hi. My main fuse board has a 16amp mcb feeding the garage. In the garage I have a small fuse board with the main MCB rated to 100amps.
It now seems that on the odd occasion the combined load of a 13amp hot tub, fridge and flood light causes the 16amp mcb to trip in the house. combined startup load?
My question is, should the garage MCB be rated to 100amps? Should the downstream breaker be less than the upstream breaker? In my case, the 100amp breaker will never trip, so it doesn't seem to serve a purpose.
The 100A m.c.b will never trip, because its not an m.c.b at all. It's just an isolation switch rated to carry 100A. Just as your standard light switches are rated at 10A. You may be able to increase the 16A m.c.b to something slightly bigger like a 20A depending on the size of the cable and how/where it runs to eliminate your nuisance tripping.
The isolator of your garage unit is not an overcurrent protective device, there will not trip! The reason for the 16A MCB tripping is that the load of the circuit is too high for the device. The hot tub is demanding 13A, the fridge could be asking for 8A, plus the light maybe 0.5A. Equalling 21.5A. It could be possible to change the device at consumer unit but this will depend on existing cable size, method in which it was installed and length of cable.
I would agree largest MCB I have seen is 70A after that moulded breakers are used normally practice today is an isolator has a red switch. I have seen mistakes in the past where load master isolators have been fitted thinking it was a MCB.
The raising of MCB size in the house is not normally a DIY job as likely the impedance will need checking as well as cable size and route. I have seen quite a few houses where although steel wire armoured cable fed the garage there was a lenght of 2.5 twin and earth feeding the SWA inside the house and the route meant the max for whole circuit was 16A.
It is also common for people to buy consumer units populated to use in a garage and leave the 32A MCB's fitted even though the house MCB would trip first. The same with caravans and boats often 16A MCB in the caravan or boat consumer unit even though the whole supply is only 16A.
I have seen with caravans and boats the main isolator being changed for a 16A RCBO or even 10A RCBO so that the one in the caravan or boat will trip first. However 2 16A MCB's in series does not really work as you will not know which will trip first. There are motor overloads which can double as a MCB which have an adjustment but one they are expensive and two they are not allowed where the control is by an ordinary person.
So likely the best way to stop tripping is use the same socket for fridge and hot tub so can't be used together however than there is always a chance the fridge is not plugged back in. Other than that its a can an electrician job as to hire the test equipment will likely cost more than getting an electrician so just not worth doing as a DIY job.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!