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I was going to replace the fluorescent T12 fixtures around my house with T8 fixtures, but I noticed that the T8 bulbs seem to fit just fine into the T12 fixtures. It seems like I could save myself a few bucks and maybe some hassle if I just replace the ballasts instead of replacing the entire fixtures. Plus, I could make sure to buy higher-efficiency or higher-quality ballasts instead of just using whatever mystery ballasts come with the fluorescent fixtures at the home improvement store. Is there anything wrong with this plan?

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Disagree that T8 ballasts are a better ballast. Where I work, we're replacing T8 ballasts every 1 to 5 years. The old T12 ballasts that are coming out have been there for at least 10 years, and some as long as 25-30 years, and those were still working when removed. Yes, T8 ballasts are lighter weight, and may be more efficient, but do not last as long as the old T12 ones. – Arnie Dec 27 '15 at 16:52
Have you considered using LED bulbs instead? Most designs for straight tube fluorescent replacements just bypass the ballast -- taking it out of the circuit completely. They should save more energy than the T8. The downside is the risk of how well developed the long tube LED lights are. I have heard mixed comments about their reliability and some of those have focused on where made and by whom - essentially issues of quality of construction. – JKEngineer Apr 4 at 0:42
up vote 6 down vote accepted

Replace ballast and lamps, and for extra savings, on a four tube per bay fixture, get a 3-lamp ballast and run only three tubes.

The extra efficiency of the smaller tube plus high-frequency electronic ballast means that three T-8 tubes can put out nearly the same light as four T-12 tubes.

The ballast tends to be a lot smaller, and weigh much less than the t-12 ballasts.

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Typical wiring for three tube T-8 electronic ballast.

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And useful push-in wiring connectors for doing the rewire. These are Gardner Bender PushGard, others are available and much better than wire nuts for dealing with the small 18-20ga wire used in the fixtures.

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A complete building rewire from four T12 to three T8 tubes per fixture eliminated 38 tubes and at 32W each, was a savings of 1216 Watts in power reduction if you only count tube elimination and not also ballast efficiency over mangetic (est. 10-15w per ballast replaced). PLUS, NO FLICKER! For someone with fast vision, not seeing 60Hz flicker in the peripheral vision was almost worth more than the power savings.

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Thanks for the tips on the wire connectors and 3-tube ballasts. I only have seven 2-tube fixtures to upgrade in my house, but after I've gotten some practice at home I might upgrade the dozen or so 4-tube fixtures at my wife's shop, which run about 14 hrs/day, 6 days a week. – rob Jun 14 '14 at 12:16
Heh, the two tube fixtures are easy, follow the wiring diagram on the ballast. I had some two-tube hanging shop lights with a good fixture but a really cheap ballast (why they were so inexpensive). They used the standard ballast mounting, so it was easy to convert them from $25 to $50 fixtures by spending $14.50 plus tubes. No more flickering, weird starts, dead ballasts and old, inefficient tubes. – Fiasco Labs Jun 14 '14 at 14:32

Nope, this is done all the time.

Just change the ballast and lamps.

Pay close attention to the wiring diagram on the ballast as the new electronic T8 ballast are wired quite differently from the old magnetic T12 ballasts.

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You may find you don't save much money; often a replacement ballast is as much as a new fixture - but, as you note, you can at least pick your ballast, and part of the reason the ballast can cost as much or more than a replacement fixture will be that it's potentially a better ballast. You also have to shop like heck or you can get really overcharged on them; and sometimes you'll find that the maker has a ballast you'd like more but nobody seems to stock (or even be willing to order in.) Had a 4 bulb I gave up and got a low output ballast since the high-output version was unobtanium. – Ecnerwal Jun 14 '14 at 2:25
At $16 ballast cost per fixture, ballast costs weren't enough of a concern if we're talking about quality, built-in fixtures. If you're talking about those ultra-cheap hang from the ceiling things, maybe. – Fiasco Labs Jun 14 '14 at 2:31
Thanks for the warning about the wiring and notes about cost. I found some ballasts in the $16 price range at Lowe's and Amazon. The existing fixtures are just shop lights, but even the new shop lights I bought for my garage were $30-$35 (I shied away from the $10 Wal-Mart special). So I guess I'll still save a good chunk of money by upgrading the existing fixtures with higher-quality ballasts. – rob Jun 14 '14 at 12:08

yes you can change from T12 to T8, Just change the light, and you are done.

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You must change the ballast & bulb together. Changing to a T8 light bulb alone will not work. – Ben Jun 3 '15 at 3:40
@Ben T8 lamps will work on a T12 ballast but you will greatly diminish the life span of the lamp. For a quick fix it will work but, if you have the materials to do it right, just do it right. – WarLoki Jul 20 '15 at 0:31

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