Converting a T-12 Light Fixture to T-8

difference between a t-12 and t-8 light bulb

Look at the size difference between the T-12 & T-8 bulbs. Sharpie for comparison.

This post is going to go over how to convert a T-12 fluorescent light fixture to T-8. As you may already know, T-12 fluorescent light bulbs are a thing of the past. It’s important that as either a building engineer or even a homeowner that you understand how to modify an existing light fixture.

Hopefully by the end of this pic heavy post you’ll know what you’re looking for when you open up a light fixture, then the steps to replace the ballast and bulbs, and then how to put it back together.

Every fluorescent light fixture is different and it would be impossible to have a separate article for each type. Use the engineering side of your brain if your fixture is different and make some tiny changes to the procedure.

As always, safety is the number one priority. You might open up the light fixture, see all the wires, and immediately think you’ve bitten off more than you can chew. Trust me, converting a light fixture from T-12 to T-8 is relatively simple. Think of it like a puzzle you have to solve.

If you do attempt to tackle this project, make sure all electricity to the light fixture is off. I’m sure I’ll mention it again as we go but your safety is completely dependent upon you.

The reason you might want to know how to convert a light fixture over from a T-12 type ballast and bulbs to the T-8 type is because T-12’s are being phased out and virtually outlawed. The T-12 bulbs consume more energy when illuminated, and also they are larger physically which means it takes more resources to make them AND then dispose of them.

Another bonus is that going from a T-12 size bulb to a T-8 size does not require you to change out the light sockets, or tombstones, as they’re known in the trade.

So, if you think this is a project you can do, and building engineers you should, keep reading to find out the basic steps for conversion. If I had to rate the difficulty of this project, I’d put it in the moderate category. You want to have at least a basic handle on electricity and wiring.

How to Convert a T-12 Fixture to T-8

Step 1 – Make Sure It Is A T-12

You can do this by looking at the bulb size and comparing the diameter or circumference, the T-8 is noticeably smaller, and so forth, a T-5 bulb is smaller than both. You can use the picture in the very upper right corner for comparison, or you can take a look at this blurry picture of a T-12 light fixture.

t-12 light fixture pic

A T-12 fixture, like you can tell.

Another way to tell would be to remove the panel that hides the ballast and read the technical data, it will say if it’s for a T-12. This does require you to remove the bulbs completely and the panel. Once you do enough fixtures, you’ll be able to tell what size bulb it is from a glance.

Step 2 – Remove the Bulbs and Panel

Be careful during this step if you haven’t secured power to the light fixture. If you remove the panel and the sockets are still hot, do not brush up against them.

t-12 light picture

Bulbs removed, twist the wing nut 90 degrees on both ends.

That entire piece of flimsy metal will start to come down as soon as you release the two clips or wing nuts. Again, every fixture is different. Some metal pieces that conceal the ballast are more like troughs or gutters that snap into metal clips installed inside the light fixture. You’re smart figure it out.

Step 3 – Is She Hot?

Here in this pic I’m using my Fluke Volt Alert to see if there is any current present. You can also wave it at the tombstones or sockets before even removing the metal panel. I generally check all four sockets lol. I’m safety minded like that.

testing a ballast to see if it's hot

Using my voltage detector. No light or beep, should be good.

Step 4 – Compare Your Ballasts

Make sure that what you plan on installing is a suitable replacement.

You also want to check the wiring diagram or schematic that is supposed to be printed on each lighting ballast. Check that it is the appropriate voltage, and also that it is designed specifically for the bulbs you are also installing. Each ballast is different like that, a ballast only serves a specific class of lights.

Also check the new ballast to make sure it will illuminate the correct number of lights you want. Use the pictures below and we’ll break down the numbers hopefully.

277 volt 2 lamp ballast

A 277 volt 2 lamp ballast.

Looking at the picture above, we can see that this is a 277 volt, 2 lamp ballast. Another quick way to tell the voltage is by the color of the ballast. It used to be required that high voltage ballasts, 277v, were marked with red. Now most ballasts are universal in that they serve both 120v and 277v. But still, you need to check and make sure.

The colors – black, white, yellow, etc.? Those are the existing wire colors, the colors do not need to match though. You do need to be able to figure out what they do though, it’s easy. If they match, great, if not, no problem.

Here is the new ballast.

t-8 ballast pic

The new T-8 ballast, still two lamp & 277v.

Notice though how the wiring is a little bit different? No yellows and one less red? No problem. We can easily decipher this wiring diagram.

And here is a side by side picture so you can see the size difference between a T-12 ballast and T-8 ballast.

t-12 t-8 ballast comparison

T-12 ballast minus label on top, the smaller T-8 ballast on bottom

Step 5 – Remove Old Ballast

First let’s disconnect the incoming power, the hot wire, usually the black in this case (check to make sure,) and the neutral, or white wire. My thinking is that if we have no incoming or outgoing juice, we can’t get shocked. Only in this case though. There ARE pieces of electrical devices that store energy ever after being unplugged and can still shock you, like a dead bee can still sting you. Your television is one example.

how to replace a ballast

Hot & neutral wires disconnected and capped.

