New Garage install from SWA ring circuit....Help!

[Sparkylight]

I have a potential customer who wants me to rewire hisgarage – existing install is in a bad state with a very old DB.
He currently has 2.no 2.5mm, 3 core SWA cables feeding powerto the garage DB (in a ring circuit) from a small DB (with RCD protection) nextto his main consumer supply. The distance between garage and the consumer boxis approx 50m.
The power requirements for the garage are: garage door,fridge/freezer, dryer and at least a further 3.no double sockets (foroccasional hand tool use), and a lighting circuit.
The SWA cables also feed 2.no outside sockets in his garden(one on each cable) - which are occasionally used for the electric mower /strimmer or pressure washer.
I have asked him to replace the SWA’s or install an additionalone but he is reluctant to do this as he has recently landscaped his garden andinstalled a new kitchen - the existing cables are run under floorboards /concrete in the house and underground in the garden.
I have carried out an insulation test on the cables and bothwere fine.
The supply to the house is TNS.

1) Would I be right in thinking that as the supply to thehouse is a TNS system, I could effectively make the garage a TNS system throughthe 3core SWA cables and I would not need to install an earth rod?

2) Would two 2.5mmSWA cables in a ring be ok to feed a DB inthe garage?

3) As the internal DB has RCD protection, would I be rightto only install MCB’s in the new garage DB?

4) If I was to get my work inspected at a later stage couldthere be any earthing issues or Regs which might not be covered by supplying aDB through a ring circuit?

5) I plan to install 1.no 6A radial circuit to lights and 1.no16A ring circuit for sockets and garage door.
Current carrying capacity of 2.5mm SWA (ref method D) is - 24ampsper cable, voltage drop approx 3% (per cable) so was planning for 32A RCBO (at houseDB) – any issue with this?

Sorry… I know I’m asking a lot, however I’m a new sparky andthis one is not straight forward!
Thanks

 

[Spoon]

Hi Sparkylight.

1. Yes.
2. I take it that the DB in the garage is actually wired in 2x2.5mm SWA cables run in parellel?
3. If the customer doesnt mind walking 50 meters back to his house to reset it if the house RCD trips. You could install one in the garage but make sure it has discrimination from the one in the house.
4. There is nothing wrong with wiring a DB in parallel, as long as its to the regs.

Are the current SWA cables wired up to the regs and are still suitable for the install?
I take it that you know all about the regs and are just having a brain freeze????

 

[Engineer54]

Hi Sparkylight.

1. Yes.
2. I take it that the DB in the garage is actually wired in 2x2.5mm SWA cables run in parellel?
3. If the customer doesnt mind walking 50 meters back to his house to reset it if the house RCD trips. You could install one in the garage but make sure it has discrimination from the one in the house.
4. There is nothing wrong with wiring a DB in parallel, as long as its to the regs.

Are the current SWA cables wired up to the regs and are still suitable for the install?
I take it that you know all about the regs and are just having a brain freeze????

Both your 1. and 2. doesn't apply to this installation as far as i can see. If there is one garden socket out on each side of the garden then these SWA are not being run in parallel and don'tfollow the same route. Far more likely, that one SWA cable is going to be substantually longer than the other!!

 

[Sparkylight]

Hi and thanks Spoon,

Yes, SWA cables are in parellel - with a garden socket installed on each (comeout of ground at two points)
Have checked cables, where I could, and found no issues with regs - but wanted to make sure i wasn't having a brain freeze ! - have to take customers word that they are buried over 600mm deep!

 

[Engineer54]

Hi and thanks Spoon,

Yes, SWA cables are in parellel - with a garden socket installed on each (comeout of ground at two points)
Have checked cables, where I could, and found no issues with regs - but wanted to make sure i wasn't having a brain freeze ! - have to take customers word that they are buried over 600mm deep!

How is that then?? Do you know what run in parallel means?? From your description this is basically a ring circuit, both cables taking separate routes to the garage.

 

[Sparkylight]

Hi engineer 54,

Cables run down both sides of rectangular garden and seem to be approx the same length (give or take a few metres)

 

[Sparkylight]

yep - i did say it was a ring

 

[DieselShunter]

This is currently a 2.5mm RFC with a couple of outdoor sockets on it, and a CU in the middle of it. So how are you going to describe that circuit? As a RFC or as a sub-main/distribution circuit?

My suggestion is to remove the garage CU and replace it with a sufficient number of sockets, and with an FCU for the lights, and possibly another FCU for the door actuator, and in so doing maintain it as a ring. Then protect it with the 32A RCBO as you intended, and call it "Outdoor Ring" or "Garden/Garage Ring".

As far as the earthing is concerned, so long as your worst Zs on that ring is better than the necessary 1.44 ohms required for a 32A circuit, you'll be OK. The cable R1+R2 should be about 0.4 ohms at mid way, so it should be within spec. If not, then it would imply the Ze is higher than the allowable 0.8 ohm allowed for a TNS.

Edit: If the R1+Rn (which shold be the same as the R1 +R2 in this case) is greater then 0.36 ohms then the RCBO should be derated to 20A to ensure the Vdrop is in spec for a 100m RFC.

 

[Engineer54]

Cables run down both sides of rectangular garden and seem to be approx the same length (give or take a few metres)​

Then the cables do not meet the criteria of a parallel supply, as one line (as i suspected) is longer than the other, as they are not following the same route. Also, a parallel circuit should run from point of supply (cu) to point of new sub CU . It doesn't have 2 other points of utilisation along the way, ...ie, the 2 garden sockets.​

 

[Guitarist]

Why might you be getting your work inspected at a later date?

 

[Spoon]

Both your 1. and 2. doesn't apply to this installation as far as i can see. If there is one garden socket out on each side of the garden then these SWA are not being run in parallel and don'tfollow the same route. Far more likely, that one SWA cable is going to be substantually longer than the other!!

I will have to switch my brain on more next time EG54.
I just was the "He currently has 2.no 2.5mm, 3 core SWA cables feeding power to the garage DB (in a ring circuit) from a small DB (with RCD protection) next to his main consumer supply. The distance between garage and the consumer boxis approx 50m." and took it that there are 2 cables going from a DB in his house to a DB in his garage. Reading it again, Im not quite sure how there cables also feed the two outside sockets.

 

[telectrix]

from what you say, the SWA might be 2 radials fed from the same MCB. do a r1,r2,rN to see if you have end/end continuity. if not, then you'll need to bridge across the open ends to make a ring or reconfigure as 2 radials, considering the reduced CCC.

 

[Sparkylight]

Yes, Diesel Shunter...thats prob the best way forward, I will carry out full survey to check readings

Thanks:smug:

 

[Sparkylight]

from what you say, the SWA might be 2 radials fed from the same MCB. do a r1,r2,rN to see if you have end/end continuity. if not, then you'll need to bridge across the open ends to make a ring or reconfigure as 2 radials, considering the reduced CCC.

I did check this and it def is a ring - thanks though