How to Change a Ballast

how to change a ballast

Execution time old ballast.

Have a light blinking in your home? Does it stay on for a few minutes, turn off, then come back on, over and over? You probably have a failed ballast. Sure it can be intimidating but read this article and by the end you’ll be a pro.

I originally had this article up over at ezinearticles, it was my most popular one over there.  Near the bottom of every article an author is allowed to have a resource box – one where I can write “click here to check out my site.”  Since this site is new, I wanted to change my resource box on the how to change a ballast article to link to here, buildingengineertraining.com.

Over the past like 3 days, they have rejected it 4x.  I got this right?  Ezine won’t let me link back from my own article that I gave to them?  Um, okay.  I shall just take it away then and have it up on my site?  Hello?

Important steps to remember when changing a ballast:

  1. Verify that your circuit is dead, there is no need to take an unnecessary risk inside your own home.
  2. Make sure you get the correct replacement ballast from the hardware store.
  3. Double check the ballast wiring diagram against your own mental picture or sketch.
  4. Don’t reuse wire nuts, ever.

Keep on reading the very long post below if you want the details on the 4 steps above on how to change a ballast.  Sue me, I like to write sometimes.



A Really Long Back Story – slightly humorous?

My submarine logo

My submarine logo

The year – 1998. The location – Seattle, the basement office of the Chief Engineer. I had been out of the military for maybe two weeks when I had my first job interview. I was 24 years old and had just finished my Naval service aboard a U.S. submarine as a propulsion plant operator. They had three different rates, I was on the mechanical side. And in the military at the time, don’t know if it’s still the case, there wasn’t much exposure to the technical aspects of other jobs. In other words, I wasn’t learning much about anything other than steam and turbines, things like electricity. I had applied for my first job at a Seattle hospital as a maintenance engineer. Pay and benefits were awesome, the commute was rough, and I was excited. Perhaps the third question in, I was stumped and thought, “well, so much for getting this job.”

The reason I mention my service on board the U.S.S. Alabama is because it was a submarine. Submarines have to pay attention to many things in order to survive, i.e. not sink, one of which is ballast. We have ballast tanks and based upon how heavy the submarine is, how heavy the water in the ballast tanks is, where that water is, etc., determines your buoyancy. In submarine life, ballast = sinking or diving = life or death = going home or not.

changing a ballast

The sockets of a light fixture – referred to as tombstones in the biz.

In real life, well how about in a normal life, ballast refers to a device in a light fixture. It controls the amount of current that passes on to the fluorescent tube so that it works properly. If your light flickers, goes out for a few minutes and then comes back on for three, then back off again, you probably should replace the ballast.

The chief engineer then asks me to “walk him through changing a ballast.” I had never heard of a ballast except in the context I described for you above. Instantaneously these thoughts raced through my brain: “What? We have to worry about ballast here? Surely we can’t have ballast tanks. Why the hell would there be ballast tanks in a building? No, there can’t be. There is no way I am going to talk to him about adjusting buoyancy in a very grounded, and landlocked, set of buildings.”

My answer was “Um, I don’t think your ballast is the same as my ballast.” His answer was supposed to be encouraging as in go ahead, give it a stab, you probably just need a little encouragement. It was “try me.” His answer caused me to internally panic. “Sweet. Now I get to look like a donkey in my first interview.” “Mr. Bishop ballast can be used to assist with depth and buoyancy… ” “Hmm” he said, “go ahead and stop right there.”

Turns out Mr. Bishop was a former Navy guy himself and completely understood my lack of electrical knowledge. I got the job, excelled if I do say so myself, and haven’t looked back. But now I’d like to pass on a basic guideline and some simple steps so that you can change your own ballast or at least not look like an imbecile in an interview.

Be safe when changing a ballast

Electricity is no joke. Personally I choose to not work on anything hot, some call it live, all either of those mean is that electricity is still present. But you’ll sound way cooler at the hardware store if you use hot or live. “Yeah, I don’t feel like working on 480 hot.” You’ll sound like a journeyman. However don’t follow that up asking where the little cute curly twisty things are.

To me, especially as a father, it is not worth the risk when finding a breaker or fuse might take a few extra minutes. If you want to ignore electrical safety and jeopardize your health, go for it. You have nobody to blame but yourself.

