Hi all,

Currently rewiring the house at present but need a little advice.

Currently have the main consumer unit inside the house feeding the house as expected. There is a 6mm cable on a 32A mcb feeding the garage consumer unit which has sockets, lights and the old shower circuit.

Now the garage unit has a 100A main switch and an RCD for the sockets, lights etc.
The starting 32A mcb in the main consumer unit isn't RCD protected and goes to the garage consumer unit.

Now as part of the rewire I'm not sure exactly how to proceed on the here.

My garage consumer unit will be getting renewed and have 2 additional circuits. 16A to supply the caravan and a 30A for a welder connection. TBH probably only need 16A for the welder but probably better to over spec just in case.
Now the welder I'm sure would require a 32A type C RCBO with 6mm cable as in trunking/conduit fed from the 100A main switch as an independent circuit.
The rest of the garage would be on the 80A 30mA RCD with individual mcb.

I'd imagine I'd need 16mm cable to feed the garage consumer unit through the ceiling vid through joists.

The question really is what do I feed the garage with from the main consumer unit?
Is it ok to have a 100A main switch on the garage consumer unit even though at present it is fed by a 32A mcb?

Originally thought about splitting the tails but the garage unit is about 10m away from the incoming mains and the consumer unit.

Any thing that is over spec or under spec let my know please.

Thanks
Ian

You're allowed to do your own electrics?  Ron B. VK2OTC
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The basic rules regarding MCBs, breakers, cable thicknesses etc is simple.

1 The breaker protecting a ciruict should never exceed the maximum rating for the infrastructure it protects. In other words, the 32a breaker for the garage is fine. Nothing downstream of this can ever exceed 32a total combined load so having a 100a main breaker is OK, although less than ideal. I would take it down as close to 30a as I could get, currently 40a I think.

2 MCBs look for current leaking out of the appliance, i.e. the difference between input to the appliance via LIVE and output from it via NEUTRAL. If this difference exceeds 30 milliamps (soon to be 20) it will trip. This is a very good thing. So, if your 32a feed from the main board is on the non-MCB side you should protect the appropriate circuits on the garage board with an MCB. The welder is not one of those BTW but every 13a socket definitely IS something to protect.

3 Cable gauges are all listed in the online version of 17th edition regs. You would do well to read these regs in depth. Whilst you can argue under Part P that you are a competent person, you would also (if needed) be asked to demonstrate an understanding of the 17th edition.

Aside from the above, assuming you're replacing the garage CU yourself, if it were me I would lay all new wiring and boxes, get it tested by a qualified Sparkie (which I was before 17th came out) and I would connect the new CU to the main incoming supply with 15mm cross section minimum on a 100a main breaker. I would definitely not connect it to the house CU. Just my 2p.

There are cable size calculators on supplier site such as TLC direct. RS components etc.

p38arover wrote:

You're allowed to do your own electrics?

The 100A main switch is not an automatic circuit breaker (I.e. It will not 'trip' on over current in the same way that a MCB would). The 100A rating simply refers to the maximum current that it will pass safely. If you pass more than the maximum rated current, there is a risk of a resistive heating fault occurring.

In the OP's present situation, the supply to the 100A main switch is restricted to what the upstream 32A MCB can supply without tripping.

Yep - you are correct - I need to wake up before posting - it's only a switch, so makes no difference

p38arover wrote:

You're allowed to do your own electrics?

It's only house bashing and he seems to have a good grasp of whats required , and it will have to be tested and a certificate issued by a qualified sparky . Australia's attitude of not letting you change a plug is all to do with union job protectionism masquerading as safety.

The UK's requirements of wiring are much safer than Australia's yet the attitude to work is a bit more relaxed , funny that  2008 MY Supercharged
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Sorry should have mentioned I am Part P, 17th edition and I&T qualified.
Although going back 5 years when I became qualified but I decided to go down the gas and plumbing path instead so never joined NICEIC etc to get self certified.

So I'm thinking a 50A RCBO type C in the main consumer unit to feed 16mm cable to the garage consumer unit.

I'd still run it direct from the incoming supply.

BTW, which side of the board is the 32a breaker you intend using? You would't want that on the RCD side if you plan on using a welder.

I'm sure there are rules on how long meter tails are allowed to be and the garage is 10-15m from the meter.

32A type c rcbo will be on an independent circuit on the garage consumer unit. So not connected to the rcd for the lights and sockets.

Nope, although 2 metres is the norm the stipulation is that when over 3 metres you just have to go up one cable gauge. my meter tails are 14 metres and all signed off OK. I just had to go to 35mm from 25mm when I moved the CU during the house extension.

I should think that 16mm would be more than adequate for the garage. 10mm would be sufficient, especially if you are keeping the 32a mcb in the main board to feed the garage.

Personally i wold set it up:

45A breaker in the house board feeding to the garage board
10mm twin and earth to the garage or 10mm underground if separate.

board in the garage with:
5A lighting with 1.0mm cable
16A caravan feed with 2.5mm t&e
16A welder feed with 6mm cable then if you find you need to put a larger breaker in you can
16A sockets feed to a 2.5mm radial circuit.
30ma RCD
100A main switch.

The 45A breaker in the main board will protect the 10mm cable from the house and diversity says you won't draw 45A at any point.

The RCD in the garage board will protect the circuits in there even if you have the MCB in the house board on the non RCD side.

Remember though that you probably only have 100A or 80A in the main fuse to the house.

usually where welders are involved a 3 phase supply is best as they can draw high currents, but if yours is more domestic and works off a plug then you should be fine. There is nothing that can't be fixed with a hammer😜😜
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I would first look at the cable run in total, how its going to be protected, armoured, trunking, conduit or clipped, also is the garage fixed to the house or a separate building? Range Rover quotes.....

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Cable run is approx 8-10metres.

I've thought of running a 10mm cable direct form the main consumer unit to the welder. Running through joists and clipped to them to the attached garage. Last 2m in conduit.
Got a 32A socket for it for the garage. Worst case is if the welder I get is 16A or smaller then change to socket as required.
Got a 40A type c rcbo for it for the main consumer unit. 10mm cable is rated as 63A clipped and 52A in conduit so more than sufficient.

Going to run a second 10mm cable direct clipped and through joist but no conduit to the garage consumer unit.
RCBO type b 50A on the main unit feeding the garage unit.

Got a 16A socket for the caravan to fit too now as arrived.

Reply when I'm not so srunk  Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.