How to Replace Fluorescent Light Ballast

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How to replace an old fluorescent light ballast.

Fluorescent light fixtures are energy efficient over the long run, but bulbs and ballasts are expensive replacement items. Typically when the light bulb flickers continuously or the fixture begins to hum you need to replace the ballast. The repair will only take about 10 minutes. New ballast prices range from $15 to $50, which may make replacing the fixture a better choice for outdated units. This is especially true if your fluorescent light uses T12 bulbs instead of the more energy efficient T8 or T5 bulbs. Note: The number after the T stands for how many 1/8th of an inch the diameter of the bulb is. T12 = 12/8 inch which is 1 ½ inch diameter, T8 = 8/8 inch or 1 inch, and T5 = 5/8 inch.

Fortunately the ballast will typically last 15 to 20 years, which in most cases means that the fixture will be replaced before it becomes necessary for the ballast to be replaced. The life expectancy will be reduced depending on the hours of usage and the power quality of the electrical service. Nearby lightning strikes or power surges can damage the fluorescent ballast and lead to premature failure. Fluorescent fixtures installed in cold locations and bad bulbs can also significantly shorten ballast life.

Tools and Materials

4-in-1 screwdriver

Nut driver; You may not need this depending on the size of the socket on your 4-in-1 screwdriver.

Wire stripper

Replacement ballast: Check the model number on the original ballast and take the information to a home center or electrical supply house to purchase the exact replacement.

Wire nuts

Electrical Tape


If you do not feel comfortable in doing any electrical repairs, hire a licensed electrician to perform the repairs.

Depending on the number of bulbs in the fixture there can be anywhere from 4 wires for a single bulb fixture to 8 wires for larger multi-bulb light fixtures. The white and black wires are connected to the incoming power from the electrical panel or wall switch and the colored wires deliver power to and from the bulb sockets.

Old and new ballasts

All residential fluorescent light fixtures will operate on standard 120 volt power supply. Older ballasts were color-coded by the supply voltage, yellow for 120V, red for 277V (commercial). Most manufacturers make universal ballasts that can be connected to either 120 or 277 V systems which eliminate this problem.

The ballast is sized by the number and wattage of the light bulbs in the fixture. Older fixtures may have the bulbs wired in parallel which can pose a minor problem with wiring. Most fixtures will have one, two, or three blue wires to supply voltage to the bulb, and only one red wire completing the circuit.

Typical Wiring Diagram

1. Removing the Bulbs and Cover

Unplug the fixture or turn off the power at the wall switch or main electrical panel.

Remove the bulbs by twisting them to clear the tabs in the socket or pull them straight out.

Twist out bulb socket

Pull out bulb socket

Circline fixtures utilize a plug off of the ballast that connects to the prongs on the circular bulb.

Circline fixture

Remove the internal cover to expose the internal wiring and ballast. Typically this cover is held in place by a few screws or tabs. The cover is either slide down to clear the tabs on one side or the cover is squeezed to clear the tabs. Some manufacturers may use clips to secure the cover.

2. Disconnect the Existing Ballast Wiring

The quickest way to remove the bad ballast is to cut the wiring. To ensure you have enough wire, cut the old color-coded wiring (red, blue, and sometimes yellow) about an inch or two from the point where they exit the old ballast. The power supply wiring, the black and the white wires, should already be connected to the incoming electrical wires with wire nuts so these do not need to be cut. Simply untwist the existing wire nuts on the white and black wires and remove the wiring coming from the ballast.

3. Remove the old Ballast

Unscrew the mounting nut to remove the ballast. There may be one or two nuts holding the ballast in place. Be careful to hold the ballast with your free hand to keep it from falling out as they are heavy and can easily damage flooring. Hold onto the old nuts to mount the new ballast.

The ground wire from the power feed is screwed into the fixture and not directly connected to the ballast. It should not need to be touched during the replacement.

4. Installing the new Ballast

Mount the new ballast in the fixture with the old mounting nuts.

Strip the electrical wiring back about 1/2 inch and connect the like-colored wires with wire nuts.

When all of the colored wires are spliced, connect the white wire to the power feed, and then finally the black wire.

With the cover still off, install one new light bulb in the fixture and turn the power on. If the light comes on you can turn off the power and reinstall the ballast cover. To secure the extra wiring you may want to carefully loop the like colored wires together and wrap them with a small piece of electrical tape. You may also want to wrap the wire nuts with a piece of electrical tape if you are worried that they may come loose.