That being said, it is recommended that you disconnect power to your light fixture. I am the type of guy that will test and re-test just to be certain because in my job I deal with voltages anywhere between 12 volts dc up to 480 volts ac. The electricity in your home should be 110 volts, not 480. And when I type 110, it can register up to 125 volts on a meter.

However, voltage is not what kills you. It is the current. Current is measured in amperage and is the flow of electricity, voltage is like the punch behind it, big misconception, but yeah, current is what ends lives. As little as 100 milliamps, not very much, can be fatal. Disconnect power to your fixture.

This can be accomplished by opening the breaker that serves that fixture or if you can ensure the light switch will remain in the ‘off’ position you can do that as well. In the commercial world plastic covers that lock, are installed over the switch so they cannot be turned on. At home if there is a risk of someone entering the room and flipping the switch, I will use duct tape to tape it in the ‘off’ position.

Couple things of note:

  1. Not all switches are installed properly, make sure that you are taping it in the position that removes power from the fixture.
  2. Verify that this is the only switch that control this light. Sometimes hallway lights will have a switch at one end and another switch at the other.
Here is how I test a light fixture to verify it is dead

First I check to see that the switch is off. I then remove the bulbs.

Next, using a voltage tester I wave it around the sockets to see if it warns me that it senses voltage. If it beeps and illuminates at the sockets, I probably have power still.

Voltage testers operate on batteries so if your tester is silent, that doesn’t necessarily indicate your circuit is dead. Test an outlet inside your home just before the light fixture to make sure it is in fact sensing voltage. If it is silent, I proceed by taking apart the inner metal covers, concealing the ballast (you might have more than one ballast too.)

how do i change a ballast

Me using my pocket tester to check a ballast at work.

Once exposed, you should see a few wires, maybe up to ten, but probably more like 5-7 wires. If wired properly, there should be a visible black wire, a white, and a ground entering the fixture. Being careful not to touch anything with my hands, I wave the voltage tester around those three incoming wires. Hopefully it is dead. Keep in mind that your home might not be wired in accordance with code, you could have any wires doing anything. So wave it around all of them, see if you receive any alert. If not, proceed.

If something is still showing up, you need to figure out where that juice is coming from.

In my first home there was a strip of track lighting in the living room. Since this was no longer the 80’s and not an art studio I decided to take them down. I was only paying attention to the conventional hot wire, the black one. I had been getting lucky up until the point when it was removed and I went to shove the three wires as a group up into the ceiling.

I got shocked across the palm of my hand and while not excruciating, 110 volts, it was uncomfortable. Either the person that put up the track lighting or the original electricians had come off the hot side of an outlet in a bedroom with a white wire which is usually neutral. So they had made their hot, live wire which is generally black, a white one.

Thinking the entire thing was dead because I only checked the black, I got nailed.

The moral? Check everything.



Now technically you have not verified anything. If my voltage tester shows me no activity, since I have done hundreds of these, I will usually proceed. To actually verify it is safe to work on, you’ll need a meter. These can called a multimeter, electrical meter, volt meter.

fluke multimeter

My own Fluke multimeter at work.

Without touching exposed wires, you will need to steadily place your meter leads onto those incoming black and white wires. Or you can place one on the “hot” wire and the other lead on the grounding wire, which should be green. If you get a significant amount of voltage, something is wrong. If you get a small reading, you should be good to go. If you are in doubt at all and feel unsafe, just stop and either do more research, hire an electrician, or live with a blinking light.

Here is where we are in replacing a ballast:

You have disassembled the innards of your fluorescent fixture and have determined that the fixture is not energized. You hopefully are looking at like a two by two or two by four metal box, a rectangular shaped box that has wires surrounding it, and some empty sockets where the bulbs used to be.

Either take a mental picture of where everything is connected or doodle a little sketch which is what I used to do. Annotate which color wire connects where. Immediately start with the three main wires. The three main ones would be: the incoming power which should be black but can be anything. Other “hot” colors by trade are blue, red, brown, orange, and yellow.

These are usually determined by the type of building they are serving, the system, and the voltage passing through. The last three I have seen only in commercial or industrial settings and are on voltages higher than 110 volts. So start with the black and progress from there.

