Replacing Tails

[Jon Bridges]

Been replacing a CCU and doing a few other bits in a flat and a few things have arisen. As a matter of course I always upgrade tails to 25mm when renewing a ccu, am I obliged to or can I leave the old 16mm ones in if the max demand isnt higher than 16mm can take.

Also, on sub mains, Ive had to run 8 meters of tails. I'm going to put in an RCD in the sub main, however as the board is going to have a 63a and 80a RCD both 30mA, what do I need to put in to ensure correct discrimination, ie does the amp rating need to be higher or mA, or both.

Thanks Guys

 

[Hawk81]

if your max demand is not higher and the main fuse is not rated higher than what 16mm tails are rated at the you can leave them. You can fit a time delayed rcd for your sub main.

 

[Jon Bridges]

Thanks Hawk. With the discrimination on an RCD though is it the mA rating that needs to be higher as well as the Amp rating?

 

[Hawk81]

the amps rating on the rcd will not make any difference. As an rcd does not protect against overload. Only rcbo's protect against overload.

 

[sonobit]

Its says you should install 25mm tails or whatever the suppiler recommends, usually a sticker on or around the meter iif there is one most say min10mm, but if no sticker use 25mm. Meter guy came to upgrade my tails from 10mm and put in 16mm.

 

[andekoch]

look at the article published by the ESC in its magazine (issue 15) page 18 & 19. link below

http://esc.org.uk/pdfs/business-and-community/SwitchedOn-Issue-15.pdf

This in summary says that you can leave the 16mm tails if there is no evidence of damage and the max demand is below what the cable can carry.

The catch comes when the consumer wants to install 'stuff' eg a new elec shower that will increase the max demand.
I had a case where the tails were 16mm and could not be easily changed (without breaking off the tiles and floor from the new bathroom ie customer refused). The customer wanted to install a 10kW shower which would have increased the max demand significantly so the option I went for was a new 80A fused main switch feeding the exist CU and the new shower RCD and mcb. That way I can better protect the tails.

 

[Jon Bridges]

thanks for the help so far. However part of the tails will need to be buried in a wall let than 50mm. The regs state that this part of the circuit now need to be protected by a 30mA rcd. As I will have to have one at the sub main and then one at the ccu am I going to run into problems?