Cable Sizing/ SC Current Rating

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Quote LA "When sizing power cable for motors, which short-circuit current rating shall be considered? Switchboard or circuit breaker that is connected in the upstream to the motor. Thanks in advance all."

None of the above. The cable size should be determined by your local code. If you were using the NEC, the cable size would be 1.25 x the FLA of the motor. The breaker is also sized by your local code. The NEC allows the breaker to be up to 250% of the FLA of the motor. In some circumstances it may be higher.

NEC doesn't directly enforce considering short circuit current for selecting cables for up to 600V system, though article 110.10 has some sort of indication. However, it enforces to do it for the cable of the system of over 600V. Here is the quote from NEC 240.100 (C):

"Conductor Protection. The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short-circuit conditions".

If the short circuit current is not calculated for the fault at the motor terminal, I would suggest you to use the bus short circuit current for selecting the conductor size. We generally do it in device coordination study in ETAP (www.etap.com) software. In device coordination, the cable damage curve must be at the left side of the upstream breaker trip curve, so that the breakers trips before the cable is damaged.

- MS

Switchboard short circuit rating should be considered for sizing an HT cable. Time duration for which a system short circuit is allowed to persist (fault clearance time) is also taken into consideration. Formula used for selecting cable conductor size is:

Ik = 0.094XA/ sq.root (t) where

Ik = short ckt. current in KA (rms)

t = Duration of short ckt. in seconds

A = Area of conductor in mm2.

Readymade graphs are also available with cable manufacturers.

Hope this serves your purpose

BB Raina

In the adiabatic equation K^2*S^2=I^2t, K depends on the cable insulation.What are the values of k for pvc,xlpe,paper,micc,enamel,mica etc.For example XLPE cable can take more current than PVC cable.

Once again, it's all in that wonderful reference, British Standard 7671.

One should consider the tripping time of the breaker too.For a given SC current if tripping time is longer more damage will be done.

consider the actual calculated fault level at that bus for the cable sizing .

size may vary depending upon the clearing time of the protection device.

what is the typical clearing time of the protection to sizing the cable.

If it is a fuse you can consider 0.01 sec.

if is is a breaker with relay you can consider around 0.2 to 0.3 sec

is it necessary to considered for worst case conditions for sizing cables. For example we considered 1st level of protection are not trip & trip at 2nd level of protection ( VCB ). Maybe we can considered 0.6 sec. Is it over protected.Any comment

For cable sizing as already said it is short ckt withstand capability of the bus bar system for a designed time duration. For example, if your bus bar system is designed for 50 KA/ 1 second, then you should select a cable size meeting this requirement by using the formula mentioned earlier.

BB Raina