11 kV Cabling

According to rough assessment I am of the view that 35 mm sq. may not be suitable.

Your's is an 11 Kv cable; you have to assess suitability of the cable for current carrying capacity, voltage drop and short circuit withstand as well.

Take into account the short circuit value at the feed source (?) in your plant and correlate with the short circuit withstand curves provided by the manufacturer for the cables given in the cable manual to find the correct size.

Easy way out for you is to call the cable manufacturer and ask for their recommendation.

Another question to consider is whether the load will always be a maximum of 200 kw (including 2 75 kw pumps). The extra cost to install a slightly bigger cable may make sense in terms of allowing for future expansion.

I also see some benefit from a bigger diameter / stronger cable.

Probably you must have done the cable sizing calculations only for the continuous current & for the voltage drops (steady state voltage drop & acceleration state voltage drop). But, have you sized the cable for the maximum available short circuit current magnitude & duration? Probably the contrcator must have considered this while recommending 95 sq.mm. But, it need not be 95 sq.mm always. If you can prove by calculations thet your 35 sq.mm cable would withstand the full fault level for the worst expected fault duration, there no need to upsize the cable.

I guess I (can) learn something every day. To the best of my recollection, I never sized cables for their maximum available short circuit current magnitude & duration. It can probably make sense (I'm wondering if it makes sense at low voltages).

Hmm, maybe we did it in reverse, we did consider the let thru current of things like breakers when considering downstream circuit protection devices, but not the actual wires / cables.

Is this now a code requirement? If so, NEC, or some other code?

Your distance is 5 km

Your maximum load is 250KVA, whic gives approx 16A per phase.

Your load appears to be 3 phase and the transformer input is Delta with a star output.

You need to look at the tables for the Cables and check:

What is the Voltage drop factor for 35 sqmm and work it out for 5 km with a maximum of 48A starting current for both pumps. If the drop is within 5%, it will be OK.

What is the Current load for such a cable and derate it for the 5km.

The Short circuit condition should be dealt with at the start with adequate Breakers or Fuses. The length and rating of the cable has nothing to do with that, but the contrary is more true: You protect the cable! You select the cable to suit the load and the distance.