4 core SWA cable

[turningtim]

Hi All

Just found that our garage (separate from the main house by about 20m) is fed by 4 core 2.5mm swa in a 50mm duct about 350mm under the lawn from a 40A mcb in the main house CU Sounds good so far but all 4 cores run to a 2way CU with a 30A mcb on one pair and a 6A mcb on the other two . 30 runs a ring main (4x 2gang 13A sockets) the 6A a lighting circuit for 4x twin florescent tubes. Earth bonding uses the armour in to metal glands and on to the bus in each CU in 4(?)mm.
Is that OK? or should the 4 cores run back to the house CU as a ring main with 2 more for the lights or ....what? Any suggestions?

Thanks

 

[mikeyd]

It could be OK. Is there anything that looks obviously wrong?

Photos always help..

 

[333rocky333]

2.5 swa is not rated at 40 amps for a start.

 

[HullMarc]

2.5 swa is not rated at 40 amps for a start.

This. Maybe whoever installed it meant to double up on the cores, giving 5mm², but didn't bother/someone made modifications somewhere along the line.

OP, the cable runs directly into the MCB? Where's the neutral?

 

[ban-all-sheds]

Surely 2 cores come from the MCB, and 2 from the house CU neutral bar?

So what the OP has is a 2.5mm 40A radial - not good. OK - you can argue that the branch supplying the lights is OK as the 6A will prevent overloads and the 40A is OK for faults (TBH that's a guess though).

But the 30A branch is not OK.

Fixes would be to replace the garage "CU" with one that has a switch, and supply it through both SWA cores in parallel, or replace the 30A breaker with a 20 or 25A one. Both notifiable.

Tim - where's the RCD protection?

 

[1john]

how about putting the two cores into the supply side of the 30A MCB with a link over to the 6A one. Do you have RCD protection??

 

[turningtim]

Just to clarify 2 cores from the 40A mcb in a split load CU in the house (thats where the RCD protection is) and 2 from the neutral bar. The two neutrals go to the neutral bar in the garage CU, the two others to a 30A and 6A mcb. There is no live bus in the garage CU so each MCD is fed from the SWA cores, which looks like a bodge as the islolator switch does nothing, not even connected. Then to a ring main from 30A and normal radial lighting from the 6A

Is the best plan then
Pair the cores up and use the isloator
reinstate the live bar properly
Reduce the house CU mcb to 30A (or 20A)
Leave the ring and lighting as is or should that MCB always be less than the one in the house.

or remove the garage CU run the SWA as a ring from the house CU and pull another length in for the lights (or spur from the ring perhaps?)

Thanks again one and all

 

[ban-all-sheds]

Pair the cores up and use the isloator
reinstate the live bar properly

Yup.

Reduce the house CU mcb to 30A (or 20A)

No need - the 2 x 2.5mm² cores in parallel will be fine on the existing 40A.

Leave the ring and lighting as is or should that MCB always be less than the one in the house.

Leave them as they are.

or remove the garage CU run the SWA as a ring from the house CU and pull another length in for the lights (or spur from the ring perhaps?)

No - leave the garage CU in place.

If you have space on the non-RCD side of your house CU you might like to think about moving the garage supply to that and either swap the garage CU main switch for an RCD or the two MCBs for RCBOs.

 

[ericmark]

How about change 40A MCB for 20A MCB?

 

[ban-all-sheds]

Or make it a 32A and do away with the one in the garage?

But surely the favourite is to try and get it to a conventional design of an MCB in the house supplying a normally wired CU in the garage with breakers rated lower than the house one?

 

[Spark123]

Is there a single point of isolation in the outbuilding?

 

[ban-all-sheds]

No