Garage wiring

[benabbyg]

Hello, just joined as this looks like a very useful site!
Straight in with a question....
My garage currently has 3 double sockets in series which is a spur from the kitchen ring. I'm about to add another 2 double sockets in series. I was going to fit a 13amp RCD fused spur to the 2.5mm cable as it enters the garage. It would be fitted before the first double socket. Total length of 2.5mm cable is about 25mtrs from the kitchen junction box.
I'm wondering if the fused RCD spur will give me good safety protection from a potential cable fire and shock. These sockets would not be used all together, the most would be 2 or 3 sockets for simple devices such a the drill charger, small bench light, small car jet wash etc. No heavier users such a tumble drier, freezer or kettle.
I'm always being told by friends to fit a garage CU, but hoped that the 13amp fused RCD (which I've got in my spare box of bits) would be sufficient safety. I don't know any electricians to ask and can't find any books to help with my concers.
Any advice would be very much appreciated
Thanks! Phil

 

[securespark]

If a 2.5mm² spur from a ring final is all you have, then 13A is the maximum load you can place on that spur.

 

[rebuke]

If you've currently got 3 sockets on a spur, without a fused connection unit, then your installation is dangerous, as you could draw more than the rated capacity of the 2.5mm cable in the shed, causing it to melt and potentially catch fire...

 

[sparkyspike]

Is your kitchen ring not already protected by an RCD? If so, no need to use an RCD spur, just a normal fcu. If you have no RCD protection on the kitchen sockets, then consider fitting one, which will cover the garage too.

Is the garage connected to the kitchen? If so, you may wish to extend the ring to include the garage sockets, giving you more scope for use.

 

[NoSparks]

The kitchen ring is a separate ring because of the potential loading from such as washing machines, dish washers, irons, kettles etc. Under normal operation with realistic diversity, a kitchen ring can quite easily approach 20 Amps. Taking the garage service from the ring may well cause the final circuit to overload, even if you do not load it too heavily in the garage.

Assuming you have satisfied yourself as to the kitchen ring loading you must then consider the voltage drop to the garage. What is the R1 value for the kitchen ring and therefore the voltage drop to the FCU for the garage spur? Once this has been determined you can start calculating the voltage drop to the garage assuming say just a 13 amp load. 25m is an awful long way for a 13amp circuit on 2.5mm cable. (I don't have my OSG to hand to do the calculation for you) Now is the total voltage drop within the 3% allowed for lighting and the 6% allowed for other purposes? If not you might find your lighting failing, or your drill burning out!

If the voltage drop is within spec, then the FCU should be at the kitchen end of the spur, not the garage end since otherwise the cable from the kitchen to the garage would not be properly protected.

Finally how does the cable get from the kitchen to the garage? If there is any open space between the two is the cable run overhead or underground. If overhead is it high enough? if underground is it properly protected?

I guess what I am saying is that you need to do a proper design, and not just throw things together.

 

[benabbyg]

Thanks for all the advice. It's not worth me risking a fire! Did like the idea to extend the kitchen ring to include the garage, but the garage is about 3mtrs from main house and thought this would cause even more work. Got the option to run a new cable to the main house CU, so maybe this is my best option, but will become a big job!
Going to get an Electrician round to see what I need.
Just glad I asked you first before it was too late! They do say ''little knowledge is dangerous''......so true when it comes to electricity!
Thanks again! Phil