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My garage was cut in half by the previous owners who converted one half into a bathroom. Pictured below is the fuse box in the garage. The incoming power to this comes from the main downstairs socket circuit from the house. I presume this was originally just one socket in the garage, but has been used for all power in the new bathroom.

The two fuses are then shared with five outgoing cables as follows:

Fuse 1.

  • A spur with one double socket (top left)
  • A fused spur powering the boiler and the under-floor heating

Fuse 2.

  • Spot lights
  • Extractor fan
  • Another double socket

I don't see why this has to be so complicated. The boiler is on a fused spur, so why does it additionally need its own fuse box? The circuit is already RCD protected at the main consumer unit.

Could this be simplified? Could the circuit not have been simply extended to add the new fittings (lights, sockets and fan) and then the fused spur added for the boiler etc.. without the need for the fuse box?

Even if the fuse box remained, why not add that (top left) socket below the fuse box on the incoming circuit which is after all for sockets? I don't get the logic.

I'm asking because I want to add a light in the garage (probably on the second fuse, either directly or by extending the current spotlight wiring). I'm wondering if this is a good opportunity to tidy up the mess. Is there good reason for the way it's been done? If not, how would you rearrange it?

Garage fuse box

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  • The plug's cord going into the same channel as the fused spur feels weird to me, That's something I would change to separate fuses explicitly and a regular switch if that is needed. Commented Oct 4, 2022 at 14:00
  • I don't follow. Are you referring to the flex sharing the same trunking? Or do you mean that the socket spur and the fused spur should not exit the consumer unit from the same channel?
    – Tim
    Commented Oct 4, 2022 at 14:06

1 Answer 1

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This seems like a bit of a bodge job to me. It's generally considered a good idea to have a separate consumer unit in a garage as it reduces the distance you have to walk when something trips out. Adding it as a spur to a socket ring probably doesn't meet building regs (I am not an expert, but it feels wrong). Also the unit has 2 circuits, looks like 6A and 32A trips, so that would be a light circuit and a socket ring.

As I see it you have several problems that need rectifying:

  • The incoming feed should be a 4mm T&E cable, not a connection to an existing socket ring
  • The unused space in the unit should be blanked off
  • Your spotlights and extractor fan should both be connected to the same breaker and this should be the 6A breaker. This circuit is the one to extend for new lights
  • The sockets and fused spur should be on the other circuit, which should probably be a 20A circuit as it looks like it's wired with 2.5mm T&E
  • The junction box on the cable coming out of the fused spur may need replacing. From the size of it, it may only be a 6A junction box. It needs to be rated for the same or higher current than the fuse on the spur. (The rating should be marked on the back)
  • You should check the size of cables that have been used. Building regs get complicated but the general rule of thumb is:
    • 5A lighting circuits need 1.0mm or 1.5mm cable
    • 20A Radial socket circuits need 2.5mm cable
    • 30A Radial circuits need 4mm cable
    • 30A Ring circuits need 2.5mm cable
  • Given there's space in the unit I would add another MCB (probably 20A) and put the heating spur on that.
  • I'm not sure about correctness with regs, but the heating isolator should be a double-pole type, not all fused spurs isolate both poles, so check this.
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  • Thanks for the detailed answer. The fused spur has a 13A fuse in it, so I'll certainly downgrade the MCB to 20A, or lower to match the fuse? I don't see 13A at my local stockist, but they have 16. The junction box says 3x1.5mm so I assume not sufficient for even 13A, so I'll upgrade that too. I'll also add a third MCB so I can have lights on the 6A, then sockets and fused spur separated.
    – Tim
    Commented Oct 5, 2022 at 13:17
  • As for whether the cable leading to the boiler (behind that trunking) is rated sufficiently I can't tell. It's stranded/flex although looks roughly the same thickness as the solid core. Nothing on the sleeving that I can read.
    – Tim
    Commented Oct 5, 2022 at 13:19

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