How to Tell If Ballast is T8 or T12? (w/Images)


Written by

Thomas Carter



Marcos E. Williamson

how to tell if ballast is t8 or t12

When repairing or retrofitting a fluorescent fixture, you will have to get a compatible ballast. A common question that comes up in this process is, “How to tell if ballast is T8 or T12?”

So, in this blog post, we will walk you through all the steps you need to take to check, and by the end, you will be able to pinpoint whether your ballast is for a T8 tube or a T12 tube.

How to Differentiate Between a T8 & T12 Ballast


The T12 ballast is the older version of a T8, therefore, it’s thicker and wider, yet less efficient. FIY, the T5 is the latest model of ballast, so it’s the smallest, measuring about 0.625 inches in diameter.

Now, let’s see what ballast you need for your current light, prepare the following tools for your checking:

  • A screwdriver
  • A measuring tape/ruler

Step 1: Turn Off The Power


Turning off the power will ensure you do not get electrocuted when poking around your light fixture and ballast. Do this at your breaker and not toggle the flip.

Step 2: Take Off The Cover Of The Light Fixture


Any fluorescent light fixture will have an outer cover. To access the interior components, such as the ballast and tubes, you must take the cover off. You can press on the two sides and pop it right off. In some cases, though, you might have to use a tool to unscrew it.

Step 3: Remove The Light Tubes From The Light Fixture


Once the cover is off, you will find the light tubes. Detach these from the fixture as well. Set them down on a flat surface carefully to avoid breaking them.

There are two methods to find out what you need:

  • Grab a ruler or measuring tape to measure the diameter of the tubes. A T12 lighting tube is 1 inch in diameter while a T8 fluorescent tube size, diameter-wise, is 1.5 inches.

The ballast should match the type of tube. So, looking at the sizes of fluorescent tubes is one way of distinguishing a T12 vs. T8 ballast.

  • You can also look right at the lamp’s lettering, often near the prongs, as it will say 40 watts, which indicates you need a T12, or 32 watts that refers to the T8 ballast.

Step 4: Remove The Casing/Housing Of The Ballast & Examine The Ballast

You can go further and examine the actual ballast to know for sure.

Still leaving the fixture tubes detached, remove the casing or housing of the ballast. Usually, it is screwed in, so you will need a nut driver or socket wrench to help with the removal.

After the casing or housing is out of the way, you can see the ballast. You can take your ballast down for easier examination or leave it attached while studying its labels.

Most ballasts will indicate whether it is made for T12 or T8 bulb dimensions. For example, in this picture, you can see that the ballast is labeled as a T8:


After identifying your ballast, do not forget to reassemble your fixture’s setup. You must reinstall the cover of the ballast, the light tubes, and the cover of the whole light fixture.

Switch the power on after you have reassembled everything and check that the lights run as usual.

Alternative Methods to Tell Between a T8 & T12 Ballast

Method 1

T12 ballasts are magnetic ballasts, while T8 ballasts are electronic. As such, you can determine if you have one or the other by determining if your ballast is magnetic or electronic.

You will need:

  • A smartphone camera

What you need to do:

  1. Point your smartphone camera at the light while standing under or close to it.
  2. See if there are any dark strips that flicker in your camera. If there are none, you have an electronic ballast. Otherwise, it is a magnetic one.

Method 2

Alternatively, check to see if your ballast has starters. Typically, there are two and they would be on the frame of the light fixture. If your ballast does come with starters, it is a magnetic one, and thus, a T12.

Frequently Asked Questions (FAQs)


Do T8 bulbs and T12 bulbs use the same ballast?

No. T8 bulbs and T12 bulbs do not use the same ballast. There are T8 ballasts for T8 bulbs and T12 ballasts for T12 bulbs. However, you might be able to place a T8 ballast into a T12 fixture.

T8 vs. T12 interchangeable. What happens if you put a T8 bulb in a T12 ballast?

You can exchange T8 and T12 ballasts, however, it might not a good idea to use T8 bulbs in T12 ballast and vice versa. Doing so will wear the ballast out very quickly, costing you time, effort, and money.

The tubes themselves will also have shorter lives as they deal with the levels of currents that they are designed to handle.

How long will T12 bulbs last with a T8 ballast?

If you change ballast from T12 to T8 in T12 light fixtures, you will underdrive the fixture. Its lumens chart will drop, interfering with its lighting capabilities and your visibility.

In addition, it will reduce your light’s service life. Instead of running 20,000 hours to 36,000 hours, it might only last from 4,000 hours to 5,000 hours.

What is the difference between T8 and T12 tubes?

Because T8 and T12 tubes share standard fluorescent tube lengths, they do not appear very different at first glance.

However, they are, in fact, different in many aspects, like in terms of T8 vs. T12 brightness and efficiency; the former is better due to it being the new version.

In fact, you might save up to 35% electricity used when installing a T8 instead of a thicker T12.

Are T12 fluorescent tubes banned?

The production of T12 fluorescent tubes has been banned since the U.S. Department of Energy’s lighting mandate in 2012 took effect.


Now, you know exactly how to tell if a ballast is t8 or T12. To recap, get the diameter of the accompanying light tubes with a ruler or a measuring tape. T12 bulbs have a larger diameter than T8s – 1.5 inches versus 1 inch.

Then take a closer look at the ballast and check the labels, which will specify the type of bulb the ballast is designed to support.

You can also distinguish the two by determining if your ballast is magnetic or electronic with a smartphone camera or a search for starters.

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