i want to run 2 f8t5 on one 20 watt preheat ballast how do i do it? im useing this for my display case,
could i get a schematic ?

This won't work. The combined arc voltage of two F8T5s in series is too high for a simple choke ballast on 120V. They might work OK on an F13T5 ballast.

the ballast i would be useing is a f20t12 preheat choke
or i have a f20t5 ballast ?

I'm not familiar with the F20T5. Do you mean F14T5 (electronic)?

oh sorry i didint mean f20t5 i meant f28t5 this runs 2 bulbs but will this overdrive my f8t5 to much?

Try it and see.

alright ill try

There are two important parameters:
Ballast output current
and ballast OCV (or rated lamp arc voltage).

The first is critical (it dictates, on how much power the lamp will be operated), the lamp arc voltage may usually vary quite a lot without influencing the current too much.

The power label on the ballast is just informative for what lamp type it is intended, it has no direct link to the real electrical parameters of the ballast.

So e.g. in Europe, where is 230V mains, a single "18W" series choke ballast will allow the two F8T5 to start, but it will heavily overload them:
The F18T8 is a 0.37A ballast, that means (once the OCV is sufficient - ant that it is with 230V mains and 2xF8T5 series), it will feed exactly that (may be tiny bit less, but the difference is very small; so about 0.35A or so) current to the lamps.
As the F8T5 is rated for 0.17A, the  F18T5 will feed them by 0.35A, so about double the rated current. So each F8T5 will run at about 16W, with 32W total. The fact the ballast label says "18W" means really nothing (it only indicates the ballast was primarily designed for a single 18W lamp).

This was just one, simple example (a simple series choke ballast). More complex topologies (e.g. the US transformer based ballasts,...) have way more complex characteristics, making the thing even more complicated, so harder to get enough information about assessing the compatibility. Still the real power delivered to a load matches the rated power ONLY when the lamp charcteristics match the lamp the ballast was designed for. Otherwise it may be easily a factor different.

oh wow okay that helped alot and i found a choke that runs a f15t12 with a current of .24 amps and i feel like it would be able to do it but not entiraly sure, i will test it and tell you what happens and if it dose work it will under power them, like you seid the f8t5 is rated for .17 amps and this choke is .24 so idk i may just go and find another ballast so i will used 2 ballast for 2 lamps lol, and this is the schematic i found for this and it looks right but i dont have a cap?

That is the standard series connection (practically the only one that may work, if the mains voltage is sufficient).
With that circuit the capacitor is only correcting the input power factor, so for the functionality is not needed (its presence/absence does not influence the fluorescents at all).


But on 120V mains you will more likely run into problems with voltage headroom, normally with a series choke ballast the lamp arc voltage should not exceed about half of the mains voltage.
The F8T5 are about 50..55V lamps, so two in series may work on a 230V mains (with corresponding ballast), but I don't think they will work on 120V.
That is the reason, why this series connection is quite common (for lower wattages) in Europe, but practically unseen in the US...

There about couple handfull but only works for manual
Preheatig 2 x 4 watts on 8 watt chokes

2x 14 watt T12  in series in floor lamps again manual start

U can do 2 X 8 on  F13T5 ballast with starters

2 x 20 watt T12 on F40 preheat ballast  with starters

F10T8 on choke is strickly manual start due OCV.

Main reason two starter set up wont work cause it need 80 volt to tigger the starter.  For series to work right
That is rule of thumb that u need twice the arc voltage of lamp for it to start but also for ballast loss.