Low Voltage lighting query

Discodog

I should know how to work this out but I forgot most of my Physics years ago. I am installing some 12 volt garden lighting. Because of the layout I have to run two 10 watt halogen lamps at 80 metres from the transformer & I can't loop.

Obviously I am going to get a voltage drop over that distance & a dimming of the lamps. Can I overcome this by replacing the 10 watt lamps by 20 watts ? I have loads of capacity in the transformer.

If I attach more lamps can I spread the voltage loss by putting 10 watt lamps near the transformer & higher wattages further away ?

M cebee

I dont think you can overcome the voltage drop problem easily without oversizing cable or moving trafo closer

maybe led will work

Discodog

But surely a 20 watt lamp will burn brighter that a 10 watt even if the voltage drops ?

2011

Discodog wrote: »

But surely a 20 watt lamp will burn brighter that a 10 watt even if the voltage drops ?

No.
If the lamp is a higher wattage it will have a higher volt drop.
Under voltage will cause the higher wattage lamp to be more yellow (less white) and it may even produce less light.

The best solution is to size the cables correctly or install the transformers local to the lights to limit the volt drop.

M cebee

the volt drop for a 12v 20watt bulb and 80mts of 1.5 swa seems to be 4v


you could maybe try an led equivalent on the end of a roll of cable-to see if it works

seems like a lot of trouble anyhow

Discodog

M cebee wrote: »

the volt drop for a 12v 20watt bulb and 80mts of 1.5 swa seems to be 4v

It would be a great help if you could link me to the calculation.

M cebee

20/12 * 80 * 0.031

watt/volt * distance * voltdrop per amp/per metr

Discodog

M cebee wrote: »

20/12 * 80 * 0.031

watt/volt * distance * voltdrop per amp/per metr

Thanks but how do I adjust that for different thickness of cable ?

M cebee

the last table column 3 gives you the rough values
http://www.csecables.com/power-cable/swa-cable.asp

bit pointless as the larger cables will be prohibitively expensive -will be cheaper to site trafo closer to lamps for 12volt

Discodog

M cebee wrote: »

the last table column 3 gives you the rough values
http://www.csecables.com/power-cable/swa-cable.asp

bit pointless as the larger cables will be prohibitively expensive -will be cheaper to site trafo closer to lamps for 12volt

I know. Unfortunately moving the transformer is not easy. It would mean running outdoor mains cable which wouldn't be cheap either - might as well use mains lights.

I would imagine that it is going to be a problem for a few people. I was in B&Q yesterday & their lighting display tells you to pick a transformer & the relevant number of lights - there is no mention of distance !

If I could get an LED replacement bulb would it draw less current for the same output ?

M cebee

80m of cable for 12 volt lights will be more expensive than the equivalent for 230v

you still need the copper and cable has to be suitable so it doesn't degrade


those garden kits might come with say a 60va trafo and 3* 12v 20watt halogens -the hallogens will have a lead of 5 or 10m or something like that

they wont work over 80m

2011

Discodog wrote: »

If I could get an LED replacement bulb would it draw less current for the same output ?

Yes

Discodog

M cebee wrote: »

80m of cable for 12 volt lights will be more expensive than the equivalent for 230v

you still need the copper and cable has to be suitable so it doesn't degrade


those garden kits might come with say a 60va trafo and 3* 12v 20watt halogens -the hallogens will have a lead of 5 or 10m or something like that

they wont work over 80m

The kits in B&Q, made by Ring, are separate units that you match up. The cable comes in 75 metre bundles for about €30. It's only when you get home & read the in box instructions that distance comes into play. They say that a 75 metre ring is OK but the cable is pretty thin.

I think that it's worth me experimenting with an LED "bulb" before I start digging up garden & patio :eek:

Discodog

M cebee wrote: »

yes

theres's a big difference in voltage drop between a light at the end of a 75m radial circuit
and a light on a 75m ring
if the light is centred on the ring the volt drop is 1/4 of the radial but the light is only 37.5m max from trafo then

OK so it's not a linear voltage drop. I would of imagined that a lamp in the middle of the ring ie 37.5m from the transformer, would suffer 50% of the drop of a lamp at the end of 75 m.

M cebee

sorry there i was editing that:

theres's a big difference in voltage drop between a light at the end of a 75m radial circuit

and a light on a 75m ring

if the light is centred on the ring the volt drop is 1/4 of the radial but the light is only 37.5m max from trafo then


a ring would be the best wiring arrangement for those alright if the layout of the lights suited it

M cebee

Discodog wrote: »

OK so it's not a linear voltage drop. I would of imagined that a lamp in the middle of the ring ie 37.5m from the transformer, would suffer 50% of the drop of a lamp at the end of 75 m.

no it's a quarter

if you go back to the equation -distance is halved and csa is doubled

Discodog

M cebee wrote: »

a ring would be the best wiring arrangement for those alright if the layout of the lights suited it

Unfortunately the return leg has to cross a patio & there is no grout so no easy option.

Michael Collins

I'm not aware of the regulations in this area but is it possible Discodog could use a 24 V power supply and a 12 V regulator at the lamps? It would certainly solve the problem...

M cebee

don't think there would be any difference in regulations

i assume they're both classed as 'pelv' as you couldn't guarantee separation from ground on the wiring

M cebee

Michael Collins wrote: »

I'm not aware of the regulations in this area but is it possible Discodog could use a 24 V power supply and a 12 V regulator at the lamps? It would certainly solve the problem...

ebay seems to have 24v halogen and led in mr16 anyhow so that would be simpler

2011

M cebee wrote: »

don't think there would be any difference in regulations

I would agree, I just dont think it would be worth the hassle or espense.

M cebee

Yes there isnt much the op can do except install the 12v kit as per instructions

Michael Collins

M cebee wrote: »

ebay seems to have 24v halogen and led in mr16 anyhow so that would be simpler

Yeh that'd be even better, but I guess it's too late now if the kit has already been purchased.

Discodog

Had I of seen them I might of gone for 24 volt but then again there is less risk with B&Q. A transformer is a heavy lump to send back if it's faulty. First I am going to try some LED bulbs & if that doesn't work then I will have to cross the patio to complete the ring.

I have done some really nice layouts before using a 12 volt transformer & car bulbs. They can look nice in old bottles & old lamps etc. It's also really easy to vary the wattage as there is a good available range.