DC lighting transfromers

[greenjeans]

Hi, I have a very long run of low voltage lights between townhomes. trying to bid lighting the walk way which is very dark, and only 12" of dirt to dig in(loaded with all utility for 16 units) digging 6" might for tuff. my longest run will be 200' from the transformer, and someone suggested using a DC transfromer for the long runs. Installing 16- 18 watt path lights on three runs. found a 24VDC (3x100w output) transformer, but not sure about how to calulate the voltage drop. Was told dc didnt have any. any thoughts. thank you, Joe

 

[dicklaxt]

You got me confused asking about a DC transformer so I guess you are speaking of a package that has an AC transformer and a rectifier network with a DC output.

Yes you do have VD on DC and it is dealt with just as AC,factors of distance,load and resistance.
dick

 

[WinZip]

I would use in ground transformers along the run fed by 120v of course then branch out to your 12v lighting.

 

[WinZip]

Hi, I have a very long run of low voltage lights between townhomes. trying to bid lighting the walk way which is very dark, and only 12" of dirt to dig in(loaded with all utility for 16 units) digging 6" might for tuff. my longest run will be 200' from the transformer, and someone suggested using a DC transfromer for the long runs. Installing 16- 18 watt path lights on three runs. found a 24VDC (3x100w output) transformer, but not sure about how to calulate the voltage drop. Was told dc didnt have any. any thoughts. thank you, Joe

This calculator may help you if you split the run using two in ground transformers or three.

http://www.residential-landscape-lighting-design.com/calculator.asp

 

[hbiss]

...someone suggested using a DC transfromer for the long runs... not sure about how to calculate the voltage drop. Was told dc didnt have any.

That someone must be from the Twilight Zone. Maybe there they have such a thing as a DC transformer and DC exhibits no voltage drop.

Do what Win Zip says.

-Hal

 

[mxslick]

I would use in ground transformers along the run fed by 120v of course then branch out to your 12v lighting.

...someone suggested using a DC transfromer for the long runs... not sure about how to calculate the voltage drop. Was told dc didnt have any.

That someone must be from the Twilight Zone. Maybe there they have such a thing as a DC transformer and DC exhibits no voltage drop.

Do what Win Zip says.

-Hal

I agree that the DC isn't gonna make any difference with the VD issue.

But if the OP is saying that there's only 12" of soil and getting 6" deep is gonna be difficult, don't you guys think that getting a conduit for 120v to required depth is gonna be even harder?:roll:

 

[WinZip]

The way I calculate it you would need 8/2 LV cable connected to 17 volt multi tap tranny to get aprox 11.5 volts to the fixtures , problem is when a lamp burns out there will be more voltage on the balance of the fixtures that are working then as another lamp burns out the voltage goes higher then blows them all out, then daisy chaining you will see voltage drop near the end of the run meaning lamps will be dimmer near the end , not a good way to go - in my opinion I suggested in ground transformers maybe 2 of them and if burial depth is a problem with the 120v you can run IMC 6 in deep to transformers.

 

[WinZip]

Hi, I have a very long run of low voltage lights between townhomes. trying to bid lighting the walk way which is very dark, and only 12" of dirt to dig in(loaded with all utility for 16 units) digging 6" might for tuff. my longest run will be 200' from the transformer, and someone suggested using a DC transfromer for the long runs. Installing 16- 18 watt path lights on three runs. found a 24VDC (3x100w output) transformer, but not sure about how to calulate the voltage drop. Was told dc didnt have any. any thoughts. thank you, Joe

I made a small drawing of what I would do but will explain here.

use 2 150 watt in ground transformers 12v -

tranny #1 4 fixtures on each side of tranny

tranny #2 100 foot away from #1 tranny with 4 fixtures on each side of it you could use 14/2 LV cable but I would use 12/2

For your 120v circuit I would run IMC 6 in deep from electrical panel or your power source to tranny #1 an from there to tranny #2 and drop the LV cable in the same trench.

 

[LarryFine]

I wonder if there's a legal way to run, for example, ten 12v lights in series on 120v. Of course, LV wire wouldn't do, but as far as doing it. I understand runway lighting is done that way.

