Tri Rated Cable BS6231

[ndservices]

I came across this installation the other day and would be interested in anyone elses thoughts on it because i consider it to be very poorly designed.
The local electricity board have just installed a new 400A TP&N supply and the electrician has used 1x150mm tri rated cable per phase and neutral to go from the cut out to his own 400A isolator. This cable is within metal trunking. From this isolator he has used 2x50mm tri rated cables in parallel per phase and neutral to a 400a changeover switch again within metal trunking. The length of trunking that these 50mm cables run through also contains standard cables rated at 70c.
I have stated to the customer that all these cables are undersized and that there will be significant issues with overheating. Not only of the cables but of the main isolator and changeover switch.
The electricians response has been that the cable rating for 50mm tri rated is 204A and therefore two in parallel is adequate for a 400A supply.
Although this is correct according to manufacturers specifications,this is for a single cable in free air with an ambient temperature of 45c. He has made no allowances for derating the cable for grouping,being within metal trunking,other cables within trunking of a lesser rating,temperature rating of main switch and changeover switch etc. As far as im concerned the 150mm is woefully undersized and that the parallel 50mm`s are even more so.
Any thoughts or additions to the above would be gratefully received because as it stands at the moment its come down to my word against his.
Neil

 

[Engineer54]

Not too sure what your problem is with these Tri rated cables. Why are you thinking they are woefully undersized?? By the way, how long is this trunking that these cables are running in??

 

[malcolmsanford]

Have to ask .......................has anyone ever actually seen a run of trunking/conduit where the rating factor of .......... if I can get another 3 x 1.5mm cables in, it a good'un?

 

[Knobhead]

And I’ve got to ask, where does the change over switch fit in to the scheme of things.

Really you should be talking to the electrician, I’m not going to make the bullets for you to shoot.

 

[Darkwood]

Im in the assumption here that that the OP is angling at the fact the tri-rated cables are possible 90degree rated and thus have an higher tolerance and current allowance but this has to be ignored and derated to 70degrees if the switchgear (as most is) is rated at 70degrees ... regs 523.1 through to 523.5.

Although id use cable manufacturers tables for current rating and not the regs in this case de-rating them to 70degrees if confirmed connected equipment and other or other grouped cables are rated at this; i cant really comment too much on your set-up here as id personally want to weigh it up myself so as above dont be taking my bullets for your target practice.

 

[pepparz]

This is almost exactly the same installation as I am witnessing in the industrial premises where I work.
For the CCC I think the sizes should be derived from table 4D1A as per table 4A3.