I am replacing a ballast for a single tube. The new ballast I purchased is designed for two tubes but states that it is also compatible with a single (more powerful) tube corresponding exactly to what I need.

They apparently forgot to draw the wiring diagram for the single tube setup though. The only diagram I have is for two tubes and seems to be typical:

             Power Switch    +--------------------------+
 Line 1 (H) o----/ ----------|Black    Dual Tube     Red|-----------+
 Line 2 (N) o----------------|White      Rapid       Red|--------+  |
                       +-----|Yellow     Start      Blue|-----+  |  |
                       |  +--|Yellow    Ballast     Blue|--+  |  |  |
                       |  |  +-------------+------------+  |  |  |  |
                       |  |                |               |  |  |  |
                       |  |       Grounded | Reflector     |  |  |  |
                       |  |      ----------+----------     |  |  |  |
                       |  |    .----------------------.    |  |  |  |
                       |  +----|-     Fluorescent    -|----+  |  |  |
                       |  |    | )      Tube 1      ( |       |  |  |
                       +-------|-       bipin        -|-------+  |  |
                       |  |    '----------------------'          |  |
                       |  |    .----------------------.          |  |
                       |  +----|-     Fluorescent    -|----------+  |
                       |       | )      Tube 2      ( |             |
                       +-------|-       bipin        -|-------------+ 

How do I convert this to a single tube?


6 Answers 6


EDIT I'm doing this from memory and don't have a ballast to look at so I checked a manufacturer bulletin. I was wrong in the beginning so I edited it out.

Wiring diagram


It's not a question of "typical". It's a question of what the ballast instructions tell you to do. You may need to google the ballast's data sheet. Or it may be on the sticker.

There are several diagrams which are likely, or typical, but the only one that matters is the one on your ballast.

You must use all electrical products according to their labeling. Failing to do so is a codevio and likely to cause a fire.

Sometimes, especially with rapid start ballasts, the answer is "you can't".


I bought a 2 Lamp ballast and it has 2 yellows coming from the same side as the black and white wires and 2 red with 2 blue coming from the other end.

The ballast that I replaced had 2 red coming from one side and 1 white, 1 black, and 1 blue coming from the other end. The 2 red went to one side of the tube and the blue and the white went to the other side, where the white also connected with the common. The black of course went to the hot.

For the new ballast, I hooked 1 red and 1 blue to the side of the tube that the old 2 reds went to and 1 yellow to the other side. Then black with black and white with white. Light works fine now, instant on and off.

  • 1
    Your old ballast was instant-start. They have 1 wire to each end of the bulb, and the lampholders' 2 pins are shunted together internally, which cheaply blasts the bulb with high voltage to start it. You have a "Rapid start" ballast which uses 2 wires to each end to preheat the cathodes for a gentle start and -much- longer bulb life. You surely have this wired wrong, question is what fails and when. Get non-shunting lampholders and wire it to manufacturer spec, you'll get your ballast's full capabilities and a better light than you started with. Commented Jan 14, 2016 at 1:18

I have a garage with 4 t-12 8 foot fixtures with two lamps each. Ballast are costly, and I have changed one fixture to an electronic ballast. I am adding to the last post where a two lamp ballast is used for one lamp. I have three magnetic 2-lamp ballasts that have only one line each working (ie one lamp lights up, also checked the OHM readings). Following alone the line of the previous post, it appears that I can install two ballast in one fixture (lots of room) where each 2-lamp ballast only has one line working. The input to each may be a challenge, but the independent output to each lamp is simple. Working out the input now. Stay tuned. Great way to keep semi-defunct ballasts out of landfills a little longer, if it works (and postpone expenses).

  • 1
    Try to follow the instructions that are on the ballast labeling or the ballast instructions. Not only is this required by Code, it's more likely to work. Commented Aug 13, 2018 at 5:05
  • Sorry all (yes, I'm Canadian), but I am confused by the sequence of posts. my recent (Aug 2018) post seems to be among posts for latter years.
    – Dan B
    Commented Aug 14, 2018 at 15:24
  • Hit return, not finished. The comment "follow the instructions on the ballast diagram" seems to be an answer to my post about using a two lamp ballast for one lamp, when one "line" in the ballast doessn't work (per - no light, - OHM reading on ballast line). So the instructions on the ballast detail linking to two lamps, with no info I can interpret for linking to one lamp only with good "ballast line"
    – Dan B
    Commented Aug 14, 2018 at 15:28
  • often to see the full list of supported combos, you must go to the instructions / data sheet. Commented Aug 14, 2018 at 16:34

I had the same scenario, and purchased the exact same type of ballast and asked the Home Depot some questions and he said it will work but, again I was sent to the same diagram. I found on "electrical 101" that if you leave the two yellow wires alone/capped off, then, hook up the two blue wires to one end of the tube and the two red wires to the other. Don't forget the white w/white and black w/black. This will work for the single tube light!

  • You can't count on that being true. It only works for ballasts it's designed to work for, and the sticker on the ballast must be your guide. Commented Aug 13, 2018 at 5:04

Careful! Wiring this can be done a number of ways. If you had a double-lamp fixture (with it's double-lamp ballast) and two lamps hooked up and running, and then one of the bulbs burned out early, the other one would still work fine. This happens all the time and is no big emergency. Look up at the ceiling almost anywhere (the store, schools etc.) and see if any of the fixtures have a bulb out. Chances are, sure, there's one or two dark ones here or there. No one calls the fire department or the code inspection officer if one of the bulbs is burned out. So logically, if you planned it this way you'd have a two-lamp ballast, but would only hook up one bulb. Cap off the other wires that would go to the other lamp. Electrically, a dead, burned out lamp is the same thing as capping off the wires. So take your ballast and stick it in there, hook up a bulb, and you'll be in business. As for labels, and hard-core authority and electrical police - well come on. Mattress tags that must not be removed. Shampoo bottles that demand you use it twice. NEC Codes, Commercial regulations, state laws and Federal mandates and written by people, updated by people, some continued and some discarded when proven not useful. Some codes are written for no other reason except to generate revenue for the local labor and manufacturers. If you say "I'd like to purchase a ballast that will power just one bulb", someone will take a two lamp ballast, snip two of the wires and box it all back up professional-looking, and sell it to you for twice or three times the price, but it will say "Approved" on it with a fresh sticker. So - you do have to use your head and think - think - think. Sometimes the answer is "You Can".

  • Not all ballasts support what you're talking about... Commented Nov 25, 2017 at 22:34

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