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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Over-spec cable https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1237304400 Message started by markb on Mar 17^th, 2009, 3:40pm

Title: Over-spec cable
Post by markb on Mar 17^th, 2009, 3:40pm

I need to run a new lighting circuit from the consumer unit to an extension, but lifting carpets throughout the house could be avoided if I use an existing cable pre-installed for a shower unit. At the consumer unit it's protected by a 30A MCB. Can I replace the MCB with a 5A and use the existing cable (which is obviously well over the 1.5mm T&E needed) for a radial lighting circuit? Basically, is using an over-spec'd cable permissible? Any guidance appreciated.

Title: Re: Over-spec cable
Post by plugwash on Mar 17^th, 2009, 5:06pm

Using overspec cable is fine just make sure whatever you use to join it to the new lighting circuit cable is both suitable for the cable size you are using it with and (if it has screw terminals which it almost certainly will) is accessible for inspection.

Title: Re: Over-spec cable
Post by Y3 on Mar 17^th, 2009, 5:10pm

You could keep the 30a mcb and install a 4 gang consumer unit in place of the old shower and run your light circuit from there with a 6a mcb? Would that be ok? I would just like to add that if the existing 6mm cable is buried in a wall at a depth that your unaware of then the 6mm sub-main (which is what it will be, if done this way), will need to be 30mA rcd protected. Your earthing and bonding arrangements need to be assessed before carrying out the work.

Title: Re: Over-spec cable
Post by markb on Mar 17^th, 2009, 10:19pm

Thanks for the help guys. The shower cable is coiled in my loft, so running down into the extension is a breeze. I did think about the mini consumer unit option as I have to put a double 13A switched outlet in the same room (I was going to add as a spur off the ring in the room next door), so that may be the way to go. I suppose if I were to add a switched outlet off the 4 gang consumer unit then I would need to protect the entire circuit with a 30ma RCD on the main board? Right?

Title: Re: Over-spec cable
Post by Y3 on Mar 17^th, 2009, 10:38pm

Yes,

but if the shower cable is concealed anywhere along its length within a wall at a depth less than 50mm, unprotected by an earthed metallic enclosure of some kind then the circuit will still need to be rcd protected at the main cu, regardless of a socket being installed or not.

Title: Re: Over-spec cable
Post by Zambezi on Mar 17^th, 2009, 10:56pm

Since the 17th edition of the regs came out recently, pretty much every circuit in a house needs to be RCD protected, there are some exceptions but not normally done in residential properties.

In an ideal world your new lighting circuit will be RCD protected (whether you use a new cable from your consumer unit, extend your existing shower circuit or turn the existing shower cable into a sub-main).

If your extension is new and subject to planning permission/build regs then they will probably want it to meet the 17th edition.

Title: Re: Over-spec cable
Post by The_Trician on Mar 17^th, 2009, 11:05pm

Hi,
Since your 6.00mm cable is already RCD protected -

"......use an existing cable pre-installed for a shower unit. At the consumer unit it's protected by a 30A MCB."

Then using this to feed a small consumer unit will mean that any sub ccts from the new consumer unit will already be protected by the existing RCD.
This will keep the cost down, since you won't need another RCD-equipped consumer unit to wire the sub ccts in your new extension.

I'd go for thee new consumer unit option, make it a small 4 or 6 way, and wire your new sockets and lights from that, using 6A & 16A mcbs with radial cct arrangements.

TT

Title: Re: Over-spec cable
Post by markb on Mar 18^th, 2009, 8:14am

Blinding, I'm good to go. Thanks all.

Title: Re: Over-spec cable
Post by Twobarrows on Mar 23^rd, 2009, 11:53am

The_Trician wrote on Mar 17^th, 2009, 11:05pm:

I read that as 30A MCB, not a 30mA RCD, so he will need an RCBO surely? (in which case he may as well replace the entire box, unless RCBOs have coem down significantly in price)

Title: Re: Over-spec cable
Post by Y3 on Mar 23^rd, 2009, 5:28pm

I second that twobarrows, i was confused with that one too. You can get rcbo's for £17 now, 20 if its starbreaker. So, pretty cheap.