Cable size.

Discussion in 'Electricians' Talk' started by Nick watson, Jul 31, 2020.

  1. Nick watson

    Nick watson New Member

    I have a garage attached to my property and wish to fit a 2way garage type consumer unit for lights and a maximum of 6 sockets. There is no external wiring involved as the cable can run under floorboards and through the house wall into the garage. What would be the size of cable required to run from the main consumer unit to the garage. Thanks.
     
  2. Comlec

    Comlec Screwfix Select

    You don’t really need a garage CU for a few sockets and a light. If you have a spare way in your CU that has RCD protection the a radial circuit using 2.5 t&e on 20a MCB would suffice. Put the lights on a FCU. Job done. Well apart from the testing, certification and LABC part P notification.
    Good luck
     
    Nick watson likes this.
  3. marwen

    marwen New Member

    Are you really sure about that?
     
  4. unphased

    unphased Screwfix Select

    I agree with Comlec. Why are you questioning it Marwen?
     
  5. Coloumb

    Coloumb Screwfix Select

    If you want something more beefy you would have run 6mm swa to a two way cu in the garage. Things then start to get quite a bit more complicated. Really depends what you want to do in your garage. If it's just to charge up your Black and Decker it's pointless to running vast amounts of swa when 2.5mm would do.
     
  6. rogerk101

    rogerk101 Screwfix Select

     
  7. Coloumb

    Coloumb Screwfix Select

    It might be easier roger if you actually had a better grasp of the regs. The chances of being able to run a sub-main in twin without rcd protection under the 18th is very, very very small.
     
  8. rogerk101

    rogerk101 Screwfix Select

    I will gladly stand corrected when you point out (a) why SWA is necessary, (b) on what you're basing the 6mm2 requirement, and (c) where it is mentioned that there is no RCD protection already in place.
     
    Hfs likes this.
  9. Mike58

    Mike58 Screwfix Select

    In my early career I was doing some work on a fairly remote island, It did have power though.

    Those who arrived first had run 100m of 2.5 T&E from a farm, down a hedgerow and into an old lorry that was being used. It worked fine for what was needed.

    Eight years later, I went there on holiday and wandering down a path next to the field we had used, there in the hedgerow was the original T&E and no visible signs of degradation.

    I am not saying it was/is totally safe, however it did what was needed and the cable stood up to the elements fairly well.
     
  10. Nick watson

    Nick watson New Member

    If I don't need a second cu for the needs that I specified then would it be permissible to just extend the house ring circuit to the garage? But if as mentioned by Coloumb about wanting something more beefy or more to the point adding some flexibility for the future then I would need 6mm. However, as I mentioned in my original question there is no external cabling neede so why did he specify SWA.
     
  11. Bazza-spark

    Bazza-spark Screwfix Select

    SWA doesn't need RCD protection, so you can put the RCD at the garage end with your garage CU as you wanted. That way when it trips you don't have to go back to the house to reset it. If you want to add extra at a later date the supply is big enough. T+E will need RCD protection so the RCD would have to be at the house CU.

    If you are running a new circuit from the CU it is notifiable so really should be carried out by a spark registered with one of the scam providers, or done under a building control notice submitted before you start along with the appropriate fees.
     
    Nick watson likes this.
  12. Mike58

    Mike58 Screwfix Select

    Added to wrong thread ...
     
  13. unphased

    unphased Screwfix Select

    Hi Mike. I thought you were implying that, rather than connect through to his garage directly it would be a good idea to take the supply cable outside, run it around his garden boundary hedge and back in. When you posted this I realised it was in the wrong thread. :rolleyes:
     
    rogerk101 likes this.
  14. Bob Rathbone

    Bob Rathbone Screwfix Select

    Roger, I think the Coulomb is considering that the twin & earth may be buried in plaster or concealed in a wall without the necessary mechanical protection such as an earthed metal shield such as conduit. You may be thinking that the twin and earth is just run under the floorboards below the suspended ground floor. In that case no RCD would be required if it could be shown that the cable would not be accidentally damaged by any cutting or drilling of the floor surface. My personal view is that RCD's on sub mains create a hazard as the RCD is remote from the user of the sub main and may be inaccessible. But the Regulations must be slavishly followed, or should they?
     
  15. rogerk101

    rogerk101 Screwfix Select

    Bob, thanks for that explanation.
    When I had a two storey, 2 bedroom, 1 bathroom extension built at my house, it was going to be pretty disruptive to the rest of the house to get electricity supply into the extension. The existing (only) socket ring in the main house couldn't be extended as it was already close to full capacity, and I wanted numerous double sockets all around the extension. There was, however, an existing 6mm2 T&E cable that had already been taken pretty close to the extension for a power shower that was no longer in service. That cable now feeds the new sub main serving the extension. The 6mm2 T&E runs through the loft and is readily visible, so unlikely that anyone will whack a nail through it. Besides, it is fed from a 40A MCB on the RCD-protected side of my main consumer unit, so is safe.
    I saw (and still see) my situation above as pretty much the same as our OP has. At no point has he suggested that he doesn't have an RCD in his CU. Even if he didn't have one, he could either add one or just use an RCBO for the feed for the submain.
     
  16. Nick watson

    Nick watson New Member

    Thanks for all the information, now have a few options. Rogerk 101 was interesting regarding RCBO. The spare fuseway in the main cu is not protected by RCD so would a 32A RCBO suffice.
     
  17. rogerk101

    rogerk101 Screwfix Select

    If it's 2.5mm2 cable from main consumer unit to sub main consumer unit, then it can't be protected by a 32A RCBO. It'll need something like a 20A one.
    32A RCBO would be fine for a 4mm2 cable; 40A for a 6mm2 cable.
    Maximum current tables for different cable sizes are found all over the web. I tend to err on the side of caution, as there's always a chance that someone might stuff a whole lot of insulation into an area where a cable is running, and this in effect downgrades the maximum current rating of that cable.
     
  18. dobbie

    dobbie Screwfix Select

    That is only if the 6mm is clipped direct, reference method C which is not that often.
    It needs to be clipped direct to be able to be on a 40amp breaker, any other method is lower than 40amp so it does not comply.
     
  19. Nick watson

    Nick watson New Member

    Thanks everyone for your time and patience. I will use the 6mm with a 40 amp RCBO.
     
  20. Coloumb

    Coloumb Screwfix Select

    You need to use a 32a mcb on 6mm swa, as per dobbies post. It's extremely unlikely you will need 40a in a garage, that amount of current is verging into light workshop use. Its also notifiable. RCD'ing a submain is also a big mistake.
     

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