James wrote:
It is actually an inverter split type that I am having. But is is a 5kw, and
according the the installers it is a legal requirement for it to have its
own circuit with a 20amp trip, if it was a 3.3 kw, this would not be the
case. I do have a lot of equipment in my house running off the plug circuit
anyway.
[ ... ]
Regarding andy's reply. I have not had any loop or drop calculation done,
nor have they been suggested yet. I would say that the original plan for the
cable route would be approximately 26 meters from the fuse box. When the
electrician comes to do the fuse box I am going to ask if it is feasible to
route it around the side of the house (outside), where there are cables
already, like TV and sky ones, at least part of the way. I would say that
run is about 15 meters. Is a loop and drop calculation a standard
electrician thing, is there any expensive or specialist equipment required?

It's a standard electrician thing, needing no equipment and only
multiplies-and-divides-and-values-from-tables; but even more standard is
for an electrician to avoid doing it by using one of the 'known good'
Conventional Final Circuit arrangements listed in the On-Site Guide (the
'simple consequences of the Regs' book for domestic/light-commercial
installs). For a 5kW load with a starting surge (I'm guessing here from
general characteristics, not a deep knowledge of 'inverter' aircon kit!)
I'm guessing a Type C MCB. The relevant maximum lengths from the OSG a

2.5/1.5mmsq, type C MCB: 17m with TN-S earth, 27m with TN-C-S (PME),
only suitable for surface-clipped cable mounting
4.0/1.5mmsq, type C MCB: 19m with TN-S earth, 42m with TN-C-S (PME),
clipped-direct and in thermally-insulating wall OK
6.0/2.5mmsq, type C MCB: not listed for 20A MCB, but values for 30A MCB
are 27m with TN-S earth, 42m for TN-C-S (PME)

The 2.5mmsq as well as being marginal for loop resistance, is right on
the edge for voltage drop (26m, 20A, at 18mV/A/m drops 9.36V - too close
to the 4%-of-nominal for comfort. So as expected, 2.5mmsq is too weedy
on a number of grounds. The 4mmsq is closer to the mark, but as standard
4mmsq has the same size earth conductor as 2.5, doesn't give much longer
as maximum length for a TN-S earth. 6mmsq will be comfortably within
limits.

If you do decide to route outside, don't use 'normal' T&E - many people
do, but I'd claim it's not adequately mechanically protected. (For a
couple of feet running to a PIR 'security' light, massively overrated -
no real issue; but carrying 20A - no.) You'd want to use either Hi-Tuf
or metal-armoured - both of which have lower-impedance earth conductors
too (for Hi-Tuf, all three cores are the same size; for SWA, the armour
has a much greater cross-section than the live cores. Even with the
shorter route, I'd be tempted to go for 4mmsq minimum, rather than the
2.5mmsq you could probably get away with: the (true) difference in
material costs is small, and voltage-drop matters for motors.

All this should be taken into account by your electrician (whether
that's the guy replacing your CU, or the aircon installer later
connecting up to a spare way). And I hope you're planning to sleep in
the room next door to all that kit and the aircon - I know I couldn't,
with all that racket going on!

HTH - Stefek