Wiring up a Shed

Armoured cable (if running anywhere externally) from a spare breaker in existing board. Make sure this breaker is less than the max sustainable load for the cable.
New board in shed with separate fuses for a ring and lighting circuit. A new metal bodied garage consumer unit is about £30, you could probably get an old stock plastic one for less but it won't comply with current regs (no less safe though in my opinion)
Don't forget the armoured cable needs the external wire protection earthed.

No reason you couldn't do it the way you have suggested though as long as adequate protection in the way of fusing is in place. You would need to run the wire to the fuse switched lighting spur in the same thickness wire used in the main feed to the shed, don't be tempted to feed the spur with the 1mm sq you would normally use for a lighting circuit.

Thanks both, I'm wanting a fully compliant solution.

Did consider extension cable option but most of the effort is running/protecting the cable. Once that's done the incremental effort to do a proper job is minimal. Where cable is external I can contain it in Galv trunking so assuming I don't need SWA in that case. Trunking will also be used to carry extension of the House alarm system.

As mentioned above, the ideal cable to use would be an armoured cable (SWA), you could use other types of cable provided they are adequatly protected and also if exposed that they are UV stable. Putting a cable in galvanised trunking would work but you need to be careful that it doesn't end up full of water.

Personally I would use an SWA whether buried or cleated to a wall/fence etc.

Next you need to consider protection to the circuit, you must make sure that the circuit breaker feeding it is rated at less than the cable, this is where it gets tricky, there are many factors that affect cable rating so you need to consider not just what cable you use but how you install it. For example if you use a 2.5mm2 PVC SWA its rating would be 28A if clipped direct, 31A if on a tray or in free air and 29A if buried. Other types of cable will of course have different ratings.

You also need to consider the grouping of the circuit (i.e. how many there are together, whether is passes through any thermal insulation, what the ambient temperature of the spaces its running through are (For example if it runs with the heating pipes and they arn't insulated you need to consider this).

This all sounds complicated and it is but in general terms if you use a 2.5mm2 PVC/SWA (ideally 3 core with the third core as earth) and it runs on its own (not grouped), no thermal insulation and no ambient temperature issues they you should be fine with a 20A MCB (I would use a type C if you can).

You can then run this to a socket in the shed and provide a spur fused down to a maximum of 10A (preferably 5A or 3A) and wire a light in 1.0mm2 or 1.5mm2 twin and earth.

Unless the run is really long or the load really heavy you shouldn't have any issues with voltage drop with a 2.5mm2 (I used this for my garage and the cable is around 30m in length and I have no problems running welders off it.

You also must ensure that the supply is protected by a 30mA RCBO or RCD.

On final thing which won't apply to you but will apply to others is that if you have a steel framed building or have other incoming services to the unit (gas or water for example) and you main building/house supply is PME (will usally be written/labelled on the cutout) you must not connect an earth from the house to the outbuilding and instead must connect the outbuilding building as a TT supply which means that you have to isolate the incoming earth, provide your own earth rod and provide an RCD on the incoming supply to the outbuilding.

Thanks homersimpson really helpful.

Sean

As others have said, if its run externally I would be looking at a steel wire armoured cable, then you need to consider the total maximum load on the circuit, the method of installation of the cable and the length of run.
Its not as simple as just banging in a 2.5mm2 T&E and flicking it on.

Solar panels on the roof?

I won't quote it, but hompersimpson's advice is very good. He obviously knows his "regs"

You can do what you want in your own property, however there may be issues when you come to sell it as obviously the work isn't certified. The way around this to reassure any potential buyer would be to get a periodic test carried out which would say the values are correct but obviously they can't comment on wiring runs or if you have done something daft like spurring of another spur or powering the electric shower using a 2.5mm cable etc.

Also if the place burns down with no apparent cause it's always listed as an electrical fire which may give your insurers a get out if they are made aware of any non compliant work.

Despite what is widely talked about no electrician is able to sign off sometime else's work without the whole install coming back on them, There's no option on the form to say I just tested and certified this.