3 phase cable calc and fuse sizes for motors

[peterhardacher]

Hi am currently employed as testing and inspection on a industrial site just wondered if you could give me some advise
What it is I got a 110 kw motor 180 amp 100 m run full load now I got a 75 mm cable supplying it . I have checked table 4d4a in the regs as it is 3 core armoured . Ref method E . According to this it says am allowed 207 amps without correction factors .

I have also done a cable calc 110000 w / ( 1.73 * 400v ) = 158 amp without power faction correction is this correct ?

I have been in Internet on cable calc program's for cable manufacturers which is telling me 75 mm

Now what it is the cable is protected by a HRC motor fuse 315m 355a according to Ib < In < Iz the fuse size is greater than the cable max amps

Also what concerns me is local to motor the 75 mm goes to a JB and reduces to 25mm I think because there is a lack of space for the bigger cable as the motor has other things around it . Now a 355 amp defo does not protect that cable in case of overload

Now one final thing this motor goes through a variable speed drive but even if it was set at 80 % I still have the problem of oversized fuse . Now I pointed this out to my boss he says its acceptable but I think he's wrong and as am signing the sheets I would great fully apreciate your professional advise on this thanks

 

[lancselec]

basically as the motor fuse is only providing shock protection the cable sizing can be reduced. the energy let through of the fuse resticts the s/c current and as long as the cable can withstand it for the duration of the fault it is acceptable. the motor starter has overload protection built in so this is not therefore needed to be built into the cable calc.

 

[peterhardacher]

The motor will pull 180 amp full load as I said even if the VSD is at 80% still to much current for the smaller sized cable ???

 

[johnboy6083]

From my experience it is prudent to use the mechanical load rather than the electrical load. Motors tend to be oversized.
the VSD will provide overload protection, the fuse provides short circuit protection.

i personally have not seen a motor (supplying pumps) pull FLC.