am i using the wrong flex?

[mrmanor]

i have fitted some garden low voltage lights. i have two circuits - each with 4 x 20w lamps.

the transformer is rated as 8amps.

the flex that was supplied was too short for where i wanted to place the lamps...so i have used 3amp as it is only 80watts going through it.

the circuit is very long.

what i have found is that the lamps that are fitted furthest from the transformer are very dim.

q. if i replace with 6 or 10amp flex - will this enable all the lamps to be equally bright?

please help.

 

[bernardgreen]

so i have used 3amp as it is only 80watts going through it.

Watts do not go through cable. Amps go along the cable and as they do they create a voltage drop due to the resistance of the cable

q. if i replace with 6 or 10amp flex - will this enable all the lamps to be equally bright?

Probably not. From your description the cable lengths between each lamp and the transformer are different so the voltage dropped by the current on its way to the lamps is different for each lamp.

Best option is to wire the lamps with a lead per lamp to a central point where the leads from lamp to central point are similar in length. Then from that central point take a thicker cable ( as thick as you can use ) to the transfomers. That way all the lamps suffer the same voltage drop as each other and will be the same brightness.

 

[AdrianUK]

You have bumped into the same problem that Edison did & that eventually killed DC low voltage distribution.

Firstly, your lamps require 80W @ 12V which is 6.6A so.... your 3A flex is not suitable!

Secondly, the reason that the flex between the transformer & the lamps is short to keep the volt drop low..... whenever you pass a current along a wire there will be a voltage dropped along the length of that wire. This is directly proportional to the cross sectional area of the wire & its length ergo the longer the wire, the higher the volt drop - the volt drop is subjected from the source voltage to get the voltage at the end of the wire. This is why your distant lamps appear dim.


Solutions:

1 use thicker wire

2 wire each lamp directly back to the transformer - dont daisy chain. If you daisy chain then the first section of cable carries the current for all lamps instead of just one.

3 How long is 'long'? trying to go, say 50m @ 12V is never going to be very successful unless you have a very thick cable.


Adrian

 

[Lectrician]

80watt at 12 volt will be around 6amp.

You need to keep the cable between the transformer and fittings as short as possible. If extending the lengths too far, you would need to use large cables - even 4, 6, 10mm!

These kits are usually supplied with the correct length of cable (maximum).

You should look to extend the mains supply with some SWA etc.

If the transfomers are electronic, you are not meant to run long distances at all - you will be emitting EMF everywhere! Wire wound is used in these instances.

 

[mrmanor]

thank you for all advice.

the furthest lamp is maybe 25m away from the transformer.

i don't have the option of making the distance between the each light and the transformer the same - due to where the plug is.

from the advice.... can i clarify: if i replace with a thicker cable - maybe 1.5mm, and wire each lamp separately to the transformer - i would see a marked improvement?

 

[Taylortwocities]

1.5mm will not make a bit of difference.

If extending the lengths too far, you would need to use large cables - even 4, 6, 10mm!

These kits are usually supplied with the correct length of cable (maximum).
.

These are the sort of cables used to hook up big showers and cookers so be prepared to pay lots for all that thick copper.

Maybe time to rethink your approach.

 

[Lectrician]

Is it one cable looping past the lights? 25m to the furthest?

Even 1.5mm would be too small IMO over that distance.

The trouble with 12V Halogen lights is they are actually very voltage sensitive. Under-voltage and over-voltage have a dramatic effect on the lamp life.

 

[ericmark]

Much does depend on the power supply there are systems which control amps (to within limits) rather than volts so it will better compensate for volt drop along the cable. But in the main the power supply regulates the voltage. Again some clever ones have sensing wires to measure the volts at the lamps but most are very simple and are designed to be very close to the lamp.

Using longer cables can be a problem as it will depend on the frequency of the supply and one could end up at a null point. It then would not matter how thick the cable was the lights will still be dim. Likely you are using a high frequency inverter not a transformer and although Nikola Tesla won the battle of the currents today even at 50Hz long distance power lines as used in South Africa do have problems and DC is used.

So there is only really one option and that's keep the extra low voltage cables to manufactures recommendation and cover the distance with low voltage and re-site the inverters.

Although it may work with heavier cable since you likely don't know the frequency output of the transformer extending on the high frequency extra low voltage side is likely to produce problems.

 

[ban-all-sheds]

i have fitted some garden low voltage lights. i have two circuits - each with 4 x 20w lamps.

the transformer is rated as 8amps.

the flex that was supplied was too short for where i wanted to place the lamps...so i have used 3amp as it is only 80watts going through it.

1) You don't know that 80 ÷ 12 is more than 3.

2) You don't understand voltage drop.

3) I'll bet what's left of my pension that you did not apply for Building Regulations approval before starting this work.

4) Please get an electrician.

 

[SimonH2]

the flex that was supplied was too short for where i wanted to place the lamps...so i have used 3amp as it is only 80watts going through it.

Which means that it is no longer an "extra-low voltage lighting system which is a pre-assembled lighting set bearing the CE marking referred to in regulation 9 of the Electrical Equipment (Safety) Regulations 1994(a)" (if it ever was) which it would have to be in order to be exempt from notification for building regs. Thus the work is notifiable (my LABC would charge around £225 for this ) - or must be done by someone who is in an appropriate scheme and can self notify (for a few quid over and above their time etc.)

 

[blackranger]

how sensitive are the lights too voltage fluctuations?

Can he not alter the voltage to compensate for the long run?

This is common place with ptz camera's over long runs of rj45(if i remember correctly) . Mind you the cameras i have used are 12-24v so plenty room too play with.

 

[SimonH2]

how sensitive are the lights too voltage fluctuations?

Very. Under voltage makes them go dim, over voltage makes them blow.

Can he not alter the voltage to compensate for the long run?

I've never seem a transformer with such adjustment. Plus, if you do that, then as soon as a lamp blows, the rest will quickly follow as the voltage will go up.
There are (someone mentioned them earlier) some transformers with remote sensing. These will compensate for the voltage drop - at least to the first junction box.

But as already pointed out, running long, low voltage & high current, cables is the wrong way to do it. It would probably be better to put the transformer close to the lamps - this can be done as long as the right equipment is chosen and it's installed correctly. It would be notifiable work, but so is the OPs installation now he's modified the lighting set and no longer able to use an exemption from Schedule 4.