T12 and T8 bulbs interchangable?

I have several four foot fluorescent light fixtures in my basement. I recently found that some are marked that they take either T12 or T8 bulbs. Several others are not marked and currently have T12 bulbs.
Are these bulbs interchangable, or with T8 bulbs only go into specific fixtures?
I intend on opening up the fixtures to check the ballasts next time I'm up in the ceiling, but I'd like to know.
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I think that some of the new ballasts will work with both. If so, it'll say so on the ballast case.

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The older magnetic ballasts are pretty limited as to which lamps will work but the new electronic ballasts handle a much wider spectrum. Pretty soon there will be "one ballast fits all"

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No, they are not. Even with the electronic ballast, they will not work for both. You will notice the ballast model number/letters on top. It'll usually read F32T8, or something along those lines, indicating the length and type of bulb.
A universal ballast can usually be used for one or two bulbs and can work with 120volts to 277volts. But they are not universal for T12 and T8.
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Completely incorrect answer: Here are the specs from a Keystone ballast: KTEB-240-1-TP /C (Electronic Fluorescent, 120V) 2x CFQ26W/G24q , CFTR32W/GX24q , F25T8 , F30T12 , F32T8 , F34T12 , F40T12 , F40T12 (25W)

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My apologies. We come across many types of ballast within my employment and have yet to see one which will work for a T12 and T8, though many will work on a voltage range and bulb quantity. Therefore, I did not think they existed, just yet. I stand corrected.

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This is all new stuff. I was serious about one size fits all ballasts coming soon

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Check out the fine print too. It's at: http://www.keystoneballast.com/techdocs%5CKTEB-240-1-TP.pdf If you just want the tubes to light up, fine. If you want them to operate at manufacturer-rated values, then look further.
The ballast operates standard 4-foot F40T12 lamps at 75% of rated output, but standard F32T8 lamps at 5% more than the rated value.
TKM
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