I am installing some of that low voltage "rail" lighting on a remodel job we are doing. It is the stuff which can be bent to form smooth curves and has a 12 volt remote transformer.

The designer/GC has provided all of the rail, hardware, heads etc. and I am to locate the appropriate transformer based on the load. The rail is a 2 circuit track with ckt A - common - ckt B. During the rough, I ran an 8-3 romex to the canopy location in the ceiling from the basement where the transformer will be located. From the line side switchbox I ran a 14-3 to the basement so the rail can be operated by 2 separate switches.

Here is my problem. I cannot find a 2 circuit transformer which has 2 separate inputs. They all seem to have 1 input and a split output such as a 600 watt transformer with a pair of 300 watt outputs like this:
http://www.seagulllighting.com/Ambiance®-Lighting-94064-12.html

Is there a legal and safe way to join one leg of two separate transformers to create a common leg without frying the transformers?

The manufacturer has not been able to help me. My supplier said he thinks some specialty transformer company might have a product that may work to the tune of $1600.

Please help!

Thanks.

[This message has been edited by leespark (edited 09-13-2006).]

If you really want to use 2 transformers, you could, by tying the 2 common (center tap) connections together, and running each rail from one end of one transformer.

A cheaper method might be to switch the 2 LV circuits through relays controlled by the switches.

Yes there is.

Pick two ISOLATION transformers of the appropriate ratings [voltage and current] to power up each half of the light bar.

Wire up the primaries to share a common neutral and use each wall switch to apply power to one transformer.

The secondaries will have to be wired in SERIES so that HIGH side of the first transformer is connected to the LOW side of the second transformer.

The LOW side of xfmr 1 is connected to circuit A.

The HIGH side of xfmr 1 AND the LOW side of xfmr 2 are both connected to common.

The HIGH side of xfmr 2 is connected to circuit B.

This way, the common of the light bar should not see more current than either side A or side B. This is just like the 120 / 240 Volt single phase house feed.

Obviously the transformers are fed from the same circuit or they MUST be fed from the SAME PHASE!! Verify the output voltages BEFORE you connect them to the lights.

You should get about 14 Volts from circuit A to common and about 14 Volts from circuit B to common AND about 28 Volts from circuit A to circuit B.

Larry

Hmmmmm..... I have a question....
If they are providing it, why worry about cost?

I'd say you could use two transformers, and wire them one side of each as a common much like a single phase transformer set up.... Remember to check polarity on them so you get 12/24.

Yep it can be done - but.... I would not use dimmers on them! I'm sure you could, but would be skeptical of the result with any ol' transformer.

But why not try something that was intended and listed for it(See artical 411) from the manufacturer of the monorail as a system????? (Not sure what brand you have???? They should make one.)

Tech Lighting There instructions down there.... Still pricey though..... $400-800+ (I'm sure that covers thier liability insurance......)

If you can't get something from the original manufacturere, may I suggest Q Tran units. I have been very happy with them, you could get a DUO unit that has 2 coils in one case. THey are not the cheapest but I think they are the best units out there. I have done setup similiar to what you have before, works fine, you just have to reverse the polarity for one of the circuits, check "neutral" loading with your clampmeter to make sure everything is good. I've never dimmed with this setup, I'd be curious to know how that whent!
http://www.q-tran.com/

You guys are incorrect. If you connect two 12v transformer secondaries together via one end of each, and do not care about getting 24 volts, polarity does not matter.

As for the primaries, they could either share a single circuit or be fed from a multi-wire (120/240) circuit, and again, since there's no interest in 24v, phase doesn't matter.

In other words, you can use any standard transformers of adequate capacity.

[This message has been edited by Larry Fine (edited 09-14-2006).]

IMO you must use transformers listed for the purpose of suppling LV lighting.

[This message has been edited by iwire (edited 09-14-2006).]

Bob that is what I was trying to elude to as well. However there seems to be some descrepancey if it need to be listed from the same manufacturer for a certain fixture(s) or just listed as a lighting transformer. (i.e. "associated equipment~") Or what seems to me to be just listed as a lighting transformer. (Acceptability debatable) As many of us alreadry know, most of the lower end listed transformers are simple "buck and boosts" with new paint, labels and a secondary CB... Some still have the old labels inside....

While on the subject of high end transformers like q-tran, Semper Fi Power Supply as makes a comparable, or better line than them IMO.

From a purely electrical point of view, you need to have things connected up so that the two "hots" have 24V between them when they are both on. Otherwise, you are going to have 2X the current, and 4X the heat generation, in the "neutral" wire and track conductor.

As far as what Code allows/requires, I have no opinion at this time.

Okay, yes, that's true. The "neutral" conductor will carry the current of both halves of the circuit if wired that way. Howeever, when it was mentioned that #8 was used, I hadn't envisioned its capacity to be taxed.

At 12 volts, 300 watts is 25 amps. 50 amps would be a stretch for #8, but I can't imagine the two halves of the circuit being loaded to capacity. I concede that, in theory, polarity does matter, but not for functionality.

Larry 120/240 or 12/24 nothing changes the rules remain the same, we must protect the conductors within their capacity.


I doubt you would say the same thing about a multiwire branch circuit and that is just what the OP wants to do.

Hi Mark, to be honest I only read the first three paragraphs of you previous post as it seemed you where on the 'make it yourself' bandwagon.

Now that I read the whole thing I see you had pointed out the listing issue.


It is my opinion based on the definition in 411 that we need a power supply that is listed to supply LV lighting. We do not need all the parts from one particular manufacturer.

leespark If you follow the letter of the code and listings I think you have a problem.

Two listed for the purpose LV lighting transformers could be rigged to provide a 24/12 circuit on your 8/3 but that will likely be a violation of 110.3(B).

I think it is likely that you will need to run new cable to the fixture.

Also considering the transformer is in the basement and the tracks on the first floor ceiling it is likely that 8 AWG is to small for the job. If you plan on loading each circuit to capacity you will have noticeable voltage drop.

If you use a standard "dual output" transformer, that is doublespeak for a Center Tapped Secondary. You can control your circuits with 2 relays in the secondary lines to your lights, that way the neutral/common of your #8 carries zero current, assuming that both strips of lights are the same wattage. This is just like 1 phase 120/240 V. services, which are created by a 240 V. center tapped secondary where the neutral carries only the difference between L1 & L2 current...basic transformer theory.
My experience with LV lighting is that your copper losses (i.e. voltage drop), are excessive @ 12 V. and I have found that 25 amps over 60 feet distance (120 ft. circuit length) of # 10 will only give you about 11 volts across the filaments...your bulbs will last longer, and the wire won't ever freeze in the winter. I x R of supply and return length gives you volts drop, # 8 should be adequate.
If the customer wants a dimmer on these two circuits of lights, this could be interesting...hope it's T & M...

[This message has been edited by Almost Fried (edited 09-15-2006).]

[This message has been edited by Almost Fried (edited 09-20-2006).]

Oh! I hate it when the wire freezes in the winter! Those cold, cold mornings with the blowtorch, thawing out the frozen wires...

Fried, we know we can make it work.

The question is how can he do it and meet all NEC requirements.

Unless there is a center tapped UL listed low voltage lighting transformer available he will not be able to use the 8/3 cable for two circuits.



[This message has been edited by iwire (edited 09-16-2006).]

iwire: I believe that both the seagull and Ruud transformers referred to are listed for the purpose considered.