Low voltage lighting systems

[geordie8]

Its my first post so please be gentle. We are wanting to set up a low voltage lighting system in the garden to run 160W (16 x 10W halogens) the total run length of cable is 35 metres to reach the last light. The cable isnt looped. Please can someone help with the voltage drop. The cable we have is 1.5mm outdoor flex cable, is this sufficient for this design.

thanks

 

[PrenticeBoyofDerry]

Low Voltage or Extra Low Voltage?
There is a difference!

 

[geordie8]

Sorry, forgot to mention its a 12V system.

 

[Stoday]

Are you using a 12V transformer for each light or feeding the whole cable at 12V or something else? If else, what?

 

[geordie8]

Hi

Yes I'm going to spur off a internal socket via a RCD & then armoured 240V cable out to transformer. Using a 200VA 240/12V transformer mounted outside then running 1.5mm2 cable a total of 35 meters. Connecting off this 12V cable will be 16 x 10W halogens. Total current 13A and I understand the capacity of 1.5mm is 15A but wasnt too sure about the voltage drop especially towards the end of the cable and the heat losses running this amount of current. Unfortunately the cable is already cut in 16 sections (its ran in a wall) so we cant loop it. Just wanted some advice on the setup.

thanks

 

[davelx]

VD for 1.5mm 2 core is 29mv/A/m

Each lamp takes 10/12 = 0.833A at 12v but as you move along the cable the voltage drop will mean that each lamp actually takes less than that.

The first cable segment from tx to lamp will therefore carry 16x0.833A
the second cable segment (lamp 1 to lamp 2) will carry (15x0.833A)- (a bit) the third segment (lamp 2 to lamp 3) 14x0.833A - (a bit more)

etc etc etc

You need to know the length of each cable segment. Then you can work out the voltage drop for each segment and the cumulative drop at each lamp along the run. Excel would make this easier.

A quick calculation suggests to me that you will only be getting about 6V at the far end so you are going to need bigger cables, or run at mains voltage and use luminaires with integral transformers,

 

[ban-all-sheds]

Sorry, forgot to mention its a 12V system.

The IEC definitions of voltage bands are:

• Extra Low Voltage: AC below 50V and DC below 120V
• Low Voltage: 50 - 1000V AC or 120 - 1500V DC
• Medium voltage: 1kV - 35kV
• High voltage: 35kV - 230kV
• Extra-high voltage: >230kV

This is not some pedantic technical point - if you start getting involved in doing your own electrical work, and start learning about regulations etc you'll come across references to "Low voltage", and it'll be no good you thinking that that means 12V...

Yes I'm going to spur off a internal socket via a RCD

So your existing sockets are not RCD protected?

then armoured 240V cable out to transformer.

How will that be installed, i.e. where will it run?

How will you terminate it at the socket and at the transformer?

then running 1.5mm2 cable

What sort of cable?

How installed and mechanically protected?

How will you terminate it at each light?

Connecting off this 12V cable will be 16 x 10W halogens.

Will you mind if they are all different brightnesses and colours?

understand the capacity of 1.5mm is 15A

Why do you believe that?

wasnt too sure about the voltage drop especially towards the end of the cable

It's about 29mv/A/m

and the heat losses running this amount of current.

Why do you think that "heat losses" are an issue? Is it a heating cable?

Unfortunately the cable is already cut in 16 sections (its ran in a wall) so we cant loop it.

Oh well - you're stuck with it then, no matter how unsuitable or dangerous it is.

Just wanted some advice on the setup.

You did notify LABC before you started all this, didn't you?

 

[geordie8]

Thanks to everyone for their help and advice on this. Learnt a lot, much appreciated. Could anyone give me the voltage drop for the 2.5mm2 cable please. Guess we have to re-think our installation.

 

[Lectrician]

You keep saying the cable ISNT looped?

Do you mean you have 16 cables coming back to the transformer, or do you infact mean that the cable IS looped past each fitting?

This makes a HUGE difference.

 

[electronicsuk]

You keep saying the cable ISNT looped?

Do you mean you have 16 cables coming back to the transformer, or do you infact mean that the cable IS looped past each fitting?

This makes a HUGE difference.

I've got a feeling the OP is suggesting that it isn't a ring circuit, but the supply is looped in and out of each fitting.

 

[ericmark]

Thanks to everyone for their help and advice on this. Learnt a lot, much appreciated. Could anyone give me the voltage drop for the 2.5mm2 cable please. Guess we have to re-think our installation.

There are correction factors but around 18mV/A/m for 2.5mm cable so assuming even spacing between each lamp 18*0.7*35 = 0.44v approx.

 

[ban-all-sheds]

Could anyone give me the voltage drop for the 2.5mm2 cable please.

So are your existing sockets not RCD protected?

How will the armoured cable to the transformer be installed, i.e. where will it run?

How will you terminate it at the socket and at the transformer?

What sort of 2.5mm² cable do you now plan to use for the lights?

How installed and mechanically protected?

How will you terminate it at each light?

Did you did notify LABC before you started all this?

Guess we have to re-think our installation.

Help is available here.

 

[geordie8]

The circuit isnt a ring circuit, I was trying to work out the worst case voltage drop for the 1.5mm2 and the 2.5mm2 and from this work out if it was necessary to install a ring circuit.

Pardon my ignorance but worst case scenario assuming:-

2.5mm2 cable
13.3A (all lights on & between transformer & 1st light)
spacing of 2m
correction factor of 0.7

we get a voltage drop of 0.336V between the transformer and the 1st light.

Can anyone point me to a web site which lists the voltage drops & conversion factors for various cable sizes.

 

[ban-all-sheds]

Pardon my ignorance

To what extent should it be pardoned when it's of the topic of cable calculations and it's leading you to use formulae which are incorrect?

Can anyone point me to a web site which lists the voltage drops & conversion factors for various cable sizes.

Can anyone answer these:

Are your existing sockets RCD protected?

How will the armoured cable to the transformer be installed, i.e. where will it run?

How will you terminate it at the socket and at the transformer?

What sort of 2.5mm² cable do you now plan to use for the lights?

How installed and mechanically protected?

How will you terminate it at each light?

Did you did notify LABC before you started all this?

 

[geordie8]

Thanks for that much appreciated. Apologies im not sure how to reply with your questions embedded so ive repeated them below.

So are your existing sockets not RCD protected? No

How will the armoured cable to the transformer be installed, underground & glanded into the transformer enclosure.

How will you terminate it at the socket and at the transformer? The elecrician will check and test this bit but via a RCD socket & then out the wall into a junction box to connect to the armoured cable.

What sort of 2.5mm² cable do you now plan to use for the lights? Outdoor 3 core type. The exterior wall is an L - shape with 17m & 8 lights on one length and 18m and 8 lights on the other. Because of the unmanagable voltage drop Im thinking to locate the transformer in the center with two radial circuits each with 8 lights on. The cable is routed within the double wall cavity but we can remove some bricks to gain access and replace it if necessary. It has 1.5mm2 cable ran in at the minute.

How installed and mechanically protected? The 12V cable is within a cavity wall.

How will you terminate it at each light? Each light is IP67 rated (although no seal on the mount) There is a small junction connector to terminate the 12V cable & I was going to use self amalgumating cable as extra water protection.

I just wanted to check the advice which we received from the electrician who said the 1.5mm cable would be okay.