Sizing Medium Voltage Cables

[CuriousElectron]

Hi Guys,

When sizing medium voltage cables, it seems like ampacity may not be the main consideration. Are there any other criteria that would govern MV shielded cable sizing. What role is fault current plays into cable sizing? My understanding is that the cable needs to survive the max. system fault current and so needs to be protected by the protective device but also be able to withstand the time required by the breaker to clear the fault?

Any fundamental knowledge shared on this topic would be greatly appreciated.

Thank you,
EE

 

[waross]

Size according to ampacity and then check the withstand ability.
Some codes use a separate ampacity table for cables over 5000 Volts.
Upsize if required.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

CuriousElectron,you are partly right in saying that the SC rating decides the cable size in MV systems. This is true when it comes to minimum size of the cable that can be installed in a given MV system. It happens in MV systems with high SC rating.
But, for large feeders where the current carrying capacity requires a larger size cable than the minimum stipulated size, it is the current rating that decides the cable size.

 

[CuriousElectron]

Thank you guys..

Does a medium voltage fuse typically clear the maximum SC current under 2 cycles?

I guess I'd have to look at the curves for that, these are not always readily available..

Best Regards,
EE

 

[RRaghunath]

With fuse backed MV contactors, you will be able to use much smaller cable sizes as the fuse let through current will be much smaller than the rated SC level.
Also, the duration to be considered is 0.2s max (for fuse blowing with margin).

 

[dpc]

The IEC world seems to be much more concerned with cable damage from through faults compared with those of us in the US. When I look at the cable damage curves that we typically use, cable protection from short circuit protection is almost never an issue - the fault is cleared long before the cable is theoretically damaged. Just curious - other than shields, has anyone experienced a cable failure that was attributed to through fault damage?