Garden Electrics

[MB]

When landscaping my back garden, I decided to install some electric cable for future lighting and power requirements. I was advised to install 4-core cable which would run from a separate consumer unit in a shed, itself fed from the main consumer unit positioned in the house.

I am now at the point of wanting to connect the electrics up, but have now been told that I needed a 5 core cable to feed both lighting and power sockets. Originally, I was told that it would be acceptable to operate a common neutral, leaving me with 2 lives (one for lighting and one for power), and the earth.

If this method really can't be used, is it acceptable to use the outer armour as the earth, freeing up a cable for a second neutral?

Forgive my apparent ignorance, but I want to try and understand my best options (short of digging up the garden and replacing the cable with a

5 core!)

Thanks for any advice.

[Owain]

You can only use a common neutral if the power and lighting are both from the same MCB, ie the same circuit.

This may be possible if you are not using SES or SBC lampholders, and all your luminaires are suitable for a 16A circuit (and a 16A circuit is adequate for your power requirements).

If you wanted to use separate lives for switching, you could use X10 or similar remote switching system that sends the signals down the mains wires.

It depends on the characteristics, in particular length of cable, of the installation.

Owain

PS In England and Wales, Part Pee will apply.

[John Rumm]

In simple terms, and assuming that the cable is a steel wire armoured one, then yes you can use the armour for the earth.

Outdoor electrics are not always straightforward, and there are a few gottchas. We would need a bit more background to give any really useful advice. In particular, what sort of lights and sockets are you feeding? How is the shed CU fed with power, and what sort of earthing system does your house use. Also what cable lengths are involved.

[meow2222]

Life is so much simpler if you run 12v around instead. No regulations, no significant safety issue, no rcd trips, no wiring failures due to water ingress, run the cable where you want, fuse it how you want, plenty of lights readily available that will run on it etc. You could keep your cable for 240 power and add another for 12v lighting.

NT

[The Natural Philosopher]

I would still be tempted to put an earth spike in and earth to that AS WELL.

[tinnews]

... and you need very thick cable for any runs more than a few metres long which have significant power consumption through them.

[MB]

Thanks everyone for your comments.

In answer to the queries:

1. I don't know what type of earthing the main house uses - sorry, I am not an electrician (and it probably shows). However, if it helps, the house is only a few years old - so this might indicate the type of earthing used?

1. The CU in the shed would be supplied from a spare 32A fuse on the main CU in the house. I ran a 6mm 3 core armoured cable underground to supply this from the house to the shed.

2. The distances involved are quite large. I don't have the figures to hand, but it is c. 40 metres from the house to the shed. From the shed

5 cables radiate out across the garden. The furthest point that I am looking to supply is about 80 metres from the shed. The cable for these runs is 4 core 2.5mm. Voltage drops were calculated at the time, and I know that it worked out considering the likely level of loading.

1. I intend to use low voltage lighting, but due to the distances involved, I had to run 240v to as near as possible to the point of use. Naturally, I needed 240v for the power outlets.

I am intrigued about this idea of using separate lives for switching - my original intention was to have separate lighting zones in the garden, being able to turn each on independently. However, as I want to incorporate power sockets in these same zones using the same 4 core cable this is where the problem starts.

If I am allowed to wire both lives from the 4 core into the same MCB in the shed, then presumably to allow the switching, I would pass the live for the lighting through a separate switch, while the live for the power sockets runs uninterupted?

Alternatively, is it possible to 'link' two MCBs (a bit like the di-pole switch) so that if any maintenance is to carried out on one circuit, both have to be switched off at the same time?

Thanks everyone for their interest - please let me know if you need any more clarification.

Best regards

Michael.

[dingbat]

Then you should fix both of these (to a small, but useful extent). Go to your local wholesaler and splash out a tenner or so for a copy of "The On-site Guide". This explains pretty much all you ever need to know regularly for most situations, including the rules you need to follow for your earthing here. Well worth a read.

[MB]

Thank you for your advice.

I should make it clear that I have no intention of installing this myself - and will be looking for the help of a qualified electrican - I value my life too much. However, I do like to understand broadly what can and can't be done before paying the professional.

[MB]

Sorry, John, I neglected to answer one of your questions.

The power sockets to be supplied would be single gang 13Amp sockets, probably the MK Masterseal type which have a good IP rating.

I intend to use low volatge lighting, probably 12v - no SBS or SES type arrangements.

[Andy Wade]

Incorrect. The requirements of Part P and BS 7671 apply to the whole of an electrical installation, including ELV circuits. There's an exemption in Part P (Schedule 2B paragraph 3(a) for:

"telephone wiring or extra-low voltage wiring for the purposes of communications, information technology, signalling, control and similar purposes, where the wiring is not in a special location;"

but not for ELV power circuits. The work would still be notifiable since wiring in a garden counts as a "special installation" (as defined).

Significant risks of overheating and fire could exist if proper overcurrent protection is not provided. Protection against electric shock requires correct application of the many regulations in BS 7671 relating to SELV (or other ELV) systems.