Disconnect the white or neutral, then the green or grounding wire. If you can, disconnect the wires by unscrewing the wire nuts at the ends. Do not re-use the wire nuts when you re-assemble. If you don’t want to mess with the wire nuts, cut the wires using a pair of insulated wire cutters to cut as close to the ballast as possible.

how to replace a ballast

Old ballast with wires hanging.

We do this so that we ensure we have enough excess wire left over for the new ballast. It is awful to be almost done, wiring it up and be like one inch too short and have to install a connecting wire.

Now you should have no new electricity being introduced into the fixture. For standard ballasts, the ones I’ve seen, the wires that are still connected inside the fixture are only servicing the lights. And with no voltage present, there ought to be no danger. You need to still respect the risk though.

By practicing safe habits all the time, you won’t make a mistake in the future and become complacent. Again, cut these socket “supply and return” wires as close to the ballast as possible.

You should be completely disconnected now and just have a rectangle mounted inside your fixture with little nub wires hanging out of the sides. When you go to the store, make sure you are getting an almost identical replacement.

For this example, let’s use an F40 T-12(4) ballast. Here are the wires you’ll probably see. Black – incoming voltage, white – neutral, green – ground, two reds – off to the sockets, two blues – off to the sockets, and two yellows – back from sockets. What is that, nine wires?

Remove the ballast once it is free and clear from the wiring which is usually accomplished by removing one screw at each end. Sometimes you only have to remove one screw and simply loosen the other. You have the ballast down and are ready to install your new one. Just reverse what I wrote above. Good luck!


Installing the new ballast

how to change a ballast

New ballast in place.

Just kidding but re-assembly will be much quicker. Put the ballast in place and tighten the mounting screws. I am a neat freak when it comes to work so I like to now straighten out all of the wires, get them all free and liberated so I can position them as I want.

If you look at the front of the ballast, you should see a wiring diagram or schematic, like a blueprint. It shows you how their specific ballast is to be installed.

I always look at it to verify I am about to put it in properly. Looking at the diagram, find the fixture or lamp as a starting point. As more of a mechanic, I prefer to look at the wiring schematic like it’s piping. You have an incoming, a supply, and a return.

What I am writing next is a generic guideline and should used as such while you are looking at the diagram on the actual ballast. On the ballast sometimes the word “line” will appear. This signifies that this is the wire where incoming voltage should be connected, it stands for line voltage. I start with the same wires that I ended with on dis-assembly. I start with the yellow ones and connect them to my existing yellows. Do the same with your reds and blues.

At this moment, you have your ballast connected to your sockets, supply and return. You only need to hook up your incoming now. Land the ground/green where it was before, connect the neutrals together. The last wire should now be your hot one, connect that.

Wirenuts for changing a ballast

Wire nuts for changing a ballast.

For connections you can use new wire nuts or butt connectors. Wire nuts are much easier to use. Butt connectors look a lot neater and cleaner when finished because you can make your wire lengths exact.

Testing the new ballast

Before I reassemble the fixture I like to test it. It is a bummer to put it all back together, install the tubes, flip the switch and have to take it all back apart to troubleshoot.

So before I tidy up those incessant wires and conceal the ballast, I like to install all the tubes and flip the switch while standing back. Standing back is merely a precaution and it is important that you insert all of the tubes as some fixtures won’t illuminate at all without the complete path through all of the tubes.  So if you were to just install one, and not get light, you might think something else was wrong.

If it tested properly, clean up the wires and install the ballast cover if you can without removing the fluorescent tubes. If not, take the tubes out, replace the cover, return the tubes to the sockets. That’s it.

Pretty simple but it sure can be overwhelming or intimidating at first. From my days as an ex-submariner and not knowing anything, to now after doing hundreds, they are almost fun. Sometimes there are so many tubes and ballasts it’s like a treasure hunt and puzzle all in one as you try to decipher what goes where, etc.

Important steps to remember when changing a ballast:

  1. Verify that your circuit is dead, there is no need to take an unnecessary risk inside your own home.
  2. Make sure you get the correct replacement ballast from the hardware store.
  3. Double check the ballast wiring diagram against your own mental picture or sketch.
  4. Don’t reuse wire nuts, ever.

I hope this article has steered you in the right direction and helps you avoid having to hire an electrician or your neighbor, me, to change a ballast for you.

Click here to check out a cool little multimeter comparison chart I put together.  The best sellers from Amazon in a sortable table.

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