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Heating & Cooling

Ballast change from T12 to T 8

OK, bought GE T8 light ballast to replace older T12 Ballast in older troffer. Took light apart, and I "have" (2) T12 ballasts in light fixture.  Each ballast has red/blue yellow wiring, which I understand how to rewire to (1) BALLAST. But, each ballast also has a Black/White (Hot/Neutral) wiring going to it. The kitchen light has (2) on off switches on either side of the room. I know I cannot put both "hot" wires to the ballast, as 2 hots = 220 Volts. So how do I wire in, and not lose a light switch/or whatever and/or do I just go off (1) Black/white, tie in both or what?


I really do not want to lose a switch if can be avoided.




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Posted 2013-12-13T21:34:47+0000  by tbird42 tbird42

your switching in the kitchen will remain as you should only have one set of hot wires coming to the fixture from one of the switches. just put black to black,, white to white and you will be all right

Posted 2013-12-13T21:57:29+0000  by jgelect

Hello tbird42.  Welcome to the Community!


In this case I will have to answer your question with a question.


Most of the time, a troffer of lamps that can be operated by either of 2 switches will be wired using 3-way switches.  These switches have no on/off printing on the toggles, and either one will turn the entire lamp on or off independently.


Your concern that you could end up with 2 hot leads out of phase (220 volts) is possible but unusual.


Question:  If you turned on the troffer from one switch, did you have to turn the light off from that same switch?


If the answer is Yes, then you will have to rewire the switches, replacing them with 3-ways, and then you will only need one hot lead to the lamp.  This Yes answer means that each ballast was separately wired to its own single pole switch, and only half the light worked when one switch was turned on.


If the answer is No, then you have 3-way switches installed already.  The reason you have two blacks is that each ballast needs power to operate.  It does make me wonder though if these 2 black leads came from the same source.  Normally the black would be one lead from one of the two switches with a splice either inside the troffer or the electical box above it to feed both ballasts.


I will likely need more info from you to diagnose this further.  If you still have doubts about where the wires come from and what they do, a multimeter test map of the leads can help greatly.  Also, pull the switches out of the wall and look at the terminals.  Do the wire colors going out to the light match what you see at the troffer?  


Here is a Home Depot Project Guide on 3-way switch wiring:



Still not sure?  Have an electrician take a look at this.  It will only take a few minutes to sort out what's going on, and that is really the safest bet.


Is this what you are looking for?





Posted 2013-12-13T22:14:46+0000  by Chris_HD_CHI

NO,  I do not need to turn off the Other Switch. Turning on either switch turns on all 4 bulbs. Turning off either switch, turns off all 4 bulbs.


I bought the GE T8 ballst to replace the T12's, did not know there were (2) up there. 

Posted 2013-12-14T00:05:08+0000  by tbird42

If either switch turns on the entire fixture, then it's an ordinary 3-way, 120 volt circuit.  Connect the two black wires from the house to the black wire from the ballast.  Same for the white wires.  Wire the ballast to the lamp holders as show in the instructions or on the ballast itself.  Grounding is important for fluorescent ballasts, so make sure the fixture is grounded properly.



Posted 2013-12-14T00:32:11+0000  by Adam444

OK< got all that.


FINAL PROBLEM:  Each T-12 Ballast has their OWN Black/White going to the ballast.  Which set of B/W wires do I use?  DO I tape off the other ones? Not sure what is hot or not, or are both HOt.

Posted 2013-12-15T20:29:44+0000  by tbird42

Hello tbird42.


"Which set of B/W wires do I use?"


It does not matter.  You can do what Adam444 said and connect both blacks coming from the house to the black wire coming from the ballast, and do the same for the whites to white.  OR, you can tape off one set of black and white wires and connect the other set to the single ballast.


The reason it does not matter is that the 2 sets of black wires are duplicates.

This is because of your reply:


"NO,  I do not need to turn off the Other Switch. Turning on either switch turns on all 4 bulbs. Turning off either switch, turns off all 4 bulbs."


Therefore they are on the same circuit, coming from the same switch, and probably spliced at the junction box behind the fixture you are repairing.  The white neutral wires all go to ground eventually, and so they are common in any event.  Black should be hot, white is neutral.






Posted 2013-12-17T14:45:43+0000  by Chris_HD_CHI
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