Powering the Garage (and maybe a car)

Discussion in 'Electricians' Talk' started by SupposedlyDIY, Feb 1, 2020.

  1. SupposedlyDIY

    SupposedlyDIY New Member

    Hi all,

    First post here so go easy on me. I thought I'd found my answer on another post on this forum, then realised it didn't fit in the available space, so seems like the right place to be. This might be a long post but from reading others it seems best to provide as much info as possible to get a sensible answer.

    Let me start off by saying I'm not a spark. I've done plenty of simple stuff in the past (extending rings, putting in lights etc), so I know which wire is which. But this is clearly a "new circuit"/notifiable area which I will be getting a professional in to wire up. Although I might do some of the donkey work (underground cables etc).

    It would be nice to have some impartial advise so as to know what I'm getting myself in for financially.

    Fundamentally I'm looking to lay a patio in the garden this summer and that's between the garage and the meter box and house consumer unit. l would like to be able to fit an electric car charger (32a 7kw type) to my garage in the future, however the cheapskates that built my house only fitted a 1.5mm swa cable between the house and the garage. Obviously it's easier to lay a cable before putting a patio down

    Currently my consumer unit in the house (100a split load type with 2 63a RCDs supplies a 20a MCB which feeds a cable through the house (I assume T&E) out to a plastic terminal box outside the back door, the swa is connected to this and then runs under the garden in a plastic tube, pops up in the garage and terminates in a garage consumer unit. This feeds a 16a radial socket circuit and a 6a lighting circuit.

    In addition to the car charger I'd like to have another 16/32a socket in the garage to be able to plug in a lathe or welder in the future. Therefore in the garage I would like to have a consumer unit supplying 2 32a MCBs a 6a and a 16a. Assuming diversity can be applied on this board that comes to 53.6a (which for practical reasons I would assume means a supply of 63A)

    Now if I was to simply replace the existing wiring with higher current stuff and a 63a breaker in my house consumer unit I would end up vastly exceeding the 100a capability of the main switch on the consumer unit (which I understand is a big no-no). For clarity taking 63a as my biggest load and applying diversity to the rest I end up at around 125a. (Can people confirm that I'm right to apply diversity in this way on both boards?, as technically the highest single load is actually a 40a electric shower)

    The only way I can keep the existing layout seems to be to derate some of my breakers (is that allowed?). The oven breaker is currently 32a and the manufacturer says it should be 20a so that's an easy win (although maybe isn't a great idea if I ever chose to remove the gas hob), the other circuit I could change would be to derate my upstairs socket ring from 32 to 20a. (Not so much load up there, a TV and some charger and lamps). But I'm wary of doing this incase someone plugs a big PC in the office one day, or the fact it's a 20a breaker makes people think it's a radial. Also the thought of trying to get a sufficiently sized cable through the house is painful.

    All of this "fudging" in order to get to a system that's just about legal but right on the limit doesn't seem ideal to me but this is effectively option 1.

    Option 2 is to not supply the garage consumer unit from the house consumer unit.

    Happily the house builder did fit a double pole switch after the meter in the meter box (which kind of makes up for feeding the entire garage with lighting cable). I could have a pair of Henley blocks and split the supply from the isolator between the 2 consumer units. Sadly the garage is a lot more than 3m away so I believe I'd need to fit an additional fuse to keep the electric board happy. So it seemed I needed one of those Wylex switched fuse isolator units which I could connect my tails coming from the Henley blocks into and swa out the other side to the garage. (Garage unit now needs its own RCD protection). The problem with this is there isn't enough space in the meter box to fit this.

    A neighbor recently had a charger installed fed of a small unit within the meter box linked to Henley blocks, but the installer has had to remove the dp isolator to fit it in and pull the naughty fuse to do it. (Something I don't really want to have done).

    Do I really need the switch fuse unit, or will an inline service fuse satisfy the regs given there's already an isolator between it and the meter? Obviously this makes terminating the swa more of a problem but I need something that fits the box first. And I don't really want to have to install another meter box or similar just to fit a switched fuse isolator.

