Ring to Garage/ workshop.

[marcus1507]

hi all,
I am in the process of wiring electric to my garage about 30 foot from the back of the house.

i have used 10mm twin and earth to go from the consumer unit under the floor to the rear of the house. from here i was going to use a double pole switch to meet up with the armoured cable. the problem is i have been given a large quanity of 2.5mm SWA that i want to use though i am worried it will not be large enough.
can anyone tell me what sort of current i can safely expect to draw if i ring the 2.5 SWA from the switch to the garage consumer unit and back to the switch in the house.

All comments greatly recieved.

 

[securespark]

Hi Marcus

I think you're attacking the problem from the wrong end. When I talk to customers who want a garage supply, my first question is, "What do you want to use in there?"

If all you want is a light and occasional use of a power point for a garden tool, then 13 Amps is fine. If you want to use freezers, washing machines, tumble dryers etc, then you will need more.

If you want to put in a more generous supply, you could do worse than a 32 Amp radial circuit feeding a small consumer unit in the garage.

Given your choice of cable (10mm2), that is good for 60+ Amps and the distance (9 metres) is not too great so voltage drop is not a problem.

Basically the longer the distance and greater the current the more voltage drop you get, and if you get too much, you have to increase the size of the cable.

As for the idea of a ring, I suppose it would work, although I normally wire a radial to the garage then branch out from there.

A 32Amp ring served by a 32A MCB cannot cover greater than 100m2, but in a garage unless it is a big one, that should be no problem.


Whatever you do, make sure that the supply is protected by a 30mA RCD.

All circuits that may be used to feed equipment used outdoors must be protected by an RCD.

 

[yankey-sparky]

Excuse my interruption, but this is probably the best place to ask. Marcus1507 wants to use the 2.5mm^2 wire he already has, so I'm wondering if he can just run three 2.5mm^2 lines in parallel to get the same ampacity as a single 10mm^2 line? Is this acceptable in the UK?

 

[BR]

No, it is not acceptable to simply twist three lengths of 2.5mmsq together at both ends and 7.5mmsq!. However he could if he had 4 or five core cable, have two separate circuits each protected by 25A MCB's.

Securespark, perhaps i have interpreted this incorrectly, but my understanding is that marcus1507 only has 10mmsq installed for part of the run, i.e. from the front of the house to the rear. I imagine he then wants to join his 2.5mmsq armoured hear and run it straight to the garage.

This leads me on to another question; in Ireland the max breaker we can use on 2.5mmsq cable is 25A (the cable being rated for 27A) regardless whether it is radial or ring. In practice socket ring mains are always protected by a 20A MCB. I gather the system is different in the U.K.

 

[marcus1507]

thanks for the advise. things i will be running are, a couple of strip lights, a compressor, drills etc. though on occasion i do use a small mig welder.
Cheers

 

[BR]

How big is the compresser?

 

[marcus1507]

it is a 3hp single phase compressor but does not run continuously.
i have now installed the cable under the garden lawn. i have used two lengths of 2.5mm armoured cable terminated at a junction box with the 10mm cooker cable. i plan to wire a small consumer unit at the garage end soon with RCD, 16amp and 6amp mcb's. i am sure that two lengths or cable will more than carry the load.

 

[securespark]

Hi BR

Yes, in the UK 2.5 can be used as part of a ring main on a 32 Amp breaker as although the individual current rating for 2.5 is 28 amp, you are effectively doubling up on the conductors, ie 5mm2 for live conductors and 3mm2 for earth.

Yes, I understood that the 10mm2 was to terminate at the end of the building, then 2.5 from there.

 

[securespark]

BR PS I have never yet seen an installed 25A MCB in my neck of the woods!!

 

[BR]

Should you wish to install a 2.5mmsq radial circuit, what is the max breaker that that would be protected with? Thanks BR.