Hi - this is my first post, and I would be grateful if anyone can please advise me on this.

Firstly, whilst I have a reasonable electrical knowledge I am NOT a sparkie, and I would not attempt to do the work I am describing below myself! However, I would like to know if what I want to do is possible  as if not, I appreciate the costs (replacing external cable / upgrading CU's etc.) could increase hugely.

My garage currently has its own CU, wired into the main house CU via 2.5mmX3 armoured cable (BS5467). The MCB in the house CU that it is wired into is rated 16A. The garage CU has a master switch (DP) rated 63A (this seems excessive? Although I appreciate probably not actually a problem) with the socket MCB 16A and the lighting 6A. I understand that the total maximum draw in the garage is therefore 16A, limited by the 16A rating of the MCB in the house.

My main question is this. From the tables I have seen on-line, that 2.5mmX3 cable should be capable of carrying a load of over 30A. Therefore, could the MCB in the house CU be up-rated from its current 16A to 32A, and the socket MCB in the garage CU then from its current 16A to 25A (or even 32A)? I appreciate I will need to check the cable from the garage CU to the sockets is sufficiently rated for this higher power. My concern is whether I have correctly understood that the 2.5mmX3 armoured cable can take this higher current?

On the assumption I CAN do this, some subsidiary questions:

My main house CU main switch is rated 100A (standard, I believe). The total of the individual MCB's is already 98A. From what I have read I THINK that it is not a problem for the total MCB value to exceed the switch value so long as the actual draw does not exceed 100A - is this correct?

The current MCB in the house CU that runs to the garage CU is not connected through the main RCD (correct, I believe) but there is ALSO (I do not think) no RCD in the garage CU. I understand this is INcorrect - is that right? How serious is this? For any garden or power tools I would always use a plug-in RCD as protection but is this a matter of law? I BELIEVE (but am not sure) the garage CU installation was a professional job.

Finally, with my current arrangement,  am surprised that the socket MCB in the garage CU and/or the MCB in the house CU has not tripped due to over-load. I have a total of 3700W (16.1A) of heating alone plugged into the scokets, plus (when they are all on - which they have been) at least 500W (over another 2A) of lighting spread over the lighting MCB and socket MCB (but all of course going through the overall 16A MCB in the house CU, of course) plus sundries totalling at LEAST another 250W. PLUS, I have ALSO run a camping kettle (another 650W) in there WHILST the heating is all on (and I mean actually heating, not "on" but the heating elements off due to internal thermostatic control) - this all adds up to AT LEAST a 21A draw from a MCB rated just 16A. Does this indicate a problem with the MCB (or perhaps MCBs - plural) and further, if itself, does this indicate the 3X2.5mm cable is indeed capable of supplying higher levels of current (or have I just been lucky thus far to "get away with it")?.
The MCB's definitely do work as when turning on 3 CRT computer monitors at once from the same switch (i.e. they are all in the same 4 way adaptor and when switching that on from the wall, with the individual units already on), the garage socket MCB AND the house MCB BOTH trip, but they are fine when the monitors are turned on individually, presumably due to high initial draw which when 3 are turned on together?

Apologies for the length, any assistance appreciated! Many thanks.

I think if this was a new installation an RCD would be required. The regs have changed over the years so it was probably ok when installed. Changes to the regs aren't usually retrospective until you change the installation for other reasons.

I take it the garage isn't a very long way from the house?

There is no limit to the amount of MCB's you can have, and these days having more circuits than too few is preferred (for convenience when there are faults).  Having 3 or 4 ring final circuits in a property is not uncommon, and that is more than 120amp straight away.

It all boils down to "Diversity", and the regulations allow for this - The on site guide as some guidance tables on maximum permissible diversity, but you can still go beyond this.

I will give you a non domestic example.

A caravan park has a field of 8 hookup points for touring caravans.  The hookups are all rated to the standard 16amps, and fed from circuit.  If you total (8 x 16) you get 128amps.  In reality the circuit will most likely be protected by a 63amp fuse or breaker.  Diversity is applied as not everyone will be using the full 16amp at once.

As your SWA goes directly from the CU to the outbuilding, you can omit RCD protection to the SWA and use a 25amp MCB (if you can source one for your CU).  You would be better then fitting a 20amp MCB in the outbuilding, and swapping the main switch in the outbuilding CU for a 30mA RCD (25amp or larger).  Although not specifically required, assuming the lighting circuit is clipped direct/trunked etc, the lighting circuit would be better RCD protected too.  This is more of my personal preference though - nothing wrong with not providing RCD protection for it.

Nuisance tripping

It means if you are in garage and rcbo trips you need to go back into house to reset.

if you put it onto rcd side of split load db then you also trip half the house at same time.

if you have rcd in garage then if it trips you can reset it there.

SIMPLES