I'll assume the lights are fairly new and have an electronic ballast (no starter, right?), they probably wouldn't light at all if they didn't. Is the shoplight designed for bi-pin tubes?

Assuming all the above, you will be driving the T8s in the region of 40W and this shouldn't cause a problem. The exact input power can only be found by measuring it, a standard T8 tube driven at the 430mA that a T12 is designed for would run closer to 36W, but an electronic T12 ballast is very likely to have a low ballast factor to work as a retro-fit and so may be running at even lower power.

Thanks Shrubs. Yes, they are electronic ballast and are the 2-pin sockets, so the T8s fit right in. I've been running them for about a week now on a 12hr cycle, and they seem fine except perhaps a bit warmer at the ends than the ones in the T8 fixtures.

George

Interesting comment on the warm ends. It might just be that they are being overdriven. Do they look any brighter? It might not be very noticeable but any difference you can see with just your eyes is actually quite significant to the plants.

Or it could be that the ballast is running preheat where it passes a small current between the two pins at one end of the tube. This is intended to heat the plasma there so that the lamp will start, but it may be sending more current than necessary for a T8.

They don't appear much brighter to the eye, but they are on different racks, and the eye is not too reliable for intensity comparison except directly side-by-side. They certainly do fine for my plants though. 3 fixtures = 6 tubes on a 20 X 50 shelf. At 10" from foliage, I'm blooming a lot of lamium cuttings I was increasing for a new bed. They are in better bloom than I've ever seen them in my garden.