Another Garage to shed wiring question

[cymru001]

I have looked through the various post on this site and whilst I've found a lot which are relative to my question, none of them really provide a direct answer.

I have a garage which currently has power for sockets and lighting. The garage is fed from a 40 amp MCB in the house CU via a 6 mm sq cable. This cable terminates into a second CU housing a main switch (no RCD) and two MCB's. One is a 20 amp for the sockets and the other is a 6 amp for the lights. (This is what I inherited when I bought the house. I had the house re-wired in 2005 and whilst the cable to the garage was not replaced, it was tested.)

I want to get power into my shed for an interior light and sockets plus to provide power for external lighting.

I am thinking of replacing the existing garage CU with one which will allow for expansion (say 5 way). Within this CU I would have an RCD (I am thinking 100 amps) I will then have a 20 amp MCB for the socket ring and a 6 amp MCB for the lights (as the existing). I will then add another MCB of 32 amps. This will be the for the shed.

I am thinking of running 2.5mm sq SWA from the 32 amp MCB to the shed (the cable will be buried in a trench).

Within the shed, I am proposing another CU which will have a 63 amp RCD. From this CU, I will run a ring for the sockets using a 16 amp MCB. (I have a pair of metal clad double sockets. they both have built in RCD protection and would like to use these to keep the costs down and to give extra protection.) I will add a 6 amp MCB for the interior light.

I also want to install exterior garden lighting operated by a remote control unit housed within the shed. There will be a total of five lights - one in each corner of the garden and a fifth illuminating a tree at the bottom of the garden.

Each of these lights will have its own 6 amp MCB and be fed via a 2.5 mm sq SWA cable. The reason for using individual MCB's is because the remote units I am thinking of using, only have a live and neutral. I obviously need to earth the armour of the cable and the light itself. In order to acheive this, I am thinking of terminating the SWA into a metal clad box with a blank cover. The armour will earth to the box via the gland. The earth wire of the SWA cable will terminate into the earth point of the box itself. In order to then continue the earth, I thought I could use a length of 2.5 mm sq earth wire (sleeved) which will 'daisy chain' from one metal box to the next. (there are five boxes in total) in the final box I propose to use a 4 mm sq earth wire which runs back to the shed CU earth.

I am thinking there will be a problem getting the shed earthed properly if I rely on the SWA feed into the shed (the cable length from garage CU to shed CU would be approx 25m). My thought process is, the earth will have to travel 25m to the garage CU and a further 15m (approx length of cable run) back to the house CU. To resolve this, I thought I could insert an earth rod outside the shed with a 16mm sq cable connecting this to the shed CU.

I'm sorry for the essay, but am trying to get all the information here.

In summary, here is what I am thinking: -

House CU 40 amp MCB - 6mm sq cable to garage - 5 way CU with 100 amp RCD - 32amp MCB - 2.5 mm sq SWA to shed - shed CU - 10 way CU with 63 amp RCD, with an earth rod in the ground outside the shed.

The exterior lighting will be 100w max. The shed sockets will only be running the lawn mower, strimmer, maybe rechargable power tools - essentially nothing with any great load to it. Obviously, the exterior lighting will be running at night and with the exception of the interior shed light, it is unlikely there will be any power taken from the sockets.

I am aware the will be some voltage drop but have no idea if the drop will prove too much for the set up I have in mind.

Is this acceptable? If it is belt and braces I am happy. If not suggestions please!!!!

I know all this would be notifiable and the reason for using a 10 way CU in the shed, a 5 way in the garage and 2.5 mm sq SWA between the garage and shed is because I already have them along with 6 metal clad blank boxes a rake of MCB's of varying amperages. I have amassed this kit over a number of years with a view to doing this work.

I think this installation is ok, my only concern is the existing 63amp RCD within the house CU.

Having read through this post, I think some pics may help to clarify my intentions.

