Protecting a 2.5mm spur

[motorbiking]

Some time back electricians installed a whether proof box with what I thought was an external radial using a 2.5mm cable running from the consumer unit (supposedly to a 16A MCB). This was to allow me to connect a 2.5mm underground armoured cable to my garden room.

Unfortunately it doesn't look like the radial cable was ever connected to the consumer unit and is probably lost somewhere under the floor. I've therefore fitted a new whetherproof box at the back of an internal double socket on the main and extended the main through to the box, where its connected to the 2.5mm armoured cable. The garden room already has a 5A fuse switch for the lighting and ring main for the sockets, but because I was expecting the radial to be protected by a 16A MCB, there is no protection for the 2.5mm single armoured cable.

I therefore need to put a 16A fuse in somewhere. Is the easiest option to put a mini garage consumer unit in the garden room and re-wire the ring and lighting in to a 5A and 16A MCB, or is it better to protect it at the point the spur meets the ring?

There are too many sockets on the garden room to leave it to chance, though its highly unlikely I'd get anywhere near 20A.

thanks for advice

 

[ban-all-sheds]

I therefore need to put a 16A fuse in somewhere.

No you don't.

You've now made it a spur from the house socket, so you need to start it from an FCU with a 13A fuse.

That you did not know that is an unequivocal sign that you should not, not, NOT be doing this work.

 

[motorbiking]

Sorry, I meant MCB not fuse, but I now realise that my "garage consumer" idea will be over the cable limit with a B16 MCB + B6 MCB. I'll keep it simple and put a 13A FCU on the incoming garden room supply.

I think I have some of these type RCDs kicking a round..
http://www.screwfix.com/p/british-general-13a-rcd-fused-spur-white/8250p

With regards getting a qualified electrician. Yeah been there done that, ended up with a dead cable.

 

[winston1]

If the circuit you are spurring off is already RCD protected at the CU you DON'T need or WANT a second one.

 

[ban-all-sheds]

What about the testing required before and after the cable to the shed is energised?

 

[ericmark]

When I found a socket was already a spur rather than part of a ring, I used grid system to allow fitting a 13A fuse and retain a socket at the original location.

The ring final system permits items to be fused to a maximum of 13A, it does not allow 22A even if the cable is rated 22A, the problem with the ring final system is it forms parallel circuits of unequal length, so it is possible to draw over 22A on one leg and under 10 amp on the other, as a result although it does not completely stop this from happening the devices taken from a ring final are limited to 13A.

It is possible in the centre of the ring you could draw 32A without causing an overload, but the problem is others would not realise what you have done, and although it may be the centre when you install it, you don't know if it will remain centre.

That said as the designer you can design that's why you are considered as the designer, but you would need to be able to show you had the skill, and to put it bluntly you don't, so you are stuck with a 13A limit.

I have argued with my son over if the designer should be considering what others may do in the future, however this a just a debate, in real terms doing something which others will not expect is asking for problems.

 

[motorbiking]

The simplest option may be to remove my weatherproof junction box and replace with one of these, giving the added benefit that I can leave the roof downlights on and switch the whole garden room off from the house end.

regarding testing - my brother qualified as an electrician and has the gizmo to do that. He's just not local or willing to issue paperwork, given that my paperwork already covers it.