Short Circuit Current used for Cable Sizing Calculation
Short Circuit Current used for Cable Sizing Calculation
(OP)
I have the question that when we do cable sizing calculation based on short circuit current, the short circuit current used is the short circuit current at the end of cable(terminal of motor orload) or it should be at the beginning of cable connection to circuit breaker ?
I understand that the point for calculating the short circuit current should be at the end of cable as it should be through fault ? because if we use the point at the beginning of cable connection with CB, it seems to be unrealistic as this current does not pass through the cable yet.
Is there any standard recommend the point of short circuit current for the cable sizing application ?
Thank you
Best regards






RE: Short Circuit Current used for Cable Sizing Calculation
RE: Short Circuit Current used for Cable Sizing Calculation
RE: Short Circuit Current used for Cable Sizing Calculation
First I thought I should consider the through fault since it doesn't make sense considering the current of a fault at the beginning because the cable must be changed anyway.
But a faulted cable ought not to be a cause of danger or damage also for the equipment at the beginning. So it must be able to withstand to a fault close to its beginning.
So my last and actual opinion is that I must consider the fault current at the beginning of the cable.
RE: Short Circuit Current used for Cable Sizing Calculation
It can make sense to consider chunks of cable where portions could be replaced if damaged, but if you have a cable fault at the source end the cable is toast whether it was above or below the withstand rating. Any fault before the first point the cable is accessible and could be spliced is going to require removal and replacement anyway, so use the through fault current; just choose the right through fault.
RE: Short Circuit Current used for Cable Sizing Calculation
Thank you very much
RE: Short Circuit Current used for Cable Sizing Calculation
Reference ANSI/IEEE STD.242-1986.
Hopes this clarifies the issue.
RE: Short Circuit Current used for Cable Sizing Calculation
RE: Short Circuit Current used for Cable Sizing Calculation
Why is cable sizing being determined by SC availability?
The purpose of knowing the SC current is to properly size the interuprt rating of the OP device...right? When you change the conductor size....you change the SC availability all the way downstream of that cable.
Please let me know if I'm missing something here...
Thanks,
RE: Short Circuit Current used for Cable Sizing Calculation
I used SKM and eventually got them to add an N-1 option for cable damage; for a long time it had been 1 or N as the only choices.
RE: Short Circuit Current used for Cable Sizing Calculation
For cable withstand rating, calcaulate SC at the end of the cable, as it is for through faults only. The cable is not supposed to be damaged in delvering the fault current.
The fault current at the beginning of the cable is for the breaker/fuse assembly protecting the cable and not for the cable.
Senslessticker:
In medium and high voltage applications, SC withstand rating often dictate the size of the cable as normnal load currents are small (as the voltage is high, but SC currents could be very high. You need to check both.
RE: Short Circuit Current used for Cable Sizing Calculation
The utility I work for designs our distribution substations, with few exceptions, to provide a maximum of 20kA at the distribution voltage. Many transmission busses won't draw even 20kA for a bus fault. The energy content of the fault is far greater, but the current associated is often surprisingly low if one is used to building systems. We're looking at ways of keeping the distribution voltage ring bus of a potential new substation below 40kA, but that is an issue only because there will be 200MVA of transformation feeding that ring bus. But even at 40kA, the large conductors used for the distribution circuits will not have withstand issues.
RE: Short Circuit Current used for Cable Sizing Calculation
In practice we normally used the calculated currents in the bus where the cables is connected because this is the highest fault current available that the cable will withstand to lessen the effect of equipment damages if the fault occurs on the cable it self (especially in the location under the substation cable space).
Normally when we calculate cable size by load currents we rarely encountered resizing of cable size due to short circuits due to a very conservative derating factor on cable ampacity. That is my experience in Oil and Gas Plants especially Aramco projects.
Cheers!
RE: Short Circuit Current used for Cable Sizing Calculation
Thanks for the insight. I just learned something new!
Thanks,