Garage wiring - CU with greater capacity than submain?

[333rocky333]

That's another one which ought to be answered by reference to countless past discussions!

The short answer is that "opinions (often quite strongly polarised ones) vary" !

Kind Regards, John

I would still value your opinion on 4mm armoured John, Winston seems to be shirking the question.

 

[winston1]

Not really. I believe that you should use 3 core SWA and not rely on the armour for earth. Others may disagree.

 

[JohnW2]

I would still value your opinion on 4mm armoured John, Winston seems to be shirking the question.

As I said, opinions as to whether one should use 3-core or 2-core SWA (the latter relying on the armour as the only earth/CPC) are highly polarised.

Those proponents of 2-core say that 3-core is simply a waste of money and copper. Those who favour 3-core argue that, particularly if underground, it is not unknown for the outer sheath to develop defects, allowing water to get in and result is corrosion of the armour, eventually resulting in its 'failure'.

For what it's worth, particularly given the relatively small marginal cost difference (in terms of money or copper!) I am personally more inclined to the latter view, but that proves little.

Kind Regards, John

 

[ChrisZx]

Even a 20A MCB won't discriminate against a 32A MCB. Look at the tables.
Discrimination and Back-up tables • Moulded Case Circuit ...

You could follow Winston's suggestion.
32A MCB in the house
The SWA terminating on an RCD as the incomer/ isolator at the garage end.
4mm² Radial round all the the sockets
FCU with a 3 or 5A fuse in it as the supply for lights.

Job done.

Hmm guess you're right, not really any advantage to a CU

Whilst I understand your concern, I can't see how youi can have it both ways - in other words, I can't see (without the use of a relay or similar) how you could arrange that if the lighting circuit 'failed', the sockets one would be guaranteed to do so as well.

You could ger part of the way there (in terms of just earth leakage faults) by not having any RCBOs, but having both lighting and sockets protected by the same, single, RCD (with separate MCBs for lights and sockets) - but the downside of that would be that if the sockets power failed (due to RCD trip) then so would the lights. In any event, if the lighting circuit were to fail because the MCB tripped, the sockets circuit would then not die unless you had included a relay to bring that about.

Kind Regards, John

Yes sorry I wasn't thinking straight there, amateurs for you

 

[JohnW2]

I always assumed 80A or 63A was the breaking and making capacity.

As I'm implied, I'm not certain. However, if what you say is correct, those limits could fairly easily be exceeded. In particular, if an RCD operated (before the corresponding OPD) as a result of a full-blown L-E fault, it would find itself having to break the PFC, which could well be hundreds of amps. One really would need a 6kA (or more if not a type-tested CU) breaking capacity, just like an MCB.

Kind Regards, John

 

[333rocky333]

Not really. I believe that you should use 3 core SWA and not rely on the armour for earth. Others may disagree.

Ok, fair enough its your own personal view, I read it as 2core swa was unacceptable, my apologies.
Happy new year, our past conflicts were only banter, nothing personal.

 

[ChrisZx]

For my own sanity, people are saying I can use something like this https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/261284772516 (first one I found though I assume it will have to be metal?) and wire the socket straight off of it? Because overload protection is handled by the mcb in the CU

 

[winston1]

Ok, fair enough its your own personal view, I read it as 2core swa was unacceptable, my apologies.
Happy new year, our past conflicts were only banter, nothing personal.

Accepted and happy new year to you as well.

 

[winston1]

For my own sanity, people are saying I can use something like this https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/261284772516 (first one I found though I assume it will have to be metal?) and wire the socket straight off of it? Because overload protection is handled by the mcb in the CU

The metal, or rather non combustable, which of course metal is not, applies to domestic premises. It is debatable whether it applies to garages.

 

[333rocky333]

For my own sanity, people are saying I can use something like this https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/261284772516 (first one I found though I assume it will have to be metal?) and wire the socket straight off of it? Because overload protection is handled by the mcb in the CU

All your sockets will need to be done in 4 mm and an Fcu as a light switch.
Less faffing about to use a Garage Cu as you intended, Or get a small 4way Main switch Cu and fit 2 Rcbo's if you dont want total trip loss

 

[ChrisZx]

I'll have to do some research into running the lighting off an FCU. I need three separate sets of lights, could I run a 1.5mm2 lighting ring off an FCU? Or take a new spur for each set?

 

[333rocky333]

EDIT
some may say its ok to do a 2.5 ring from the rcd, though i am not too sure about that circuit

 

[JohnW2]

The metal, or rather non combustable, which of course metal is not, applies to domestic premises. It is debatable whether it applies to garages.

True but, ironically, one could probably come up with more sensible arguments for requiring metal CUs in garages than in houses!

Kind Regards, John

 

[333rocky333]

You can do what you want after the Fcu, it would only be to reduce from 32a to 5a, it dont even need to be switched, just treat it like an Mcb and wire from it, to your lights and a normal 3 gang lightswitch.
You could dismiss the Fcu totally and just use a 6amp mcb with your RCD in an enclosure

 

[JohnW2]

EDIT some may say its ok to do a 2.5 ring from the rcd, though i am not too sure about that circuit

That would be a 'lollipop' circuit, and, although not explicitly authorised by regs, it's difficult to see an electrical argument against it. However, once one has an RCD one needs an enclosure, so it might as well be a mini-CU with MCBs, thereby removing any scope for debate/argument!

Kind Regards, John