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#69526 - 09/13/06 09:16 PM Two circuit transformer  
leespark  Offline
Joined: Jan 2004
Posts: 54
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:®-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!


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

Work Gear for Electricians and the Trades

#69527 - 09/13/06 09:58 PM Re: Two circuit transformer  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
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.

#69528 - 09/13/06 10:30 PM Re: Two circuit transformer  
LarryC  Offline
Joined: Jul 2004
Posts: 783
Winchester, NH, US
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.


#69529 - 09/14/06 01:54 AM Re: Two circuit transformer  
e57  Offline
Joined: May 2003
Posts: 2,876
Hmmmmm..... I have a question....
The designer/GC has provided all of the rail, hardware, heads etc.

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. [Linked Image]

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......)

Mark Heller
"Well - I oughta....." -Jackie Gleason

#69530 - 09/14/06 09:33 AM Re: Two circuit transformer  
mhulbert  Offline
Joined: Oct 2004
Posts: 132
Anaheim, CA USA
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!

#69531 - 09/14/06 05:04 PM Re: Two circuit transformer  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
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).]

Larry Fine
Fine Electric Co.

#69532 - 09/14/06 05:19 PM Re: Two circuit transformer  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
411.2 Definition.
Lighting Systems Operating at 30 Volts or Less. A lighting system consisting of an isolating power supply operating at 30 volts (42.4 volts peak) or less, under any load condition, with one or more secondary circuits, each limited to 25 amperes maximum, supplying luminaires (lighting fixtures) and associated equipment identified for the use.

411.3 Listing Required.
Lighting systems operating at 30 volts or less shall be listed for the purpose.

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 Badger
Construction & Maintenance Electrician

#69533 - 09/14/06 07:02 PM Re: Two circuit transformer  
e57  Offline
Joined: May 2003
Posts: 2,876
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.

Mark Heller
"Well - I oughta....." -Jackie Gleason

#69534 - 09/14/06 07:23 PM Re: Two circuit transformer  
SolarPowered  Offline
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
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.

#69535 - 09/14/06 10:10 PM Re: Two circuit transformer  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
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 Fine
Fine Electric Co.

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