    One thing that doesn't make sense to me is that if you install multiple consumer units off the same 100a supply surely you run the risk of overloading the tails feeding the Henley blocks or blowing the naughty fuse. Does it not make sense to fit a fuse before the Henley blocks but after the main , dp isolator or would there be little point given you'd have to fit a fuse lower than the naughty fuse for it to ever blow first?
     
  2. jonathanc

    jonathanc Guest

    I would look at option 2. Separate cu for garage. Henley block and Fuse. Back to your original problem. Chuck in a duct and rope under the patio before you lay it and it allows you or anyone else to pull a cable whenever they want
     
  3. Bazza

    Bazza Screwfix Select

    Ignore this idiot. You’ve got the supply you’ve got. The main fuse may only be 60A. it may be 100A. But 100A will be the max.


    @SupposedlyDIY
    Sorry, don’t have the time to answer your very long post blow by blow. You are right that this is very much a new circuit/ notifiable activity. There’s too many variables to safely give u guidance on a forum, and it would be foolish to try.
    You have two options
    1. Find your registered electrician now. Tell him what you need. He can do the design tell you what you can do in preparation.
    2. As suggested above. Lay in proper double wall conduit like this
    https://www.tlc-direct.co.uk/Main_Index/Cable_Index/Armoured_SWA/index.html#Cable_Ducting
    It comes complete with drawstring.
    Put your patio down and get your electrician to pull in the right sized SWA if/when you decide to go electric car.
     
  4. SupposedlyDIY

    SupposedlyDIY New Member

    Thanks for the replies gents,

    Knowing the supply is fitted with a 100a naughty fuse and the meter is rated at 100a I figured the 180a comment was a bit of a wise crack.

    Appreciate the post was long, so apologies for that. Also appreciate the honesty Bazza. I will be doing this in the not to distant future if I decide I can justify the expense/nock on work I'll have to do.

    If going with option 2, is the Henley block and fuse scenario just a fuse or does it need to be a switched fuse isolator? The fuse and the blocks I've got space for. The switched fuse isolator I don't. (And the location of that would determine where I run my conduit to).

    What started as a few hundred quids worth of cable replacement, a bigger breaker, and a spark to connect it all, is now looking a much more expensive undertaking if I need to possibly end up making changes to the house to find places to put the equipment.

    Thanks all.
     
  5. Bazza

    Bazza Screwfix Select

    For option 2 you just run the duct and leave the decisions on how to connect to your electrician. two quick points:
    The SWA must be protected by a current limiting device at the source. The size of this will be determined as part of the cable sizing calculation by the electrician. the current must be limited otherwise the SWA will only have the service fuse in the mains incomer as it’s protection. That’s not allowed.
    While we are on the subject. You say that you know that the supply fuse is 100A. How do you know that? The fuse carrier may have a label on it that says 100A. But that is the maximum fuse allowed. The DNO may, and quite often, only fuse properties at 80 or 60amps.
    The only way to really know is to examine the fuse itself, and that means breaking the DNO fuse and putting your life at risk.
    Really, don’t concern yourself with the why and wherefor of henley blocks, fuses etc.
    Get an electrician to advise.

    I know that you want to do it yourself, but (with respect) this is outside your competence level.
     
  6. SupposedlyDIY

    SupposedlyDIY New Member

    Rest assured I won't be doing this myself. I just need to know where I'd run my conduit to.

    There's minimal space in the existing meter box. A single fuse unit and the blocks would fit in there, but I've measured up and a wylex switched fuse isolator unit won't.

    But if that's what's needed I guess it's tails to someplace within 3m that it does fit? And that means running conduit someplace else. Maybe fit another meter adjacent meter box (getting expensive)? Or I might just about be able to get to my understairs cupboard within 3m but it's tight (and my understanding is that the 3m rule varies by supplier).

    Hence I'd rather avoid that if the regs can be met with just a additonal fuse in the meter box to continue running beyond 3m.

    It was a recent new build so I've got all the documentation from the original electrical installation which lists the "characteristics of primary supply overcurrent devices" as being a rated current of 100a. So I took this as meaning there would indeed be a 100a main fuse on the supply?
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice