I'm in an odd situation, I'm currently helping the manager of the assisted living with an undercabinet light, But the T12 bulb has a tight fit, and that doesn't sound good for the fixture.
So, the question is, can I run a F20T8 on a F20T12 fixture? It's the same wattage, so it should work?
I don't want to have a bad rap because I chose a T8 over a T12 and it doesn't work.

Edit: Do F20T8 bulbs not exist? I looked at 1000bulbs.com and bulbs.com and I found nothing. I'm in a pickle!  
Edit again: I do know that my rapid-start ballast lists F20T8 as an option. Does that mean they exist?
Edit yet again: I found a F20T9 on Amazon, made by "INDUSTRIAL"
Edit again?: I found LED tubes on Amazon. That might be good, but I don't know if the people at the assisted living will see that.
Yet another edit: The LED tubes are generic Chinese replacements, I won't pay any attention to them, but I'll have to notify the manager about this sticky situation.

Use f17t8 lamps. They're brighter, and will give a sense of energy saving. I had the issue of space in a craparound I got, so I junked it.

Use f17t8 lamps. They're brighter, and will give a sense of energy saving. I had the issue of space in a craparound I got, so I junked it.

This is an assisted living, where 3 watts of energy saving is practically nothing.
And 17 watts may burn up/destroy the ballast.

Use an 18 watt T8 instead. From what I hear they are optimized for t12 ballasts.

Use an 18 watt T8 instead. From what I hear they are optimized for t12 ballasts.

Problem is, I don't know if the ballast was for T12 or not, I didn't have enough time to see it.
Could be a solution.

F18T8 is a direct energy saving replacement lamps for T12. It got slighty higher arc voltage.  Only  problem will
It work on trigger start ballast.  If fixture have a starter
Then should be no problem at all. I have been using 17 watt lamps on 20 watt ballast, but u might run into starter problem that blink tube now and then. As put starter near trigger voltage to fire the starter.

On Preheat 18W replaces 20W and 36W replaces 40W. They have nearly the same arc voltage so no difference for the ballast

F18T8 is a direct energy saving replacement lamps for T12. It got slighty higher arc voltage.  Only  problem will
It work on trigger start ballast.  If fixture have a starter
Then should be no problem at all. I have been using 17 watt lamps on 20 watt ballast, but u might run into starter problem that blink tube now and then. As put starter near trigger voltage to fire the starter.

From what I remember, it was a instant start ballast.

On Electronic ballast you can use a lamp of lower arc Voltage

I use European 18W T8 lamps on chokes and no problems, nice and bright. A bit of flicker at first start but warms up quickly. I use Advance LC-25 ballasts designed for 25W appliance lamps and they drive the 18W at (or close to) full power. They start very quickly, 1-2 blinks max.

If you have an electronic ballast in there, you could probably use F17T8 lamps no problems.

I use European 18W T8 lamps on chokes and no problems, nice and bright. A bit of flicker at first start but warms up quickly. I use Advance LC-25 ballasts designed for 25W appliance lamps and they drive the 18W at (or close to) full power. They start very quickly, 1-2 blinks max.

If you have an electronic ballast in there, you could probably use F17T8 lamps no problems.

Are you sure? I don't want to be hit with liability charges or have a bad reputation.

With Electronic ballasts stuff can go haywire even if you installed everything perfectly with the correct parts

With Electronic ballasts stuff can go haywire even if you installed everything perfectly with the correct parts

Then who do they blame?  
What will I do if something goes wrong?
Well, actually, I don't know for sure what ballast it was, I just saw a instant start start.

OK, for liability, take the channel cover off and read the ballast label. Then you'll know. If you're stuck with T12 and you want T8, then you either have to change the ballast or have a certified electrician change it for you.

If the ballast is preheat, then a European 18W T8 will work, no questions asked. Otherwise, F17T8 might work if the starter shuts off when the lamp is lit.

If the ballast is trigger start magnetic, then no T8 lamp that I know of will work and you're stuck with T12 unless you change the ballast.

OK, for liability, take the channel cover off and read the ballast label. Then you'll know. If you're stuck with T12 and you want T8, then you either have to change the ballast or have a certified electrician change it for you.

If the ballast is preheat, then a European 18W T8 will work, no questions asked. Otherwise, F17T8 might work if the starter shuts off when the lamp is lit.

If the ballast is trigger start magnetic, then no T8 lamp that I know of will work and you're stuck with T12 unless you change the ballast.

What is a trigger start ballast? I've never known.