You will probably have to check the ballast. There is the possibility that the T12 ballast will overdrive the T8 bulb shortening it's life.
T12 ballast are magnetic, T8 ballast are electronic.
Good(read old,heavy)ballasts for T12 tubes run T8 just fine.
The cheap 2 tube magnetic ballasts (common now days) have an open circuit voltage just sufficient to start the tubes with the heaters warmed up. T8 tubes need a bit more voltage to start reliably. If it works and the ballast does not overheat (you need to watch this), its just fine to use T8's.
I went to Lowes and bought a T8 fixture. They did not have bulbs I wanted, so went to Pet Smart and bought the T8 40 watt bulbs. Got home and the T8 fixture was 32 watts. Took T8 fixture back to Lowes. They did not have any 40 watt T8 fixtures, only 40 watt T12 fixtures.
Where in the world do you buy T8 40 watt fixtures?
or will T8 40 watt bulbs work in a T12 40 watt fixture?
While you had the bulbs and the fixture in the same place why did you not just try it. I have had cheap shop lights, the 6 buck special that had no real large central ballast but two smaller ballast like things on either end. This thing would only run 1 t-8 or two t-12s. It wasnt designed for t8s. I took them back. If you stick your t-8s in and they dont work just take them back and return them. If they only flicker and dont come to full brightness then your fixture can not run them.
Sorry if I sounded harsh there bookpage, that was not my intention. I tend to build my lights out of the ballasts and parts I pick up for cheap, what I did was I found some advance ballasts built for four t-8s and used it to overdrive two sylvania t-8s that are 6500 k and available cheap locally. This worked great but then I realized I could just as easily overdrive by some fractional percentage t-5 bulbs, so I bought t-5 endcaps and now run two t-5s with that affordable ballast. The ballasts are about 35 at the big box stores but I found a bunch of them for 7 bucks each on one of the forums or craigslist.or ebay somewhere. I put them in an a t-8 shop fixture and they work fine.
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