Cable ampacity calculation under short circuit condition

It's all in British Standard 7671, as usual.

Cable ampacities as far as electrical codes are concerned is that which is listed in any of various tables, and "short circuit condition" is not considered.

If you're asking, "What is the maximum current a cable can carry?," then the answer is, "Crank up the current until something catches fire or melts." Since this is not necessarily easy or safe to do, it can be calculated to an acceptable degree by looking at the power density of the conductor (which means that knowledge of the conductor properties is needed), heat dissipation to the ambient surroundings, etc. In other words, a good thermodynamic model of the system is needed before you can feel confident that you have correctly calculated the maximum ampacity of a cable.

Short Ckt. current is important for MV cable sizing, following is some guide line though it is not exhaustive..

Steps to size a MV cable as follows

1) Determine Total Load current to carry

2) De-rate the current as par actual installation conditions and select a cable based on this current from mfr table

3) Calculate Short Circuit current for this cable as Isc=K*A/sqrt t or from mfr table

4) Compare this Isc to system max Isc, cable Isc should greater than system Isc to withstand, if not go for higher size of cable.

5)Calculate % Voltage Drop as [{1.732*route length of cable*total Load current*(r cosphi + x sinphi)}/V] *100

check the acceptable level of Vd with your local standard. if it is more than acceptable limit, go for higher size and repeat calculation.

** In case of no of run, you have to do economical analysis of installation i.e. what is the cost of single run cable + cost of energy lost and what is the cost of multiple run cable + cost of energy lost AND site suitability.

hi,

A = 13.2* Ik * √t mm²

A = area of conductor in sq mm

Ik = short circuit current in K Amp.

t = duration of short circuit in seconds.

13.2 is for particular case, so you can not put it at general formula. In my previous post there is a constant "K" which is a material constant and it depend on type of material and temperature rise. you can always ask this value from OEM.

Thanks