the motor or carry the load, the trip setting is permitted to be increased in accordance with 430.32(C).

Branch-Circuit Short-Circuit and Ground-Fault Protection

The selection of the rating of the protective device depends on the type of protective device selected, in accordance with 430.52 and Table 430.52. The following is for the 25-hp motor.

(a) Nontime-Delay Fuse. The fuse rating is 300% x 34 A = 102 A. The next larger standard fuse is 110 A [see 240.6 and 430.52(c)(1), Exception No. 1]. If the motor will not start with a 110-A nontime-delay fuse, the fuse rating is permitted to be increased to 125 A because this rating does not exceed 400% [see 430.52(C)(1), Exception No. 2(a)]

(b) Time-Delay Fuse. The fuse rating is 175% x 34 A = 59.5 A. The next largest fuse is 60-A fuse [see 240.6 and 430.52(C)(1), Exception No. 1]. If the motor will not start with a 60-A time-delay fuse, the fuse rating is permitted to be increased to 70 A because this rating does not exceed 225% [see 430.52(C)(1), Exception No. 2b].

Feeder Short-Circuit and Ground-Fault Protection

The rating of the feeder protective device is based on the sum of the largest branch-circuit protective device (example is 110 A) plus the sum of the full-load currents of the other motors, or 110 + 40 + 40 = 190 A. The nearest standard fuse that does not exceed this value is 175 A [see 240.6 and 430.62(A)].





Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120
Flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net