Socket on spur with cable in conduit

[GrahamP]

Hi. I'm hoping someone can help with a question about adding a socket as a spur onto a ring mains circuit (rcd protected at CU).
Its a 2 gang socket so max load could be 26A. The 2.5 TE cable is rated for 27A but this reduces to maybe 20A if its in buried in a wall in conduit (as it will be). Its not a long run (500 mm) and its unlikely anyone is going to plug two electric heaters into it, but would is this something to worry about? Would it be OK/advisable to label as, perhaps, max 13 A total load? many thanks.

 

[Avo Mk8]

Hi. I'm hoping someone can help with a question about adding a socket as a spur onto a ring mains circuit (rcd protected at CU).
Its a 2 gang socket so max load could be 26A. The 2.5 TE cable is rated for 27A but this reduces to maybe 20A if its in buried in a wall in conduit (as it will be). Its not a long run (500 mm) and its unlikely anyone is going to plug two electric heaters into it, but would is this something to worry about? Would it be OK/advisable to label as, perhaps, max 13 A total load? many thanks.

You can legitimately take a spur as you suggest, as long as there isn't already a spur at that point!
The derating is to ensure sustained loads don't damage the cable, (and allows for two such cables in the conduit).
As you point out, it's unlikely anyone would run a 6kW load continuously from that socket (if a heater it would turn itself off after a while, unless you live in a barn!) so I think many would say it's not an issue to do as you propose, even without a label.

To comply with the derating factor, you could use 4.0mm2 T&E, though getting 2 x 2.5mm + 1 x 4.0mm into the ring socket may prove fiddly! One of these would work: https://www.screwfix.com/p/mk-logic-plus-rapid-fix-13a-2-gang-dp-switched-socket-white/814ph
An electrical wholesaler would sell you 1m of cable!

 

[GrahamP]

You can legitimately take a spur as you suggest, as long as there isn't already a spur at that point!
The derating is to ensure sustained loads don't damage the cable, (and allows for two such cables in the conduit).
As you point out, it's unlikely anyone would run a 6kW load continuously from that socket (if a heater it would turn itself off after a while, unless you live in a barn!) so I think many would say it's not an issue to do as you propose, even without a label.

To comply with the derating factor, you could use 4.0mm2 T&E, though getting 2 x 2.5mm + 1 x 4.0mm into the ring socket may prove fiddly! One of these would work: https://www.screwfix.com/p/mk-logic-plus-rapid-fix-13a-2-gang-dp-switched-socket-white/814ph
An electrical wholesaler would sell you 1m of cable!

Many thanks. Very clear!