by sparx »
Wed Feb 24, 2010 7:10 pm
oxoid,
the ring circuit consists of effectively 2 parallel 2.5mm2 cables effectively equaling 5mm2 protected by a 30/32A fuse/mcb.
A spur from the ring being only a single 2.5mm2 conductor needs to be current limited by design, thus it is considered unlikely that a single outlet even if a double socket would have say 2 X 3kW heater on it for example but if a second outlet was connected some distance away this could become the case.
Likewise a radial in 2.5mm2 has to be limited by a 20A device.
Also a radial run in 4mm2 can have only single outlet spurs in 2.5mm2 ,
So in order to protect the spur cables/connections from overload if more than one is required then a fused connection unit must be used at the start of the spur whose 13A fuse will limit the load for all the outlets, just like using a multi outlet extention lead.
Your point about cable being to required standard is the crux of the matter since the design of the standard domestic ring to which we allude states the sizes allowed by the IET.
Anyone who is prepared to design a circuit with all criterior/calcs. to demonstrate safe compliance with the regs can of course do so but this is outside the DIY forums brief I believe,
regards Sparx