Earth Fault Loop Impedence

For one problem (among several), you can run only one of these motors at a time. At a typical power factor of 0.8, one 45kw motor requires 56.25 kva. Not mentioning the voltage is a serious lapse. Circuit breakers are not sized based on partial motor loads. Etc.

As I understand of 50% duty cycle mean motor will not run continuous,rather it will run half of the time and will stay in rest for half of the time.likewise 30 minuets on time and 30minuets off time. So if your conclusion of motor not running on full load is based on its duty cycles then I think its not a right consideration.

Or may be I am reading your post wrongly.

I was too vague in initial post, apologies. Genset will feed distribution board and split into individual circuit feeding one VFD and one motor. From distribution board to VFD is maximum length 400m. From observation and measurement on similar sites it is known that each VFD/motor draws no more than 25kW with less than 50% 'duty cycle', also on other site is 120KVA genset feeding 2 x 45kW + 2 x 75kW VFD/motors on individual circuit.

British Standard 7671.

The interesting thing is the international response and the issue of differing voltages. I'm in Australia so I'm using 240/415v, without (easy) access to british standards. I think the equation i was seeking is:

0.8 x Uo x Sph x Spe / (7.5 x Ia) x 0.0225 x (Sph + Spe)

Uo = nominal voltage

Sph = phase CSA, Spe = earth CSA

7.5 for C-curve breaker as VFD = soft starter

Just in case of interest - the motors drive CSG well pumps and lower water level by ten metres and then shut down until the water rises back up. It is hard to judge the frequency of running intervals and running time, but these reduce with the life of the well. Next week I'll take some measurements on 45kW motors running at 90% torque to get an actual current draw. I hope this will confirm the lower actual currents as opposed to the thoeretical maximums.