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My house has a spur into the garage. It was installed from new (2000) by the builders. There is a simple fuse box in the garage with one mains fuse and one lighting fuse. They are protected by an 80amp RCD on the mains circuit on the house consumer unit. What will be the maximum wattage I can use on the spur? There is a single fridge and a chest freezer in the garage and I have just installed 2x 30 watt LED spotlights. I want to use a few electrical items (not at the same time) in the garage, eg pillar drill, router, circular saw and also use a heater on the cooler days. The heater would be maximum 1.5kw, but probably use no more than 500 w. I could fit a new small consumer unit in the garage, so would a 30 amp fuse be okay. The cable is 2.5mm and 1.5mm for the lighting, in the garage.

First of all a lot of your terminology is incorrect, with this being incorrect it must be assumed anything else you say also contains errors.

In order to be of any assistance I would need to know the following. (I can guess but I may be wrong, hence the following questions)

The consumers unit in the garage, what exactly does it have by means of protection? A clear, in focus picture would be useful.

The cable that supplies the garage consumers unit, what size is it, and also what is it connected to in your house? Again, a clear, in focus picture would be useful.

How far is the garage from the house.

The cable from the house to the garage, how does it get there (You can see it, I and others can not) Does it go over head, is it buried underground is it clipped to a wall, some thing else?

Sorry about my terminology, probably has something to do with me not being an Electrician. The Fuse box in the garage is the old kind that has fuse wire between two screw terminals. These fuses then push into an oblong socket. The garage is 4 mtrs from the house. The cable is attached to a fused socket on the lounge wall, passes through into a metal box on the outside wall, then into conduit that goes underground and enters the garage a similar way. The cable is either armoured or in a sheath. it is black and measures approx 12mm in diameter. This cable then goes behind a wooden board and into the fuse box.
I can take photos but it will have to wait until daylight.
Thanks for your reply.

Your reply is very informative.
The cable used sounds like it is 2 core 2.5mm steel wire armoured. It has been (badly) connected to the existing house ring main.

As it is connected to the existing ring main it will depend on what is being used in the house at the time as to how much is "available" for the garage.

A ring main is protected by either a 30A or 32A fuse or MCB. (Around 6900 watts) so if the house ring is using (for example) 3000 watts you only have 3,900 watts available for the garage,

The garage really should have its own dedicated supply, then you could have had what ever is required (Within reason) not as you have "what ever the house ring is not using"

Thanks! I found my solution here.

Thank's Mrsmith. That would probably be why one of the MCB's that has 'sockets' written above it, does not turn any sockets off, only the one that has a 32a MCB, itself attached to the 80a RCD. This one turns all the sockets off. It would not surprise me, with the builders of this house, as It has no insulation in the cavity either, just a bit of glass fibre roof insulation, in the walls, in the porch, obviously to show the Building inspector.