Question re SWA cable

[andemz]

Hi there. I have phoned up a company today in my local area and have asked them a couple of questions. I want to put them to the sparky's on this site.

What size cable SWA would one need for a 30 meter run, protected on the rcd side of a consumer unit. It will be intended to run into another consumer unit a crabtree or MK in what will be a garage converted granny flat and will power a small kitchen, living room bedroom and bathroom. there will also be a combi boiler in the accomodation. The SWA will be buried in a trench bricks either side with slabs on top to protect the cable and with black and yellow tape on top of the slabs.

The Sparky stated that i will need 16 mm2 T&E SWA RCD protected by 32 MCB.

Would this be right. I'm not to concerned with the burying of the cable at this stage as i understand this well. The reason that i am asking is because when i did the calculations on the tlc-direct website it came up with a different answer completley the calculator stated 4mm2.

Please enlighten me.

 

[plugwash]

if the garage is going to be a living space haveing it on a rcd in the house is insane

the tlc calculator asks to to specify maximum power....
personally for a submain (which is what this esentially is) i woudl always use the power represented by the maximum current the breaker can take

i would probablly go for a 40A breaker in the house
while the TLC calculator says 6mm armoured would be ok for this it is pretty close to the limit on volt drop and doesn't leave much for final cuircuits

there is also the issue of main bonding which should really be connected via an earth run of at least 10mm

i would go for 16mm 3 core armoured though for a few reasons
1: an electric cooker may be wanted in there in the future
2: it keeps your sparky happy
3: you really only want to dig that trench once

 

[andemz]

thanks for the post. I've noticed you post quite a lot on here.

I agree that this is a submain and have done a bit more reading an concur with you re the 40 A breaker. The voltage drop is an interesting point that you have raised, the sparky didn't even mention that.

Why do you say that having it RCD protected is insane? and

When you talk about main bonding are you saying to run also a 10mm earth cable from the house main (earth rod) to the other accomodation? or from the garage accomodation earth to the earth rod?

Just out of interest i will obviously get a pro in to do the work, but this is about me learning and understanding the processes.

 

[plugwash]

i mean any pipes leaving the garage need to be main bonded in the garage just like pipes leaving a house do

every building should have a point where all its main bonds connect to in this case the earth bar of the garage CU is the best place

hmm you didn't mention you had an install with an earth rod this changes the rcd issue somewhat

i presume your main CU has a main rcd (100ma type S) and a normal 30ma rcd covering sockets

if this is the case then you should come from the side with the 100ma type S rcd and in the converted garage you should fit either a normal split load CU or use a switch only CU and put sockets on a RCBO

 

[andemz]

i see what you mean. You have helped me a lot by simply explaining those few points for me. Thanks for your time and effort

 

[ban-all-sheds]

Why do you say that having it RCD protected is insane? .

Cos then you risk the problem that your Dad had with your Gran's wiring - problem her end causes his house to trip out...

 

[andemz]

yes you are right, you remembered well.

I suppose that this can be looked on as a negative and or a positive. It trips out at the old man's end indicating that he needs to attend to a situation, otherwise as she is now registered blind she would be then left for a while.

I guess though that if all things were fine then this could be a nusiance.