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Hi all,

Would appreciate your thoughts on the following.

Customer has a garage CU fed by a 6mm cable from the main CU in the house. The cable exits the house and then is stretched overhead between the house and the garage. The cable is starting to sag and the customer wants it replacing. My initial thought was to replace with swa from house CU to garage CU going underground however, the main CU in the house won’t take swa and gland without pulling it all out and repositioning it, something the customer doesn’t want to do. So I’m thinking of cutting the cable where it exits the house and connecting to swa then dropping down underground and across to the garage. The CU in the garage has 2 radial circuits, 1 socket and 1 light on 32a and 6 a mcb’s respectively. I’m thinking that 6mm cable is way over the top for this demand and looking at 2.5mm swa to connect to the 6mm in a junction box under the roof overhang. The main CU is protected by a 16a mcb. It’s not the ideal solution but I think it’s safe and reasonable - any thoughts?
 
i.d still use 6mm SWA to allow for any future increase in load. make sure armour is earthed.
 
No way is 6mm over sizing the supply to another CU.
Why are you talking about a 32 amp breaker for a single socket??
Main CU protected by 16Amcb?
 
i.d still use 6mm SWA to allow for any future increase in load. make sure armour is earthed.
Thanks telectrix. Yes to earthing the armour although I think I’ve seen on the site somewhere that it’s not strictly necessary if using one of the conductors are cpc. Is this right?
 
Is there space to put the junction box adjacent to the CU? That way the box is inside and if the CU is replaced in the future the cable could be put into the new CU.
Hi R-fur, no the CU is positioned tight in a corner, looks very neat but difficult to get at plus the cables go outside almost immediately.
 
Hi all,

Would appreciate your thoughts on the following.

Customer has a garage CU fed by a 6mm cable from the main CU in the house. The cable exits the house and then is stretched overhead between the house and the garage. The cable is starting to sag and the customer wants it replacing. My initial thought was to replace with swa from house CU to garage CU going underground however, the main CU in the house won’t take swa and gland without pulling it all out and repositioning it, something the customer doesn’t want to do. So I’m thinking of cutting the cable where it exits the house and connecting to swa then dropping down underground and across to the garage. The CU in the garage has 2 radial circuits, 1 socket and 1 light on 32a and 6 a mcb’s respectively. I’m thinking that 6mm cable is way over the top for this demand and looking at 2.5mm swa to connect to the 6mm in a junction box under the roof overhang. The main CU is protected by a 16a mcb. It’s not the ideal solution but I think it’s safe and reasonable - any thoughts?
Do a
Hi all,

Would appreciate your thoughts on the following.

Customer has a garage CU fed by a 6mm cable from the main CU in the house. The cable exits the house and then is stretched overhead between the house and the garage. The cable is starting to sag and the customer wants it replacing. My initial thought was to replace with swa from house CU to garage CU going underground however, the main CU in the house won’t take swa and gland without pulling it all out and repositioning it, something the customer doesn’t want to do. So I’m thinking of cutting the cable where it exits the house and connecting to swa then dropping down underground and across to the garage. The CU in the garage has 2 radial circuits, 1 socket and 1 light on 32a and 6 a mcb’s respectively. I’m thinking that 6mm cable is way over the top for this demand and looking at 2.5mm swa to connect to the 6mm in a junction box under the roof overhang. The main CU is protected by a 16a mcb. It’s not the ideal solution but I think it’s safe and reasonable - any thoughts?
So a 32A in the garage CU Covered by a 16 A in the main CU, your maths are a bit out there me old China, I would think again if I was you mate. poor deign equals a poor job.
 
Do a

So a 32A in the garage CU Covered by a 16 A in the main CU, your maths are a bit out there me old China, I would think again if I was you mate. poor deign equals a poor job.
Hi Pete999, sorry if my post wasn’t clear, I was outlining what is currently in place not what I’m designing. The change I’m thinking about is a partial replacement of the 6mm cable with 2.5mm swa, partial because the main CU won’t accept swa in its current position.
 
Thanks telectrix. Yes to earthing the armour although I think I’ve seen on the site somewhere that it’s not strictly necessary if using one of the conductors are cpc. Is this right?
It's not strictly necessary to connect the armour of SWA to the cpc at BOTH ends, if there's a cpc conductor inside the SWA, but it is good practice to do so, and if only earthed at one end, that should be the supply end.
A far better job would be done if a 32A metal switch fuse was placed adjacent to the CU, and 6.0mm2 3C SWA run from that, all the way to the garage. RCDs for the circuit would be in the garage.
 
Yes to earthing the armour although I think I’ve seen on the site somewhere that it’s not strictly necessary if using one of the conductors are cpc. Is this right?
The armour absolutely must be earthed. No exceptions!

Sometimes the armour is uses as the CPC and then obviously it has to be connected soundly at both ends with a proper SWA gland. However, the armour resistance is usually much higher than the copper conductor (for SWA above about 2.5mm at least) and its capacity is generally not up to the (typically) 10mm copper equivalent needed if bonding extraneous parts on a TN-C-S supply.

If you are using a 3rd core as the CPC then you only need to earth the supply end of the armour, but it is good practice to earth both end anyway.

The exception being a feed for a TT load. Then the armour is earthed at the TN supply end, but isolated at the sub-board where a separate earth rod and RCD are used to safely provide the TT supply to the loads.
 
The armour absolutely must be earthed. No exceptions!

Sometimes the armour is uses as the CPC and then obviously it has to be connected soundly at both ends with a proper SWA gland. However, the armour resistance is usually much higher than the copper conductor (for SWA above about 2.5mm at least) and its capacity is generally not up to the (typically) 10mm copper equivalent needed if bonding extraneous parts on a TN-C-S supply.

If you are using a 3rd core as the CPC then you only need to earth the supply end of the armour, but it is good practice to earth both end anyway.

The exception being a feed for a TT load. Then the armour is earthed at the TN supply end, but isolated at the sub-board where a separate earth rod and RCD are used to safely provide the TT supply to the loads.
Thanks pc1966 I will be earthing both ends of the armour.
 
I won't comment further on getting the supply to the CU as it's all been said.
You probably picked up that the design of the CU in the garage is sounding odd. Are the two radial socket circuits on 32 amp breakers wired in 4mm twin and earth?
Are you saying there's a 16A breaker at the house feeding the existing 6mm T+E circuit to the garage?
 
I won't comment further on getting the supply to the CU as it's all been said.
You probably picked up that the design of the CU in the garage is sounding odd. Are the two radial socket circuits on 32 amp breakers wired in 4mm twin and earth?
Are you saying there's a 16A breaker at the house feeding the existing 6mm T+E circuit to the garage?
Hi Tim, yes that’s what’s there currently, the garage has 1 light on 6A and 1 radial socket on 32A. The garage CU fed from 6mm cable on 16A.
 
Hi Tim, yes that’s what’s there currently, the garage has 1 light on 6A and 1 radial socket on 32A. The garage CU fed from 6mm cable on 16A.
It's making me wonder - did someone accidentally fit the wrong breaker and couldn't be bothered to revisit it, as it feels like the 32A was meant for the house CU and the 16A was for the garage radial....
In any case at least this is all being looked at again!
 

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