Proposed shed wiring

• cymru001
• 29 Apr 2010
• 1

 

[mikhailfaradayski]

Sounds like overkill to me...

Why don't you add up the power of ALL devices that you may reasonably foresee using in the shed/garage combined.

With this info, you will be able to work backwards to define whether you really need all those mcb's.
Design currents, voltdrop, disconnection times etc can then be considered.

Do you really mean 'ring' as opposed to radial?

Why would you need 100A rated RCD?
Why would you need multiple RCD's inline with each other?

 

[Monkeh]

And just why do you think you need five MCBs for the lights and five boxes to gland the SWA to?

 

[cymru001]

Sounds like overkill to me...

Why don't you add up the power of ALL devices that you may reasonably foresee using in the shed/garage combined.

With this info, you will be able to work backwards to define whether you really need all those mcb's.
Design currents, voltdrop, disconnection times etc can then be considered.

Do you really mean 'ring' as opposed to radial?

Why would you need 100A rated RCD?
Why would you need multiple RCD's inline with each other?

Thanks for the reply.

The sockets in the garage are on a ring the light in the garage is radial. The sockets in the shed along will be a ring and the interior light will be radial.

Each exterior light will be radial via the remote control module.

I thought the in line RCD's would provide 'zone ' protection for the shed and garage individually.

I could reduce the number of MCB's by paring two external lights with one module. This would mean I would only need one feed into the module. i.e. - top pair of lights fed by one module and its supply, bottom pair of lights by a second module and its supply and the tree light could then be on a third module with a supply. This obviously means three modules each with a supply so then only three MCB's. Would I have to increase the MCB rating to accomodate for this?

 

[cymru001]

And just why do you think you need five MCBs for the lights and five boxes to gland the SWA to?

I'm thinking each light will have a 2.5 mm sq SWA cable running to it. Each cable will require the armour to be earthed. I know I could probably use one large metal box and terminate each cable to it, but I already have the small, single socket sized metal boxes with blanking plates.

These have been donated to me through work as they became surplus, as has the other gear.

 

[Monkeh]

The sockets in the garage are on a ring the light in the garage is radial. The sockets in the shed along will be a ring and the interior light will be radial.

Why are you going to use a ring for a 16A circuit?

I could reduce the number of MCB's by paring two external lights with one module. This would mean I would only need one feed into the module. i.e. - top pair of lights fed by one module and its supply, bottom pair of lights by a second module and its supply and the tree light could then be on a third module with a supply. This obviously means three modules each with a supply so then only three MCB's. Would I have to increase the MCB rating to accomodate for this?

Let's try that again. Why does each module need an MCB to itself?

 

[cymru001]

Why are you going to use a ring for a 16A circuit?[/quote]

It seems like the right thing to do

Let's try that again. Why does each module need an MCB to itself?[/quote]

Agian, I just thought it right to protect each module individually. Are you saying I could have one MCB and create a ring to supply the modules?

 

[Monkeh]

Why are you going to use a ring for a 16A circuit?

It seems like the right thing to do

Why? All it's going to do is waste copper.

Let's try that again. Why does each module need an MCB to itself?

Agian, I just thought it right to protect each module individually. Are you saying I could have one MCB and create a ring to supply the modules?

There's no need to supply them seperately. And no, you don't need to use a ring. Just a radial.

 

[cymru001]

Why? All it's going to do is waste copper.

OK, a radial set of sockets it is then!

There's no need to supply them seperately. And no, you don't need to use a ring. Just a radial.

What size MCB should I use and if one light developed an earth fault, how would I trace it?

What about the rest of the earth arrangments, would the rod be acceptable?

 

[Monkeh]

There's no need to supply them seperately. And no, you don't need to use a ring. Just a radial.

What size MCB should I use and if one light developed an earth fault, how would I trace it?

6A will be more than sufficient. The maximum 100W you've stated is under half an amp. As for the earth fault, just.. disconnect the lights?

