AJJewsbury said:

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.

Agreed ... also a consideration

(Although, a fuse could be used ... there may well be fuse and circuit-length combinations for which 1.0 sq mm is OK).

considering we allow 1.5mm flex on the downstream side of the same 13A fuse that is in the plug or protecting the 1,5mm spur, there is no real electrical problem in terms of overload or fault, I will take the reduced cpc as a possible issue but only if we are expecting large CPC currents so the copper is pre-heated to 70C before the fault comes on.

M.

mapj1 said:

onsidering we allow 1.5mm flex on the downstream side of the same 13A fuse that is in the plug or protecting the 1,5mm spur, there is no real electrical problem in terms of overload or fault, I will take the reduced cpc as a possible issue but only if we are expecting large CPC currents so the copper is pre-heated to 70C before the fault comes on.

Apologies, but I don't agree.

BS 7671 doesn't "allow" that, because it's outside the scope of the standard.

I agree what you decribe may be mis-use, but as far as BS 7671 is concerned, it's "foreseeable mis-use". We can address that by co-ordinating the final circuit CSA with the OCPD appropriately. The non-adiabatic approach in the referenced standards permits that.

It may be the case, that the non-adiabatic approach, in some circumstances for fixed installations, may lead to a 1.5 sq mm conductor for wiring systems other than MICC, but in my opinion this is a "non-standard circuit", and it's it he designer's lap as to whether it's acceptable for the particular installation.

As others have said ... unlikely in some circumstances for adiabatic in any case.

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.