Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 231

Figure 9.7 Fuse and cable time versus current curves.

to be no less than 3 at the corner point. Hence a fuse must be chosen that has an operating time
no less than 63 seconds at a current of 1408 amps. A suitable fuse for motor applications would be
rated at 400 amps, see Figure 9.7, the next lower rating of 355 amps may not prove reliable.

Find the cable size to suit the starting current and the route length.
From (9.3) the R and X components can be found in terms of the volt-dropVstmand the
currentIstmduring the starting period,

rcos Østm+xsin Østm≤

V.Vstm

l

√

3 Istm× 100

Where lis the route length of 250 metres
V is the rated line voltage of 440 volts
cos Østmis the starting power factor of 0.33
and sin Østm= 0 .9440.

Therefore,

0. 33 r+ 0. 944 x≤

440 × 15. 0

0. 250 ×

√

3 × 1408 × 100

≤0.10825 ohm/km for the cable.

Assuming the cable is routed in air which has an ambient temperature of 35◦C and that the
cable is not grouped with others then its rating at 25◦C must be at least 245.4/0.92 i.e., 266.74 amps.