On 30/05/2021 19:00, John Rumm wrote:
On 30/05/2021 16:35, Grumps wrote:
On 28/05/2021 18:42, ARW wrote:

Of course 10A MCBs are available which might be suitable (if 16A is
for some reason not) and they would not trip with a 2kW heater.

Keep us updated.

So, I had a quick visit to the property (still haven't got the full
EICR yet) and had a look at the wiring behind the FCU. Well would you
believe it, it's almost certainly only 1.5mm2. I measured the cable
(with my trusty plastic vernier) and it was a little more than 8mm but
much less than 10mm.

I don't have any edition of the regs but I do see table 4D1A (which is
probably for most domestic situations) that shows max capacity of
1.5mm2 cable as 14.5A (cable is plastered into the wall).

I think you are looking at the wrong installation reference method.
"Buried in masonry" is treated as method C, same as "clipped direct". So
on current draw alone 20A would be a closer estimation. (there may be
voltage drop issues if the cable is long) See:

http://wiki.diyfaq.org.uk/index.php/...#Metric_Cables


So the electrician was partly right I think. 16A MCB is too much, but
his suggestion of 6A would be too little. When I get the report I'll
have a chat and hopefully come to a compromise of using 10A MCBs.

I would argue that 16A is fine for a number of reasons. Primarily, the
MCB only *has* to provide fault protection at the origin of the circuit,
not necessarily overload protection - that can be done elsewhere (a spur
on a 32A ring circuit being a prime example - the 32A MCB will not
provide overload protection for a single length of 2.5mm^2).

You seem to have adequate overload protection via two means - firstly by
selection of power consumption of the appliance - since this is hard
wired you don't have to consider someone "plugging in" a too heavy load.

Secondly you mention a FCU. That kind of implies a 13A fuse is the
highest that can be fitted there, and that will also prevent overload on
the cable.

The property was re-wired in 2004. Would (or did) the regs change

They have changed in that time (from the 16th to the 18th edition - so
new rules on RCD protection, and maximum permitted earth loop impedance
Cmin factiors[1])), but not in ways that would have a bearing here.


or did the original installer make a mistake?


Not really - just engineered it to meet the requirements without any
additional overspend or overbuild.

[1]

http://wiki.diyfaq.org.uk/index.php/...2C_Cmin_factor


http://wiki.diyfaq.org.uk/index.php/...-_17th_edition

Thanks for the wiki link. Very concise information.
So the cable run leading to the FCU is method C (buried in plaster in
this case), but perhaps the electrician has determined that some parts
of the run are in the ceiling space and probably that would mean method
100 or 101 applies. Either way, the 13A fuse in the FCU should make any
method acceptable except 103.
Voltage drop would not be an issue either.
Looks like I'll have to wait for the report.