Voltage=400V, Motor load=30 KW WHEN I USE A DOL STRAT THE SECTION OF CABLE=95mm2,ADD A DIFFERENTIAL PROTECTION THE SECTION WILL BE 35mm2 WHY THE SECTION REDUCED WHEN I ADD A DIFFERENTIAL PROTECTION?

what's a dol strat?

Basically because of the fact that the differential protection protects you against dangerous voltages on the housing of the motor. In case of an earth fault it will trip relatively fast.
This is why other protections may act "slower". They will take longer to trip because of the higher resistance in the circuit with a reduced cable diameter.

I think by using differential protection the fault clearing time is reduced and a lower sized cable can be used.Actually the cable should be capable of handing the high current during motor faults,If there is a delay in fault clearing the lower sized cable will not witstand the heat generated and will fail.The sizing of the cable (apart from motor full load current,voltage drop, system fault current) will also depend upon how adequately the cable is protected.

i think when used differential protection we used CT sense current and CT secondary must 1 or 5 A so current value will be small as compere to direct connection so size will be reduce

Differential protection on a 30KW motor is very unusual. Motors this small are often considered throw away items. The cost of a differential relay and the CT's is not justified by a motor so small. A simple MCP and thermal overload are more typical.

I wouldn’t say 35Kw is a small motor, I think Bill has been spoilt!! ;-} The current rating for the motor will be on the rating plate of the motor and normally a high and low voltage rating and the number of poles or the speed and the frequency. I don’t know where you got the 95mm cable idea from, a SWA cable will take well over 300amp’s. I would guess you went by about 7x the normal current for starting? Use a motor rated fuse set and a star delta starter and overload. The cable size I guess size of 16mm for SWA is more like it!! The cable will take many times the current for a while before the big fail state kicks in! Let the forum know the rating on the plate!

Good luck.

Perhaps I estimated the number wrong. But it still looks like 35KW at 400V to me and that calculates out (simplified radically) to approximately 50A. Still seems small to me. Am I still missing something?

I went to my SKM Power Tools software.
For a 400V, 35KW motor. Assumed to have a power factor of 0.8 and an efficiency of 93% (default values) the full load current will be 67.9 amps.

The retail cost of a motor this size in the US is 6,100$.

The cost of differential protection including CT's will come close to the same $$ as the motor.

Thus my initial comment.

No Bill not all. I just use only small small motors 200W to about 4Kw. I work in electronics where 100mA might be a lot current.

Hey Bill, ‘Meant to say not at all’. So from Bills more up to the mark data that puts the cable size at 16 or 25mm square for SWA depending on the method of installation. Looks like the CT’s are out of the window for you due to high cost.

Geoff