SWA Cable for run between switched fuse isolator and consumer unit

[srmoore72]

Hi all,
I am looking to replace an existing CU (ground floor) and put it in a new position (first floor).
The existing CU is on the inside of a wall to the meter cabinet (which is on the other - external - side of the wall).
The existing cabling between these two devices is not SWA/armoured. (the cable runs through the wall so there is no risk of mechanical damage).

The proposal is to install a new CU in a location which is approx 6m from the external electric meter.
I appreciate that a CU which is distance greater than 3m from the meter cabinet, needs to have a switched fuse in between.

This cable will be routed within a stud wall - so no risk of mechanical damage.

It has been suggested that the cabling from the (internally mounted) switched fuse and the new (internal) consumer unit should be SWA - however I cannot find any regulations stating that this cable should be SWA and that suitably sized T&E cannot be used?
Can anyone offer guidance?


Thank you
Scott

 

[pc1966]

If it is only 60A then 16mm T&E can be used if not at mechanical risk, but not if it is buried < 50mm from a wall surface as then it needs 30mA RCD protection (which is incompatible with a reliable CU supply due to the accumulated leakage of all circuits) or it needs some form of protection against being penetrated by a nail, etc, and causing a shock hazard. SWA fits that bill.

However, SWA has a limited bend radius so is quite hard to wrangle through any wall, etc.

 

[srmoore72]

If it is only 60A then 16mm T&E can be used if not at mechanical risk, but not if it is buried < 50mm from a wall surface as then it needs 30mA RCD protection (which is incompatible with a reliable CU supply due to the accumulated leakage of all circuits) or it needs some form of protection against being penetrated by a nail, etc, and causing a shock hazard. SWA fits that bill.

However, SWA has a limited bend radius so is quite hard to wrangle through any wall, etc.

Hi pc1966
Thank you for your information.
The CU is rated at 100A - but to follow on from your information - the cable will be deeper than 50mm from the wall surface.
That should negate the requirement for RCD protection and SWA.

Understood about the bend radius limitations for SWA -

many thanks!
S

 

[pc1966]

If the likely demand is anywhere near 100A then 16mm T&E is not an adequate solution. Typically it is fed off a 60A fused-switch for that sort of a reason.

If you need more current and, again, do not have a cable penetration risk then you might look at separate 16mm CPC along with some 25mm 19-strand flexible sheathed tails, though you would have to find someone who seels that by the metre as usual packs are 1m or 3m lengths.

 

[Electro-tech]

Looks to me like a 16mm cable in a wall < 50mm certainly does not leave much structural strength left unless it is a solid wall. I don't think I would feel happy if the cable were not protected, even an SWA cable would not stop a nail & chances are any fixings to a wall surface over 6M in length may become live & you may not know it. I'm thinking of future proving the house for other tenants that would not be aware of the cable run.

 

[swaRRR]

which is incompatible with a reliable CU supply due to the accumulated leakage of all circuits

Hi 1966 what do you mean by this?

 

[pc1966]

Hi 1966 what do you mean by this?

Well if you have a 30mA RCD then your goal is typically to keep the "normal" leakage below 9mA (30% of rating, as normal trip threshold is 50-100% of rating) so there is less risk of it tripping for no good reason. Once you have a house of cables and various electronic devices it is easy to exceed that amount.

Furthermore if that incoming RCD trips everything goes off, which is not good from a safety point of view due to no lights, maybe while cooking and hot appliances nearby, etc.

Those are the reasons most installations moved to a dual-RCD board, and in recent years to all-RCBO as their cost came down. You distribute the normal leakage over multiple RCD/RCBOs so less risk of nuisance trips, and you reduce the impact of a fault down to (in the RCBO case) only the faulted circuit.

 

[swaRRR]

Well if you have a 30mA RCD then your goal is typically to keep the "normal" leakage below 9mA (30% of rating, as normal trip threshold is 50-100% of rating) so there is less risk of it tripping for no good reason. Once you have a house of cables and various electronic devices it is easy to exceed that amount.

Furthermore if that incoming RCD trips everything goes off, which is not good from a safety point of view due to no lights, maybe while cooking and hot appliances nearby, etc.

Those are the reasons most installations moved to a dual-RCD board, and in recent years to all-RCBO as their cost came down. You distribute the normal leakage over multiple RCD/RCBOs so less risk of nuisance trips, and you reduce the impact of a fault down to (in the RCBO case) only the faulted circuit.

Thanks for the explanation i didn't know what you meant by it being incompatible with a CU supply

 

[mainline]

Looks to me like a 16mm cable in a wall < 50mm certainly does not leave much structural strength left unless it is a solid wall. I don't think I would feel happy if the cable were not protected, even an SWA cable would not stop a nail & chances are any fixings to a wall surface over 6M in length may become live & you may not know it. I'm thinking of future proving the house for other tenants that would not be aware of the cable run.

You would be very unlucky to hit the live conductor without hitting the armour first using swa.