With electronic ballasts, if the pin configuration matches and the fitting allows for the different widths then the ballast will handle either tube. Single pin T12 fittings cannot accept normal T8 tubes. Many T8 fittings cannot even fit a T12 tube in the space, and you wouldn't want to anyway since they are likely to run dim. T8s may be somewhat overdriven by a T12 ballast.

If you have magnetic ballasts, we could go into possible compatibility or otherwise, but my advice is to throw them away and get something that works better ;)

Posted thread on this a few weeks ago. Link below.

Cheapest T8 shop lights I could get were $28 per. I got 3 of the T12 fixtures for $27 and used all 3 (6 bulbs now T8-32W) on a 20" X 50" shelf. Over 3 weeks running now at 12 hours/day and no problems. Plants love it.

George

Here is a link that might be useful: T8 in T12 fixture

Interesting. Three ballasts for $27, even forgetting the fixtures, may be magnetic ballasts. Do they have starters? If so then you shot yourself in the foot.

Nope, just cheap electronic ones. Last year I got the electronic ballast T8 fixtures at this price, but for some reason this year they only had the T12 size ones. Home Depot can not be depended on to keep the same thing in stock for very long, so it's always a guessing game when you try to go for something specific.

George

That's frustrating when you know you could have had exactly what you need if only you'd needed it a month ago!