Thanks.
All three RCDs are 30mA. The armoured cable runs to the outside of the garage at floor level, then there is a small junction box outside the garage, out of which comes a grey un amoured cable running up the inside wall to the consumer unit which is above head height. I'm guessing the rcd in the house is there to protect that little bit of unprotected cable.
The old owner had a second kitchen installation done in the garage hence the cooker and appliances. Guessing they used to do some kind of catering.
As for feeds... Probably 20m of amoured cable from main CU to garage, plus 2.5m of unarmoured able as mentioned above. Then another 10m from garage CU to shed CU (all internal going through the loft of the garage and straight into the shed which is attached to the garage.
From some quick number crunching, the limitation on load may be the unarmoured bit of cable in the garage. If it's 10mm Twin and Earth that would take 64 Amps, while 10mm Armoured cable will take 85 Amps (making some assumptions about installation conditions).
Having said that, the chances of any overload being a genuine risk to the cable are slim, though it may not do the 40A MCB much good long term if it is consistently overloaded, but not enough to trip it.
In the real world, the practical loads can probably be considered quite a bit smaller. It is unlikely that the oven will be on overnight, when the car charger is likely to be, for example. And some of the other loads are likely to only be at full load intermittently.
The selectivity issue between RCDs may well be the biggest potential issue - where a fault in either the shed or the garage might trip any or all of the 3 RCDs, tripping out your house downstairs socket and lights. There is also the issue with MCBs tripping as mentioned by
@pc1966
If you aren't using the garage oven at the same time as other loads, then the only potentially dangerous issue may be the car charger, if it really is 2.5mm on a 32A breaker. That could easily be solved by uprating the cable to 4mm.
If the concern of nuisance tripping bothers you, then the armoured cable could likely be removed from the RCD at the house end and protected with with a suitable MCB (63A perhaps), or even a suitable fused unit.
Does the certificate you have say what the size of the main fuse is? Likely to be 100A, but if all this is running off a 60A supply then someone was a bit optimistic (though in reality you'll almost never pop a suppliers main fuse with an overload)
For a more recent new build I'd have rather seen RCBOs used, but electrical installation in new builds rarely reflects the selling price in my experience.
It may be worth approaching Clarkson Evans direct for the certificate for the garage, for your records in future. Perhaps the house buyer paid them separately for the garage "catering" installation as they were on site, which is why it didn't come through the house builders pack.
Many thanks.
