alanblaby said:

433.1.204 says 2.5mm minimum cable size.

Is that for the ring, and unfused spurs?

Yes for T&E,but only if the unfused spur feeds ONE accessory to BS 1363.

BUT... don't forget, the other requirement that for both ring and spur,  the current rating has to be 20 A, so for some arrangements in thermal insulation, or grouped, 2.5 sq mm might not be enough.

A fused spur would be required for use of 1.5 sq mm.

alanblaby said:

433.1.204 says 2.5mm minimum cable size.

Is that for the ring, and unfused spurs?

Yes for T&E,but only if the unfused spur feeds ONE accessory to BS 1363.

BUT... don't forget, the other requirement that for both ring and spur,  the current rating has to be 20 A, so for some arrangements in thermal insulation, or grouped, 2.5 sq mm might not be enough.

A fused spur would be required for use of 1.5 sq mm.

My point was really, as a design, the 1.5mm spur to a single socket would comply, with overload/fault/RCD protection, so why has 2.5mm been used as a minimum?

Just to keep the 2.5mm RFC as a 'standard' circuit, so no 'design' required?

Such a scenario (1.5mm spur to single socket) was brought up as a C2 on an EICR, I immediately said no fault, maybe a C3, it cannot overload, then was pointed to 433.1.204.