 

[dbuckley]

Runway lighting certainly is done this way; its a constant current system, whereby 6.6A flows through the circuit, the voltage across the circuit being enough to maintain 6.6A. The voltage can go as high as 5000V plus and minus with respect to earth, ie 10KV.

There are clip together assemblies that can make all this happen.

Given how much smaller a home lighting system would be than a runway, it may be that the voltage across the loop would be acceptbale for home use...?

Time was when street lighting was done this way too, and I seem to recall the l;ast vestige of it was in SoCal, and may even still be there...

 

[hurk27]

Here is a way I have found to do long runs, and since all lamps light at the same briteness level nobody notices the voltage drop:

A low voltage ring circuit.

For 200' run, lets say 300 watts, make two runs of #10 cord or just one continuous 400' run down to the end and back, alternate each other fixture to each other cord in the run, at the transformer connect both of these cords to the transformer paying close attention to the polarity marking on the cord (Rib wire to Rib wire) and in the case of two run do the same at the other end, this will create what is called a loop fed lighting circuit that is or was used in Europe, it places all the lights with equal voltage so if there is a voltage drop it isn't noticed, a little costly for the extra wire but worth it in never having complaints for some of the lights being dimmer, just remember voltage drop is much more profound at lower voltages then at 120 volts.

 

[broadgage]

What about LED lights?
The current would be far less for a given light output, and the better types of LED lamp incorporate a voltage regulator in order to give constant light over a range of voltages.
It might be possible to use a 15 volt supply, with nominal 12 volt lamps, thereby allowing for 5 volts drop. max voltage at nearest lamp=15 volts, min voltage at last lamp =10 volts.

Or what about a low voltage multi wire branch circuit?
Exactly the same principle as a regular 120/240 volt MWBC but at 12.5/25 volts, from a 25 volt transformer with center tap, and 12 volt lamps.
For a given load in watts, a 12.5/25 volt MWBC will allow 4 or 5 times the cable length, for the same % voltage drop., as a straight 12 volt circuit.

 

[WinZip]

Here is a way I have found to do long runs, and since all lamps light at the same briteness level nobody notices the voltage drop:

A low voltage ring circuit.

For 200' run, lets say 300 watts, make two runs of #10 cord or just one continuous 400' run down to the end and back, alternate each other fixture to each other cord in the run, at the transformer connect both of these cords to the transformer paying close attention to the polarity marking on the cord (Rib wire to Rib wire) and in the case of two run do the same at the other end, this will create what is called a loop fed lighting circuit that is or was used in Europe, it places all the lights with equal voltage so if there is a voltage drop it isn't noticed, a little costly for the extra wire but worth it in never having complaints for some of the lights being dimmer, just remember voltage drop is much more profound at lower voltages then at 120 volts.

I believe you would need to send 14 v across each # 10 to make that work.

We usE to do something similar to that with pole lighting , run three phase 1200 feet an alternate phases at each pole like this - AB CA BC - may have been a run of 15 pole lights 400 watt HPS

 

[WinZip]

I hope to get this job I looked at Friday very expensive home an must be 60 low voltage lighting fixtures an transformers all over the property an the home owner asked if we worked on this type of lighting and wants it all repaired.

It was all installed about 3 years ago , lamps burn out constantly and they only use it when having parties because of all the problems an from some of what I saw it had to be idiots doing the work , one tranny I saw 900 watt had a #10 connected to 16v terminal coming out of tranny 5 feet then connected to 5 other cables that looked to be #12 going in diffrent directions so I turned it on an went to the first fixture of a run that appeared to have 10 fixtures on it an the first seemed very bright an the last one looked dim.

I told her let me try an map this out as best I could an come up with a plan to repair all of it , front - back - and sides of the home.

 

[gar]

100919-1239 EST

hurk27:

I did not understand your description. But I believe the following may be essentially what you were descirbing:

Run 3 wires the full length of the run. Label these A, B, and C. At the far end connect B to C. Wire C will have no lights connected to it. Its sole purpose is to supply a current path to the far end. B is left open at the source. All the lights are connected between A and B. Power is supplied to A and C. To any light in the circuit there is the same supply line resistance.

