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.

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

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

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.

The fault protection was calculated, and the 1mm CPC did comply. I agree, a long piece of 1.5mm T+E would be marginal, if not a failure on the fault currents, but the scenario I asked about was a very short piece of 1.5 T+E feeding an adjacent single socket outlet from a 2.5mm RFC. No idea why 2.5mm wasnt used, but the CCC of the 1.5mm cable was certainly enough to supply a 13amp single socket, so overload should not come into it, as both the cable is rated at 20 amps, and the load is limited by the plug top fuse.

The RCD included was to show it complied with additional protection supplying the socket outlet, not for the fault protection element. So we have a calculated fault current which complies, RCD for additional protection, and the cable can carry 20 amps clipped direct, and is protected anyway by the 13 amp plug top fuse. (as an example, the actual wattage was 250W for a TV, at 3 amps).

I feel its a strange Regulation, which doesnt make much sense if a calculated result shows a smaller cable size can be safely used.

As an aside, in the electrical world, I'm tending to upsize cables for the last couple of years, especially for high use items such as  car chargers, heat pumps/immersions etc, as the cable resistance losses can be shown to pay for the cost of the larger cable within 3 years, and from then on it is cheaper electricity for the user. However, on the plumbing side which I work on around 20% of the time, the onus is on reducing pipe size to the correct size, there are similar calculations done to determine pipe size and velocity, so paying for 28mm pipe when 22mm will suffice is the way forward there, not because the 22mm will reduce flow etc, it has been calculated it can carry the correct amount of water, but to save install costs, as going the next size pipe up usually doubles the pipe cost.

alanblaby said:

I feel its a strange Regulation, which doesnt make much sense if a calculated result shows a smaller cable size can be safely used.

It's a "standard circuit" - doesn't really prevent you doing something else if you are satisfied after calculation.