What a ballast does and why it fails

A ballast is the box or cylinder inside a fluorescent light fixture that controls the electrical current flowing to the tubes. Without it, the tubes would draw too much power and burn out when ready. The ballast steps down the voltage and regulates the flow so the tubes light up safely and stay lit.

Ballasts fail because they run hot and the components inside—capacitors, transformers, and wiring—wear out over time. A failing ballast will cause tubes to flicker, take a long time to turn on, hum loudly, or not light at all even when the tubes themselves are good. Some ballasts leak a dark oily fluid called PCB (polychlorinated biphenyl), which is toxic and requires special disposal.

Replacing the ballast is straightforward if you follow the steps in order and take basic safety precautions. The job takes 15 to 30 minutes and costs $15 to $50 for the ballast itself, depending on the fixture type.

Key Takeaways

  • Turn off power at the breaker and test the fixture with a non-contact voltage tester to confirm the power is off before you touch anything inside.
  • Remove the tubes and the diffuser or cover, then unbolt the old ballast and disconnect its wires by loosening the terminal screws.
  • Buy a replacement ballast that matches your fixture's tube count, wattage, and voltage—the old ballast label will tell you all three.
  • Connect the new ballast by matching wire colors or terminal labels, bolt it in place, reinstall the tubes and cover, then restore power and test.
  • If the ballast is leaking dark fluid, do not touch it and call a licensed electrician or your local hazardous waste program for disposal instructions.

Turn off power and confirm the circuit is dead

Start by switching off the light at the wall switch, then go to your electrical panel and flip the breaker that controls that circuit. If you are not sure which breaker it is, turn off the main breaker or flip breakers one at a time until the light goes out.

Once the breaker is off, use a non-contact voltage tester (a pen-shaped tool that beeps when it detects live electricity) to confirm there is no power in the fixture. Touch the tester to the ballast, the wires, and the socket areas. If it does not beep, the power is off and it is safe to work. If it does beep, the breaker you flipped was not the right one—find the correct breaker and try again.

A non-contact voltage tester costs $10 to $20 at any hardware store and is the safest way to verify power is off. Do not skip this step.

Remove the tubes and access the ballast

Open or remove the fixture cover or diffuser. Most covers slide out, twist out, or are held by two or four clips or screws. If you are not sure, look for clips on the sides or a seam where the cover meets the frame.

Gently remove the fluorescent tubes by twisting them a quarter turn and sliding them out of the sockets. Set them aside on a clean surface where they will not roll or break. If the tubes are old or flickering, this is a good time to replace them too, but you do not have to—a new ballast will work with old tubes.

Once the tubes are out, you will see the ballast mounted to the fixture frame with one or two bolts or screws. It is usually a rectangular metal box or a cylindrical can. Take a photo of the wiring before you disconnect anything, so you know where each wire goes.

Identify the correct replacement ballast

Look at the label on the old ballast. It will list three things you need to match: the number of tubes (usually 1, 2, or 4), the wattage per tube (usually 17, 25, 32, or 40 watts), and the input voltage (usually 120 or 277 volts). Write these down or take a photo of the label.

Go to a hardware store or order online and buy a ballast with the exact same specifications. Ballasts are not interchangeable—using the wrong one will not work or will damage the fixture. The label on the new ballast must match the old one in all three categories.

If the old ballast is leaking dark fluid, do not open the fixture further. That fluid is likely PCB, which is hazardous. Call a licensed electrician or your local hazardous waste disposal program to handle the removal and replacement safely.

Disconnect the old ballast and remove it

Inside the fixture, the ballast is connected to the sockets and the incoming power wires by terminal screws. These screws hold the wires in place. Loosen each terminal screw by turning it counterclockwise with a flathead screwdriver, then pull the wire out. Do this for every wire connected to the ballast—usually there are four to six.

Once all wires are disconnected, unbolt the ballast from the fixture frame. Most ballasts are held by one or two bolts on the side or bottom. Use a wrench or socket to remove them, then lift the ballast out and set it aside.

If the ballast is leaking, place it in a plastic bag or cardboard box to contain any spills, and take it to your local hazardous waste collection site. Do not throw it in the trash.

Install the new ballast and reconnect the wires

Position the new ballast in the same location as the old one and bolt it down. Make sure it sits flat and find—a loose ballast can vibrate and cause humming.

Now reconnect the wires. The new ballast will have terminal screws labeled with letters or colors (usually red, black, blue, and yellow). Match each wire to the correct terminal by color or by comparing the label on the new ballast to the photo you took earlier. Loosen the terminal screw, insert the wire, and tighten the screw firmly so the wire does not slip out.

If the wires are damaged or the insulation is cracked, trim back the damaged section with a wire stripper and restrip about half an inch of insulation before inserting into the terminal. Do not force a wire into a terminal—if it does not fit, you may have the wrong ballast.

Reinstall the tubes and test the fixture

Slide each tube into its sockets and twist it a quarter turn clockwise until it clicks or seats fully. Make sure both ends of the tube are seated in the sockets—a tube that is not fully seated will not light.

Replace the diffuser or cover and find it with clips or screws. Go back to the electrical panel and flip the breaker back on. Return to the fixture and turn on the light switch.

The tubes should light up within a second or two. If they flicker, take a long time to turn on, or do not light at all, turn the light off, flip the breaker, and check that all wires are connected tightly to the correct terminals. If the problem persists, the ballast may be defective or you may have the wrong model—call a licensed electrician to diagnose the issue.

Frequently Asked Questions

Can I replace a ballast without turning off the power?

No. A live ballast carries high voltage and can cause a serious shock or burn. Always turn off the breaker and use a voltage tester to confirm power is off before touching any part of the ballast or its wires.

What if the new ballast hums loudly?

A new ballast may hum slightly for the first few hours as it settles. If the hum is loud or does not stop after a day, the ballast may be defective or the wrong type. Turn off the power, check that all bolts are tight and all wires are seated firmly in the terminals, then test again. If the hum continues, the ballast may need to be replaced.

Do I need to replace the tubes when I replace the ballast?

No. A new ballast will work with old tubes. However, if the tubes are flickering, dim, or near the end of their life, replacing them at the same time will give you better light and save you a trip up the ladder later.

What should I do with the old ballast?

If it is not leaking, you can take it to a recycling center or an electronics waste collection site. Many hardware stores also accept old ballasts for recycling. If it is leaking dark fluid, place it in a sealed bag and take it to your local hazardous waste disposal program.

When should I call an electrician instead of doing this myself?

Call an electrician if the ballast is leaking PCB fluid, if you cannot find the correct breaker, if the voltage tester beeps even after you flip the breaker, or if the new ballast does not work after you have checked all the connections. A licensed electrician can also handle fixtures in high or hard-to-reach places safely.