Garage supply advice

[GETWIRED]

Hi, just after some advice/opinion on a garage sub main.
basically my customer got a detached garage half built as they couldn't afford it in the end but wishing to continue it 8 months on, -I'm assuming- the builders spark has put a 6mm 3 core swa from the garage buried up to the house. The client has asked me to basically get the cable to the c/u and connect into their existing c/u in the house an out a garage c/u in the garage, nice and simple, but.

The consumer unit has one spare way on the rcd side. As the cable is run externally it needs to be protecte by an RCD doesn't it? But the metal garage cu's use a 30ma RCD as a main switch, wouldnt these rcd's not interfere with each other?

also any opinions on putting a 6mm 3 core swa at a length of 15 meters, - 5 metres run externally cleated and 3 metres buried, the rest through dwelling - on a 40a mcb? The house is a tn-s system. I'm asking as I normally wouldn't budge from using a 32a. Creature of habit.

cheers in advance fellas

 

[Octopus]

So your calcs are based on an assumed cable size? Dangerous way to start a design!

 

[davesparks]

SWA run externally needing an RCD? Which regs book did you get that from?
RCD's don't interfere with each other, but the one in the garage would be pointless and probably never operate before the one in the house.
As for the cable size, have you calculated the size required using bs7671 or just taken a guess at the rating of the 6mm?
Why would you insist on using a 32A mcb with 6mm cable? What if the calculations say you need 6mm for a 20A supply?

 

[HappyHippyDad]

Welcome to the forum..

I'd first of all make sure it is 6mm, I often use 4mm for a garage as I think thats fine for a few lights and sockets.

If its SWA then it wont need RCD protection in the house IF the joint from the SWA to T&E (in house) is not buried in the wall, so you're fine to have a normal MCB in the house CU.

From the tables 6mm SWA is going to easily handle 40A, although it would depend on the reference method in the house for the T&E supply going to the CU. Will it be surface mounted? In trunking? If it is just sockets and lights in the garage then I would have thought a 32A would be fine anyway. What is going in the garage?

 

[davesparks]

If its SWA then it wont need RCD protection in the house IF the joint from the SWA to T&E (in house) is not buried in the wall, so you're fine to have a normal MCB in the house CU.

What T&E? The bloke is doing it properly and running the SWA to the CU.
Adding unnecessary joints into a new cable is just creating a potential point of failure.

 

[GETWIRED]

My bad grammar confusing things, I'm assuming the builders spark put it there not the cable size. I know the cable size is definitely 6mm. No I haven't taken a guess, the client has insisted they want a 32a ring and obviously lighting in the garage, so would need at least 40a supplying the garage board, since there is a 6mm cable already there then I'm asking if it can be used. I cannot refere to my regs book as that's on my van and I'm working away. My work is primarily domestic, so when I run a 6mm cable it is usually t&e for a cooker or a hob lol, an wouldnt need a 6mm armoured to run 20a.

 

[HappyHippyDad]

What T&E? The bloke is doing it properly and running the SWA to the CU.
Adding unnecessary joints into a new cable is just creating a potential point of failure.

He just says its 'buried up to the house' and he has to 'get the cable to the CU'. I think without hearing more its making a pretty big assumption that the previous electrician has (a) brought the SWA into the house right by the house CU, (b) has left metres of SWA to reach the house CU if it is not brought in near the house CU, (c) the house CU is metal (which it may well not be).

 

[davesparks]

Then stick to what you know and get someone who knows what they are doing to do it.

You'll find the necessary info in the in-site guide too, which you will presumably have with you?

Why does a ring and a lighting circuit necessitate a 40A supply?

 

[Octopus]

I would be asking the client what they plan to use in the garage so you can do a proper assessment!

 

[GETWIRED]

And I'll be honest, regarding swa being on an rcd when run externally, I've no idea where I've got that from... Maybe another spark said it and it's stuck. I did double check with you guys which did show I was unsure. Like I said I've no regs book to hand until I'm back home

thaks for the advice so far fellas

ill try and post the picture of my OSG... Bag of ****e full of errors.

 

[Marvo]

I'm going to close this thread, it's a bit too close to being a step by step guide which we strongly discourage although I think you already have the answers you were looking for. Getwired, if you've got a more specific issue you'd like to discuss please feel free to make another thread.