Cable Current Carrying Capacities.

nivag · Aug 11, 2016

I'm wiring a corn dryer up and the supply is into an isolator rated at 100a 3P N and Earth then straight into 6mm and 2.5 stranded singles going off in various directions to different MCBs, contractors etc
No fuse down between cable sizes. However this is uk new production. Adding up the various MCB ratings inside the control panel (not allowing any diversity) exceeds 125A

Surely this is wrong?

telectrix · Aug 12, 2016

need more info.are these various MCBs 3 phase or are some single phase? the actual load current may be far less than the rated values of the MCBs.

nivag · Aug 12, 2016

Correct me if I'm wrong. But I thought any cable must be capable of carrying the maximum current of the supplying upstream overload device.
Also once the dryer is under full load surely the 100a isolator is undersized.

Thanks for your reply

Wilko · Aug 12, 2016

Hi Nivag - the addition of mcb ratings may exceed the isolating switch rating but the current drawn shall not. So the real question is what current will the dryer take? On the cable size changes I don't do it. Why put in 6mm at all if 2.5 is sufficient. The cct must still be mcb for the 2.5 and in future some poor sod might make the mistake of thinking the whole lot was 6mm ... Cheers, David.

nivag · Aug 13, 2016

Hi, thanks for the response, the last bit of your post didn't make much sense to be honest. Maybe it's how I'm reading it.

I understand that the maximum current will be on startup and will drop dramatically once running. I will get it all finished and use temporary 25mm tails to supply it from the genny, the. Clamp meter it and see what it's pulling before I order the supply cable. I'd guess a 16mm 5c sy flex will suffice.

On the cable sizing issue, if it was me wiring it, I'd supply it via, say, 16mm to the isolator, load side in 16mm to a 3p buzzbar then MCB it there before any cable size drop.
I think what I'll do is trace the wires to see what there actually supplying and considering its situation, the fact that I'm just connecting up to it, and it shouldnt be accessed anyone else then, So long as the 2.5 feeds, for example, the 2g 13a metal clad socket mounted to the side of the enclosure, then I know the 2.5 should never exceed 26a