Sub main and diversity.

[La Poste]

Greetings.

I am wiring a large garage for a friend.

The wiring will be done in singles enclosed in 25mm round conduit running all the way around the garage in the roof space with T-Pieces put in place that will allow cables to drop from the roof space in 20mm conduit to the switches and sockets.

The whole thing is visible with no concealed cables in walls etc.

I have bought a Wylex eight way dual RCD board and will use three of the ways for lighting, sockets and an additional radial for the fridge and freezer.

The lights and sockets will be RCD protected but the fridge and freezer will not, the socket will be marked fridge freezer only.

The supply cable is protected by a 32 Amp type B MCB at the main house and the cable is 4mm SWA with one core used as the CPC, the armouring is earthed at both ends.

There is are no extraneous metal parts in the garage, no water etc and the supply is TN-S.

Because the Sub board will eventually have a few more circuits added, probably lighting, I will use diversity to assess the overall load, when I add up all the MCB's on the sub board they will probably total more than 32 Amps.

Does this sound Ok, having a non RCD protected dedicated circuit for the fridge freezer in a garage and also using the principal of diversity on a sub board?

I cannot think of any good reason why my approach is wrong but I am wondering if anyone can see a fault in this design.

Thanks very much.

 

[MarkieSparkie]

Applying diversity to assess the sub-board loading is fine. A note of caution on not providing RCD additional protection to the fridge/freezer sockets in the garage. The garage floor is likely to be damp concrete, particularly if the garage is unheated. Any person within the garage will be in contact with this good earth with only the quality of their shoe soles for insulation. It is not unheard of for young children to walk around with bare feet, particularly in a garden or its surrounds in the Summer. You will need to decide if the risk of perished food in the event of a RCD tripping outweighs the potential shock risk due to better than normal contact with the ground in your particular circumstances.

 

[SPARTYKUS]

Hi Laposte

I have done it your way (freezer thing) in past but prob wouldnt again - I'd remove the socket (& plug) & wire the flex in- no chance of others using it then. And put it on its own RCBO, much safer.

Sounds a nice job, or rather the chance to show off one's neatness on the walls! Cheers.

 

[telectrix]

agree. RCBO for the fridge if you use a hi-integrity CU. or if not possible, FCU the bugger.

 

[La Poste]

You will need to decide if the risk of perished food in the event of a RCD tripping outweighs the potential shock risk due to better than normal contact with the ground in your particular circumstances.

Good call I found the supply cable to the house today, it is a lead sheath TN-S supply cable and it runs right under the garage.

The lead sheath is the earth for the cable and so it provides a very handy route for current to flow from the garage floor back to the transformer.

I don't know the electrical resistance of damp concrete so I don't know what sort of current would find it's way back to the star point in the event of an earth fault.

I have read the resistance of a human body is somewhere between 500 and 1000 Ohms.

Thanks for the advice everyone.

 

[davie spark]

If the garage is not joined on to the house you should RCBO the supply cable from the house to protect the cable and comply with the regs. This will then also offer you rcd protection for the rest of the garage install. As for applying diversity, if the current demand is only around 32 amps total applying diversity is great in theory but a waste of time in practice, the load is not large enough to gain any submain cable size reduction or switchgear size reduction to therefore save you money on these components. In addition to this, you have said there is a likelyhood that further circuits are probably going to be added at a later date so already knowing this negates your initial decision to apply diversity, you should be allowing for around 15/20% expansion for this, does that make sense to you?

 

[santidreamer]

Dave's right all circuits outside main equipotential bonding zone (garage, garden lights, socket used to power lawn mower) should be protected by an RCD / RCBO. I would include the cable feeding the garage to be protected.

For my part I like to tightly rate, MCBs 32Amps seems ample. Freezer 2Amps + 13A for a very large power tool + lights = less than 15A.

Does anyone have issue with the size of CPC feeding the consumer unit?

I have very similar circuits 50Amp Type B breakers feeding a consumer units in a large hotel. There is no earth bonding because of PVC pipes.

Socket Zs = (calculated then tested at) 0.5 Ohm
Adriatic equation works out 1.5mm CPC is OK.

So no reason to use 10mm earth cable, actually installed 4mm.

 

[SPARTYKUS]

If the garage is not joined on to the house you should RCBO the supply cable from the house to protect the cable and comply with the regs. This will then also offer you rcd protection for the rest of the garage install. As for applying diversity, if the current demand is only around 32 amps total applying diversity is great in theory but a waste of time in practice, the load is not large enough to gain any submain cable size reduction or switchgear size reduction to therefore save you money on these components. In addition to this, you have said there is a likelyhood that further circuits are probably going to be added at a later date so already knowing this negates your initial decision to apply diversity, you should be allowing for around 15/20% expansion for this, does that make sense to you?

Daviespark & santidreamer .... disagree, if it's an armoured cable and the earthing system is TNS (i.e. good enough for disconnection times) then it already complies with the regs!

It's in an armoured sheath - where's the problem with that?

You're idea of RCBO'ing it at source is not a good idea, unless everytime something trips in the log cabin / garage / hideaway at the bottom of the garden 200 feet from the house in the winter at night when its snowing, you fancy the 89 year old owner trapesing back to the house to .... reset it.

Please feel free to pummel me with a retort

just a thought, was this a wind up ??

 

[santidreamer]

OK agreed TNS sub main so no need for RCD.

I stand corrected.

 

[Welchyboy]

is your load really likely to be around 32A?

what is the distance from the origin of the supply to the garage consumer unit?

if its at any distance one thing i would be wary of is your voltage drop, at 32A a 4mm cable has a fair amount of VD per meter, and depending on what load/conductors/length your lighting circuits are this could well push these over the max limit, you only have 6.9v and that includes the sub main!

Also i would RCD protect the sockets for fridge and freezer, if your worried about nuisance tripping maybe use RCD sockets, if the whole installation is new i would think it wouldnt be an issue anyway really

WHY would anyone RCD an armoured sub-main??????

Dont RCD your sub main, Sparktycus is dead right!

Apart from that sounds like a nice little number


good luck pal

 

[La Poste]

I found out today the cable is actually 6mm although it didn't look like it at first, I think I need glasses.

The reason I was asking about diversity is if I were to create a radial for sockets (it's a big garage) fused at 20 Amps, then a separate radial for the fridge freezer and chest freezer fused at 16 Amps or maybe 20 Amps (I like to make lots of circuits )

Possibly give him a get out of jail free card socket that is non RCD protected and next to the fuse board to plug his freezer etc into when he goes away on holiday although I am still thinking about this one, fused at 16 or 20 Amps.

Then lights probably two circuits maybe three each fused at 6 Amps, so you see add all this together and it comes to more than 32 Amps which is the rating of the BS 60898 in the house at the other end of the supply cable.

This is why I asked the question, I was wondering if applying diversity to a garage is frowned upon.

The cable length from the main house is quite short, I would say 10 Metres maximum so volt drop won't be an issue unless the guy wants lights installed a very long way down his garden.