If each wire has a resistance of R, then the furthest lamp has a line resistance of R + R to the lamp. One R on each side of the lamp. The lamp next to the source has a line resistance of 2R + 0. Two wires of R to the lamp plus one wire of 0.

If there was 100 ft to the first lamp, then B does not need to extend to the source point, just to that first lamp.


WinZip:

Besides using hurk's suggestion consider reducing the lamp voltage to maybe 10% below the nominal bulb voltage. About a 4 times increase in life. See http://www.newport.com/Light+Sources/Quartz+Tungsten+Halogen+Sources/1/5458/product.aspx

.

 

[WinZip]

There all good ideas but personally I would do this job the way I explained it.

I'm not blowing my own horn either.

 

[hurk27]

100919-1239 EST

hurk27:

I did not understand your description. But I believe the following may be essentially what you were describing:

Run 3 wires the full length of the run. Label these A, B, and C. At the far end connect B to C. Wire C will have no lights connected to it. Its sole purpose is to supply a current path to the far end. B is left open at the source. All the lights are connected between A and B. Power is supplied to A and C. To any light in the circuit there is the same supply line resistance.

If each wire has a resistance of R, then the furthest lamp has a line resistance of R + R to the lamp. One R on each side of the lamp. The lamp next to the source has a line resistance of 2R + 0. Two wires of R to the lamp plus one wire of 0.

If there was 100 ft to the first lamp, then B does not need to extend to the source point, just to that first lamp.


WinZip:

Besides using hurk's suggestion consider reducing the lamp voltage to maybe 10% below the nominal bulb voltage. About a 4 times increase in life. See http://www.newport.com/Light+Sources/Quartz+Tungsten+Halogen+Sources/1/5458/product.aspx

.

The circuit I'm trying to explain is just two conductor low voltage cable some call ZIP cable, it has one marked conductor that is marked by ribs on the insulation, the run is simply a loop of this two conductor cable to the end of the run and back, by alternating each fixture to which side of the loop will place the fixtures equally around this loop, at the supply end both ends of the cable will terminated together RIB-RIB to terminal 1, and plain-plain to terminal two on the transformer, this allows for an equal distribution of the resistance all the way around the loop, thus allowing all lamps to have equal brightness even if there is a voltage drop,

do a search on "ring circuits" on here as it has been discussed many times.

Here is one such thread back in 2003:
lv lights dim at the end of the run.

 

[WinZip]

Here is a way I have found to do long runs, and since all lamps light at the same briteness level nobody notices the voltage drop:

A low voltage ring circuit.

For 200' run, lets say 300 watts, make two runs of #10 cord or just one continuous 400' run down to the end and back, alternate each other fixture to each other cord in the run, at the transformer connect both of these cords to the transformer paying close attention to the polarity marking on the cord (Rib wire to Rib wire) and in the case of two run do the same at the other end, this will create what is called a loop fed lighting circuit that is or was used in Europe, it places all the lights with equal voltage so if there is a voltage drop it isn't noticed, a little costly for the extra wire but worth it in never having complaints for some of the lights being dimmer, just remember voltage drop is much more profound at lower voltages then at 120 volts.

I would like to see the voltage at the last fix of the 400 foot loop run , run A 200 foot then loops back run B 200 foot there would be 8 fixtures alternating on each run of the loop A-B 144 watts each , maybe 8/2 would work but nothing less than 8/2 an that's about 90 cents per foot I believe.

Please correct me if I'm wrong.

 

[dicklaxt]

I'd like to see a sketch of all these scenarios,,something's not computing in this old noggin.

dick

 

[hurk27]

I'd like to see a sketch of all these scenarios,,something's not computing in this old noggin.

dick

Draw two circles one just inside of the other, these are your two 12 volt circuit conductors, now place a transformer at the bottom and connect each terminal to one of the circles, now place fixtures around this circle and connect each of its wires to each of the circles, as you can see this ring is fed to both sides of the ring, and the voltage and resistance will be equal around this ring circuit, of course in real world installation, the cable run in the same trench out to the end and back to the transformer, and you would have to identify which one is which before you cover them up, so you can attach each other fixture to each other cable, or like I have done is route the whole cable in a way it ends back at the transformer so both ends land on the transformer.