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Fluorescent lamps use a ballast which transforms line voltage to a voltage to start up and operate the lamp(s). Newer fluorescent ballasts are usually rated for both 120 volts and 277 volts. Some are rated for only 120 volts, others for only 277 volts (used in commercial environments).

CFLs for the home have a built-in ballast at the base of the bulb. Commercial CFLs use a separate ballast. Ballasts have a wiring diagram to show how they connect to the lampholders.

There are four basic types of fluorescent ballasts:

Instant start electronic ballasts use a high starting voltage (about 600 volts) to start very quickly (less than 0.1 seconds). There is no preheating of the electrodes for the highest energy efficiency, but are best suited for a limited amount of switching (10,000 to 15,000 switch cycles before failure). Instant start ballasts are wired in parallel.

Rapid or trigger start electromagnetic ballasts are used in T12 and older T8 fixtures and wired in series.

Rapid start electronic ballasts heat the electrodes while applying the starting voltage (about 500 volts) to start the lamps quickly in about 0.5 to 1.0 seconds. Electrode heating continues while the lamps are on and consume a little more power (about 2 watts per lamp) than instant start ballasts. They can operate 15,000 to 20,000 switch cycles before failure. Rapid start ballasts are wired in series.

Programmed start electronic ballasts start quickly in about 1.0 - 1.5 seconds. They preheat the electrodes in a controlled manner before applying the starting voltage. Programmed start ballasts minimize electrode stress and maximize lamp life for frequent starting (areas with occupancy sensors). They can operate up to 50,000 switch cycles before failure. Programmed start ballasts are wired in series.

T8 lamps with a newer electronic ballast use about 20 - 30% less energy than a T12 magnetic ballast. If a T12 magnetic ballast fails, it should be replaced with a T8 electronic ballast. T12 ballasts are available, but T12 lamps are being discontinued. Depending on the light fixture, and how it is mounted, it may be easier and about the same price to replace the fixture instead of the ballast. A new garage fluorescent light fixture could cost less than a replacement ballast.

Fluorescent Ballasts

T12 Fluorescent Tubes Discontinued

T12 fluorescent tubes are not made anymore because of poor energy efficiency. While these lamps are still in stock in some stores, replacing the ballast with a more efficient T8 electronic ballast could be a better choice.

Fluorescent Ballast Labels

The label on the ballast shows two important labels.

Fluorescent Tube Diameters TableBallast Label T8

Fluorescent Tube Sizes

Fluorescent tubes have two common shapes, straight and u-shaped. The most common types are T12, T8, and T5. The T stands for tubular and the number indicates the diameter in 1/8 of an inch. Lamp diameter is determined by ballast type. A fixture with a T12 ballast must use a T12 bulb. A fixture with a T8 ballast must use a T8 bulb, etc.

Ballast Wiring Label

Lamp Compatibility Label

Lamp Compatibility Label


Up to 4 lamps

Fluorescent 32 watts

T8 only

Ballast Wiring Label

Compatible Lamp types for this ballast

(4) F32T8 - Up to four fluorescent lamps, 32 W, T8 lamp.

(4) F25T8 - Up to four fluorescent lamps, 25 W, T8 lamp.

(4) F17T8 - Up to four fluorescent lamps, 17 W, T8 lamp.

Light fixtures with ballasts sometimes have labels that show the required type of lamp and ballast ( F32T8).

Matching a Ballast with a Lamp

There are three requirements when matching a ballast to a lamp. In the example above, lamp type F32T8 has the following three requirements:

1. Fluorescent lamp

2. 32 watts

3. T8