[Owain]

Very careful consideration will have to be given to equipotential zones.

You can still do this if you use X10 or similar mains-borne remote control switching. This would allow you to control your lights etc from the house, or multiple locations.

Yes.

No.

Owain

[MB]

Thanks for your reply Owain. A couple of questions.......

Can you explain what you mean by this?

I don't understand how the X10 or similar would allow me to use a common neutral. Or are you assuming that both lives would be wired through the same MCB?

I like the idea behind the X10 product - originally, I had intended to use a remote switch panel which used similar technology to that used in remote garage doors and gates etc. However, the range on these can be a bit limited.

Thanks for your help.

[John Rumm]

A fair chance you have TN-C-S then... however to confirm, have a read of a couple of pages starting here:

it is still not obvious then post a picture of your supply installation (i.e. where the supply cable enters the house and connects up to the meter etc) for us to have a look at.

Cable wise that sounds ok. The length of the cable would suggest that your shed will need to be a TT install (i.e. local earth spike with RCD protection on all circuits). In which case you would not usually use the exported earth from the house (it would be there to protect the cable, but not connected to the CU in the shed). If you do have a TN-C-S installation then that is another strong indication that the shed supply should be TT since otherwise you would need to export and continue the equipotential zone in the shed. Depending on its construction this may be difficult.

With that long run you will need to watch voltage drop. A 2kw appliaance for example would pull the voltage down below spec even without including the 6mm SWA run to the shed. For lighting however it will probably be fine.

In this circumstance you don't actually gain much advantage from the low voltage lighting.

Can't see why. With 4 cores and a shield used as an earth you have plenty of options. For example you could have a live and neutral for a lighting circuit and use the other two wires for the socket. You could have three switched lives (starting at the same MCB) and one neutral to provide three discrete lighting zones (and no socket).

Yes, with the caveat that the lighting fixtures you use will need to be of a design that allows the use of a supply with a high protective device current rating. (i.e. a 16A MCB), Many lamp holders require lower rating protective devices than this.

No, absolutely not.

[John Rumm]

I alluded to this in my ealier post as well. If you have a TN-C-S install, it is very important that your equipotential bonding is up to current standards. This is because there are some fault conditions, like for example a break in the neutral connection to your property, that could leave all exposed earthed metalwork in your house sat at mains voltage. This is because with TN-C-S the connection between neutral and earth is made at your house and not at the substation. Take away the neutral and everything floats up to mains voltage. The solution to this problem is to ensure that any metalwork that you are likely to come into contact with is all electrically bonded together, keeping it all at the same fault voltage, and hence unlikly to give rise to a serious shock hazard.

Now when you take power outside, things become much more trickey. Access to an independant earth is in many cases now easy (you are standing on it!) So it is much harder to prevent you from simultaneously coming into contact with an electrical earth sat at a fault voltage, and a another local earth at a much lower potential. It is possible to do this in a well made shed or garrage with an insulating floor for example, but almost impossible in a building sat directly on the ground. For most practical applications it is simpler to disconnect from the house earth and use your own local one.

You don't have a common neutral as such, you just have one live and an associated neutral. There is no need to have multiple switched circuits anymore to achieve control of the lights.

Basically you daisy chain your lights between a live and neutral on a circuit. The X10 device provides switching for the light local to the switch (not for the whole circuit necessarily - unless that is what you want). The control information that instructs a X10 device to swithc on or off passes down the mains wiring to it.

[Owain]

John Rumm has explained, but to put it very simply:

The cable (and accessory) is earthed at a point up to 120 metres away (maybe more, depending on supply).

You are earthed where your feet touch the ground.

Those earths may not be at the same potential. You could get a shock when you touch the earthed cable/accessory.

As John has said, you just use 1 pair of wires per circuit. You may well need to fuse down at the lighting accessories, to protect both the lampholders and the X10 modules (and the transformers if you use LV lights), eg

MCB-------------13A-------13A--------|--------|-------13A | SKT SKT | | SKT | FCU FCU X10 3A 3A Controller | | X10 X10---LIGHT | | LIGHT ---LIGHT

They're wireless and quite limited. X10 should go quite a long way down a single mains cable.

You can get X10 modules that do dimming as well as on/off; couple a computerised controller to banks of 3-colour spotlights and you can have a continuously changing colour wash on the foliage.

Owain

[MB]

Owain & John

Thank you for your very useful advice.

One final question (for now at least), would the X10 system work across multiple consumer units? For example, were I to plug the controller in somewhere in the house, the signal would have to pass through the main house CU, to the proposed shed CU and then down the cable to the lights.

Thanks once again for your help.

Michael.

[Owain]

It should work. I have no personal experience of X10 but some other mains-borne signalling seems to get upset by ring circuits.

I'd suggest buying a couple of units and trying before committing for a large-scale installation.

Owain

[MB]

Thanks - I'll probably give that a go.

[Andrew Gabriel]

Connect the X10 sender to a socket near to the CU if possible. That will give best possible chance of coverage to everywhere.