It helps me to think of the black and white wires, the hot and neutral, or live and neutral, as the incoming and outgoing. By this I mean thinking of it this way later helps me decode the wiring diagram if need be. Follow?

In the next picture, more wires have been cut.

how to change a ballast

The red and blue wires, snip snip.

Here the red and blue wires have been cut. In the typical light fixture, the red and blue wires run out to the light sockets. Their purpose will be shown on the wiring diagram. Use the wiring diagram picture above to help, it actually has blue and red lines running out to the lamps right?

is changing a ballast hard

Another shot of how the red & blue wires run. They have already been cut.

To actually remove the ballast, you may have to look at it. How they’re attached again is dependent upon the light fixture. This style of common, industrial ballast has like two ears that tuck into a metal fold and then the other end screws to the metal of the light fixture.

how to convert a light fixture from t-12 to t-8

Old T-12 ballast removed.

In the above photo, I have unscrewed the screw and completely removed the ballast. This is an empty light fixture except for some wiring and light sockets now.

Step 6 – Install the New Ballast

I guess for some this might be the scariest part. It’s easy though. Just reverse what you just did. Simple.

First, we’re going to mount the new ballast. Since we checked the technical data earlier we know we’re good.

how to install a ballast

The new T-8 ballast only screwed in place.

Before I mount the new ballast, I like to completely uncoil the new wires so that they hang down straightly and out of the way. Pro tip. The more you know.

You can see the black and white wires hanging down, as well as the red and blue ones.

But we had yellow before right? They run to the other side of the light fixture, to the other side of the light sockets yeah?

We need a complete path for current to flow and the red, blue, and yellow wires make that path. In the case of this ballast. This is why we use the wiring diagram, every time. We need to determine the:

  • incoming.
  • outgoing.
  • out to the sockets.
  • back from the sockets.
  • maybe others.

So our first ballast, the T-12 had:

  • the black & white – incoming & outgoing.
  • 2 reds – one light socket.
  • 2 blues – another light socket.
  • 2 yellows – coming back from the other end of the sockets.

Whatever shall we do?

Now we look at the new wiring schematic and figure out what each new wire does. So simple even a caveman could do it.

Our new ballast, the T-8 has:

  • a black & white – incoming & outgoing – check.
  • 1 blue – one socket.
  • 1 blue – another socket.
  • 1 red – coming back from the other end.

What does this mean for us and wiring it up correctly?

Well in this case, this means that 1 blue wire will take up the same role as the 2 blue wires before, so that will be connected to 2 blues.

Another socket is represented by the 2 reds right? So the other remaining blue wire from the ballast will connect to the 2 reds.

Remember, we need to basically account for incoming, outgoing, out to the sockets, back from the sockets. Also remember to simply reverse the disconnect process. Next we will connect the red and blues.

is it east to change a ballast

Red and blues out to the sockets done up nice and fancy.

Okay, so we’re good there. Next, we need to connect the one new red, coming BACK from the sockets to the 2 existing yellows doing that job now right? Unfortunately that picture didn’t upload, you’ll have to take my word for it.

Now though the only wires left should be your incoming, hot, and outgoing, neutral. I prefer to start with the neutral and connect the potential hot wire last. Since we know it’s safe, it doesn’t matter. But good practice makes good habits. If you are always electrically safe, you have nothing to worry about.

is it difficult to change a ballast

Hot & neutral connected, you can also see the yellow wires connected too.

Never mind, I guess you can see the yellow wires connected to the one red here. And of course you can see the hot and neutral connected with the new yellow wire nuts. Bingo.

Now as a tidy building engineer, I like to neaten the wires up inside the fixture. Especially if you have to try and fit a metal cover back on. The cleaner and tidier the wires are, the easier this process is.

can i change a ballast

Wires all tucked up inside like a little bird wing.

Step 7 – Metal Cover & Bulbs

Now you need to put the metal cover back in place that both hides and protects the ballast. It might be a matter of setting up those clips just right, aligning the holes, and twisting them back into place. Or it might be snapping a metal guard back into a groove.

new t-8 bulbs

The new more environmentally friendly T-8 bulbs. Same sockets.

And now you are also ready for your brand new, more environmentally friendly bulbs too. Maybe at your house, or if you’re a building engineer you could methodically go through every mechanical room and retrofit every light fixture and save your facility some money. Just a thought.


No words can describe the beauty of brand new light. Awe inspiring. Am I hearing angels signing?

how to change a ballast

Hark, so pretty.


What did we cover with this awesomely detailed post?

We went over how to convert an existing T-12 light fixture, both the bulbs and ballast, over to a new slightly more environmentally friendly and smaller T-8 type. These T-8 bulbs also produce less waste after they are done with their life of illumination.

We went over in 7 basic steps with tons of pictures how to do this yourself. From checking out what you have, disconnecting it, removal, installation, wiring, and completion. Be sure to observe the proper safety precautions, you know what they are.

You may want to consider doing this project at home or at work. Why? Because the T-8 bulbs consume less energy as do their ballasts that fire them off. Over the long run these energy savings mean energy savings to you, your property such as your home, or where you work.

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