Garage supply

Could you not go out through the back of the house consumer unit and around the outside wall of the house to the garage?

Even if the 4:00mm theoretically doesn't need a OPD for connection to a 32A ring, I would at least fit an isolator at the origin.

A possible option - If the part of the ring that you are tapping onto is about half way round the house, install a fused spur and limit the power to the shed to a 13A BS1362  fuse. 

There is nothing wrong with this design. An isolator inside the house is a good idea for the times one forgets to turn off the lights, or in my case usually the compressor, that turns on when one is in bed!

Unfortunately not as it is a terraced house. The only route from the board is up through the attic and drop back through the roof at first glance. 

This is the other option considered. However I’m uncertain of the wiring plan and what exactly is going in there. Nonetheless this is a helpful piece of advice so thanks a lot for that ??

Really, it all hinges on how much load will be used in the garage. A few lights and a double socket for the lawnmower once a week is no worse than adding the same socket indoors for a desk with sewing machine and a table lamp. Fed from the ring, please with Isolation indoors, either as a fused spur or a double pole switch, is then perfectly OK.  It can then be isolated if there is a problem.

there may be a 6A breaker and a 16A, but the load  maybe 13A or less .. (does it even need a CU, could it be just sockets with the light switch as a fused spur ?)

But if the load is significant,  perhaps a DIYer /  car enthusiast doing welding, using air tools, running a fan heater  lathe whatever, or even a washer and tumble plus that lawnmower, then the picture changes, and the risk of problems increases. If there may ever be a car charge point added, it all changes again and we worry about TT and earth rods.

If done from the ring for starters, perhaps do it in a layout so the end of the cable is available for a later upgrade  to a dedicated 32A radial if ever need be. At planning stage, that requires not much more than an over sized  box and maybe some glands.

Mike.

If the circuit supplies a CU, then it's no longer a ring final circuit by definition.

Agree with what others have said regarding loading - if that is OK, then there probably isn't a safety issue.

BS 7671 has really tied itself up in knots with the definition of final circuit it uses.

gkenyon: 
 

If the circuit supplies a CU, then it's no longer a ring final circuit by definition.

 

That started a thought experiment in my head:

Starting with a ‘ring final’ type circuit - you could  fit a FCU on the ‘ring’ then feed two (or more) further FCUs from that FCU (if you wanted a ‘master’ switch, and then further switching of additional equipment which required fusing at a lower level).  Electrically, that's pretty much the same as feeding a 2-way CU from the ring if a FCU is used.  Is this arrangement then no longer a ‘ring final’ circuit, and if not then how does it differ fundamentally from feeding 2 x twin sockets from an FCU on a ring - the fuses just end up in the plugtops and not the FCUs?

I'm not saying you should do this by the way…….

A ring final circuit can have multiple fused and unfused spurs, with unfused spurs feeding multiple outlets and ‘look’ very much not like a ‘ring’ by the time you have finished with it!

Jason.

Agreed - and by the definition of final circuit in BS 7671, a circuit downstream of an FCU where the branch cable can't be protected by a B32, is effectively a final circuit in its own right …

The differentiator is required, of course, because of the way that Reg 433.1.204 is written, basically if it's not a ring final circuit (which a distribution circuit can't be) then protection against overcurrent for BS 1363 accessories is all down to the designer, you can't rely on the “standard circuit” acceptance because it's no longer a ring final circuit.

I'd not get too hung up on that, indeed it is also possible to have a  spur from a ring that is actually a 16A RCBO, and achieve a circuit performance that is better protected than the ‘standard’  13A fused spur with a real fuse, but the RCBO is not in the back of the OSG as a standard configuration either…

Far more important is to convince yourself that the load distribution will be OK, if you like to assume  that the same piece  of wire can be both final cct for the sockets on the house ring, and in the same breath also a distribution cct for anything downstream of any fuse or circuit breaker fed by that ring, that is fine, but  it does not suddenly absolve you from combining the currents of the two functions when thinking about cable heating, and thinking about total voltage drop to the far ends including losses in all stages.

Arguably ‘the full circuit’ supplying load X ought to include the substation secondary where the electrons are excited, street mains, house fuse etc as well. If it helps pretend  you could tag one electron and follow it right round the loop, anything else is not actually a compete loop, so non-circuital. 

The last leg to the load  is only a final sub-circuit, and the rest are mid sub-circuits.. No-one bothers with that level of detail, it rapidly becomes pedantry for the sake of it.

Mike.