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.

Hi Alan. Using 1.5mm T+E cable for a small load device, as you’ve described, seems satisfactory regarding  fault current compliance, RCD protection, and additional plug top fuse in protection.
If you choose to proceed with 1.5mm T+E cable for the single socket outlet, in my opinion it would be prudent to note this as a deviation on the electrical installation certificate, clearly justifying the reasons for this choice and affirming that the installation remains safe for continued use.

Again, only In my opinion 

Andrew M K 

AMK said:

Hi Alan. Using 1.5mm T+E cable for a small load device, as you’ve described, seems satisfactory regarding  fault current compliance, RCD protection, and additional plug top fuse in protection.

I disagree. This is about overload protection of the live conductors; not sure why RCD is being brought into consideration at all, since RCDs aren't overcurrent protective devices ... we aren't considering a fault to the 1.5 sq mm cpc, or a fault L-N, which are addressed by Section 434 ?

If you have a look in Appendix 15 to BS 7671, and also take account of the wording of 433.1.204, which includes the words "with unfused spurs", you will appreciate the requirement includes that the unfused spur also has to meet the requirements of BOTH csa 2.5 sq mm (unless MICC 1.5 sq mm), AND min I_Z of 20 A.

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.

The RCD inclusion was to show the socket outlet would conform for fault/RCD and overload via the plug top fuse.

A truly single socket will be OK, a single spur to a double socket may not be. I'd be less worried about a fused spur as there is less risk of unfused 2 way adaptors and a change to a double socket later.

But the whole point of the one reel of 2.5mm and no thought it is indeed the lack of thought, One could make an optimum ring thicker near the CU and thinner at the far point, but we don't.

Mike.

I agree. While it may not conform to the stipulations of Appendix 15, I don’t perceive any safety concerns. Additionally, according to table 4D5, a clipped direct 1.5mm cable is capable of handling 20 Amps.

Fault current complies

Does it though? 1.5mm² T&E has a 1.0mm² c.p.c. - which generally isn't sufficient for the energy let-through of even a B type 32A MCB. RCD won't necessarily help unless you can be sure that earth fault current is low (e.g. <575A) as even at 40ms it likely is to be a lot slower than an MCB for fault currents.

   - Andy.

alanblaby said:

The RCD inclusion was to show the socket outlet would conform for fault/RCD and overload via the plug top fuse.

Whilst the RCD can be used for ADS, it can't be used for protection against fault current.

I just wanted to clarify the situation 100% , because the current terminology 'fault protection' for ADS (RCD is OK) and 'fault current protection' for protection against overcurrent (RCD will not do, you need an OCPD - RCBO would be OK if adiabatic met for that device) ... if you see what I mean.

mapj1 said:

A truly single socket will be OK,

Provided the length of the spur < 3 m ? (If you are referring to downstream device providing protection against overcurrent), AND adiabatic met for both L-N and L-PE faults.

HOWEVER

THe problem with socket-outlets, some would argue, is that there is no protection against the use of multiway extension leads permitting long-term moderate overload currents that might damage equipment, but not operate the OCPD ... if you subscribe to that chain of thought ... NO, it's a straight non-conformity with BS 7671