I have a fused spur which goes to an outbuilding with consumer unit etc.

It would be helpful if I could take a feed off that spur to add a couple of sockets and LED lights to my garage, as it runs under our decking right next to the garage, and would be relatively easy to do, compared to running off the outbuilding consumer unit which would require quite a lot of digging and general farting about.

Leaving aside requirements for notifiable work and inspections etc for a moment, is it permissible to tap off an existing fused spur and run another one off it? Or if I asked an electrician to do it would they say it’s not allowed/permitted?

You have answered your own question and you know it.

My experience is that domestic electricians often fudge things… ask enough people, they’ll eventually give you the answer you want.

*SHRUG*

No. You can have a radial circuit (appropriately sized cable) back to the consumer unit.

Really a fused spur is like a tidy extension cable that is permanently plugged in and best thought of like that. You wouldn’t want putam extension on an extension on 4 gang on a 4 gang etc

Really a fused spur is like a tidy extension cable that is permanently plugged in

Or like a socket without pins in it. and you would happily extend* that radial to another socket or branch it and so on. it’s just a fused isolating switch.

That being said, the outdoors part of it is more a source of query and whether a fused spur is sufficient (depending on where its fed from etc) for its current use.

Personally I’d swap the existing spur for a consumer unit* and run another radial or two from there.

*loads etc permitting.

Or like a socket without pins in it. and you would happily extend* that radial to another socket or branch it and so on. it’s just a fused isolating switch.

It’s not a radial it’s a spur so you wouldn’t branch.

It’s not a radial it’s a spur

No yes.

It’s “fused spur” with a consumer unit on the other end.

Happy to be wrong but if that’s an actual branch/spur I’ll be damned. I’d wager its a “fused spur” in so much as its a fused in line isolator – a fitting that every man and his dog calls a fused spur – on a radial, and hopefully on its own.

If it is actually a spur/branch you’re quite right, you don’t extend that ad infinitum, but the consumer unit at the far end suggests that bridge has already been burnt if it is.

Taking a branch from that feed in theory isn’t an issue (though given the consumer until etc I’d be mindful of the load, RCAs and so on)

Just noticed though the op mentions sockets and lights from it which, regardless of what form the current circuit takes is based and wants a dedicated feed, to a consumer, then split to lighting and sockets.

Happy to be wrong but if that’s an actual branch/spur I’ll be damned. I’d wager its a “fused spur” in so much as its a fused in line isolator – a fitting that every man and his dog calls a fused spur – on a radial, and hopefully on its own.

If it is actually a spur/branch you’re quite right, you don’t extend that ad infinitum, but the consumer unit at the far end suggests that bridge has already been burnt if it is.

TBH it’s completely unclear what it is – original installation was three random internal twin and earth cables coming out a hole in the wall and run up the garden to the outbuilding, another to the shed and no idea where the third went. There’s a fused isolation switch which ‘controlled’ those cables but no idea how they actually relate to the main consumer unit in the house. There’s definitely no separate circuit for the outbuilding in the main consumer unit.

The internal wiring running externally was replaced recently with proper SWA and correctly terminated in external enclosures and earthed either end, and the other random cables isolated within the house-end enclosure.

The consumer unit at the other end has been roundly criticised by another electrician I had in on an unrelated job – he said it was a cowboy job and completely unnecessary. It also mixed MCBs and RCBOs which is apparently a no-no, so it’s not like it’s in any way ideal.

The problem with running a circuit off that consumer unit is that it needs the driveway excavating, and having done that once recently for the SWA installation, I’m not keen to do it all again!

The problem with running a circuit off that consumer unit

It would seem counter intuitive to do that – the thing to do would be replace the current isolator with a consumer unit, connect the existing shed to a “30A” on that, a lighting circuit, and a mains circuit for the garage from there (I take it the existing isolator is in the garage hence the desire to branch that?).

Buuuuut…

. There’s a fused isolation switch which ‘controlled’ those cables but no idea how they actually relate to the main consumer unit in the house. There’s definitely no separate circuit for the outbuilding in the main consumer unit.

Trace and get rid of that regardless of anything else.

Run a nice big dedicated cable back to the main consumer unit for the house and put that on a dedicated “50A”

-actual mcb size as appropriate.

(how big is the existing T&E feeding the isolator, you may find it’s big enough and long enough to get to the consumer unit. From what you’ve said there’s a good chance whoever did it in the first instance has left a coil of it under the floor because they couldn’t be bothered to drag it to the consumer unit and simply branched off the sockets somewhere.
There’s also the chance given what you say they used 1.5mm or silly string or something because that’s what was to hand.
I’ll lay odds it’s the upstairs sockets because it’s often easier to get under the floor than downstairs.)

Normally you can’t run a spur from a spur as you risk overload, the exception that is when the spur is fused as the fuse protects from overload. All spurs must be after the fuse and you still risk overloading the circuit but the fuse will blow so its an annoyance rather than dangerous. How big a deal that is will depend on what you use in the shed/garage. A battery charger and a light, not an issue, a welder and a kettle more so. Not neat or tidy or good practice but as far as I am aware not against regs. Also not notifiable in England, not sure about other nations as they have different rules.

That said it sounds like you have a bit of a mess so I would definitely want to to do some investigating before adding more mess.

Can you run a wire into the garage, then run from the garage to the shed? That would be a bit tidier without too much work.

what dangerourbrain said

lose the existing fused spur, find where it taps into your house ring mains, remove those taps

get a proper feed from your main unit out to the smaller unit in the outbuilding

add some proper rings or radials from that outbuilding unit to whatever lighting & sockets you need

For a cheap, quick solution, yes, you can take a feed from the output side of the current spur.The output, NOT the input.
The spur is there to only supply 13 amps maximum, from the current spur off of the existing circuit.
If the existing circuit is a 32amp ring, then the spur will quite happily supply 20-25 amps, but there isnt a commonly available 20amp fuse, so the suggestion above to fit a small consumer unit is a decent solution, then have all sockets off a 16 amp circuit breaker, and the lights off a 6 amp breaker.
If the existing circuit breaker supplying this circuit is 20 or 25 amp, then it will be no problem branching off wherever you want, as the cable, which I am assuming is 2.5mm T+E, will be protected against overload.
If there isnt much else connected to this ciruit, and it could run a maximum of 20 amps, then just change the circuit breaker to a 20/25amp version, and you can spur off a spur and spur off again as much as you want.
More info will be required to give a more considered approach, if you give me those details I can advise you.