Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 243

Figure 9.10 Overcurrent protection of a cable and motor.

For the circuit shown the following design conditions should apply,

• Cable ‘derated’ currentIcshould be greater than the rated motor currentIm.


• In practice the motor running currentIrunwould normally be slightly less thanIm, since the motor
  should have a rating greater than that of its driven machine. The margin in current would depend
  upon the rating of the motor, see sub-section 1.6.

The following worked example illustrates how (9.12) and (9.13) are applied.

9.4.4.1 Worked example

Consider the worked example in sub-section 9.4.3.5.2 for a 160 kW motor, a 3-core 300 mm^2 XLPE
cable, and a 400 amp fuse. Assume the cable is routed in air at an ambient temperature of 35◦C.

The motor has a full-load currentImof 245.4 amps.
The fuse rating is 400 amps and a near-asymptotic currentI 2 of between 600 and 800 amps.
The ‘derated’ current of the cable is 0. 92 × 666 =613 amps (see Tables 9.15, 9.16 and 9.17).
This is the currentIc.

NowIBshould be the least ofImandIc,i.e.IB= 245 .4amps.

Inshould be the rated current of the fuse, i.e.In=400 amps

Izshould be the cable ‘derated’ currentIci.e.,Iz=613 amps.

From (9.12),

IB[245.4]≤In[400]≤Iz[613]

which is satisfactory.

From (9.13):

I 2 [600 to 800]≤ 1. 45 Iz[889]