What about the rest of the earth arrangments, would the rod be acceptable?

Without knowing more about the situation (earthing type, construction, etc), nobody can say. However, more than likely you don't need a rod. What makes you think you do?

 

[cymru001]

6A will be more than sufficient. The maximum 100W you've stated is under half an amp. As for the earth fault, just.. disconnect the lights?

I thought of that after I hit submit

Without knowing more about the situation (earthing type, construction, etc), nobody can say. However, more than likely you don't need a rod. What makes you think you do?

The test sheet I have from the re-wire says I have a TN-S system (???). The garage is brick and the shed is timber (If that helps).

The reason for the rod is because a couple of years ago, I was talking to an electrician doing some installation work at my depot. I was talking about this situation and he said I should not rely on the earth which will essentially be running from the shed, through the garage, into my house CU and away to the sub station. He felt, as the shed (in terms of the cable route) is some distance away from the house and the earth within, it would be wise to install an earth rod as a back up if you like. Would it be a good idea or forget it? I have the opportunity to get an earth rod complete with clamp (again this would be FOC ) The 16mm sq earth wire I also have here - although I only have about 4m.

 

[Monkeh]

Without knowing more about the situation (earthing type, construction, etc), nobody can say. However, more than likely you don't need a rod. What makes you think you do?

The test sheet I have from the re-wire says I have a TN-S system (???). The garage is brick and the shed is timber (If that helps).

The reason for the rod is because a couple of years ago, I was talking to an electrician doing some installation work at my depot. I was talking about this situation and he said I should not rely on the earth which will essentially be running from the shed, through the garage, into my house CU and away to the sub station. He felt, as the shed (in terms of the cable route) is some distance away from the house and the earth within, it would be wise to install an earth rod as a back up if you like. Would it be a good idea or forget it? I have the opportunity to get an earth rod complete with clamp (again this would be FOC ) The 16mm sq earth wire I also have here - although I only have about 4m.

I see no reason the existing earth would not be sufficient. Unless you have the equipment to test the earth rod is suitably installed, it could be more dangerous than useful..

 

[flameport]

What you have designed would probably work, and could well be compliant, however it is a complete waste of materials and time.

Another method:

Add a metalclad 13A FCU to the garage socket ring.
Fix a 3A metalclad FCU in the shed.
Connect the two FCUs with a length of 2.5mm SWA.

In the shed, connect the socket to the supply side of the 3A FCU.
Connect the shed light and the outside light control box to the load side of the 3A FCU.
Fix a single large metal box to connect all of the outside light SWA cables into.

Flog the rest of the gear on eBay.

 

[cymru001]

I see no reason the existing earth would not be sufficient. Unless you have the equipment to test the earth rod is suitably installed, it could be more dangerous than useful..

I see. Is this something I could test myself? If so what equipment would I need?

I work in the rail industry and know guys who install electrical equipment. I could ask them if they have the relevant test equipment.

I am thinking of going down the local building inspector route as I think it would be a cheaper option than getting an electrician in to do the work. I feel confident enough to carry out the installation and assume the inspector will flag anything which is unacceptable

 

[cymru001]

What you have designed would probably work, and could well be compliant, however it is a complete waste of materials and time.

Another method:

Add a metalclad 13A FCU to the garage socket ring.
Fix a 3A metalclad FCU in the shed.
Connect the two FCUs with a length of 2.5mm SWA.

In the shed, connect the socket to the supply side of the 3A FCU.
Connect the shed light and the outside light control box to the load side of the 3A FCU.
Fix a single large metal box to connect all of the outside light SWA cables into.

Flog the rest of the gear on eBay.

I thought that would be a simple way of doing it, I was just worried about the loading of it all and making sure it was safe!! Plus, had I thought of that a couple of years ago, I would've saved a lot of time in stock piling all I thought I needed!!

I guess I need to come here first when I get my next bright idea