I know this has come up before but was only touched on briefly. Can an electronic ballast thats only rated to drive T8 lamps also drive T5 lamps of similar length and wattage? I recently came across some F28T5 (the normal output 46" T5s) fixtures that contained F32T8 ballasts. Even though no T5 lamps of any type are on the "lamp types supported" on the ballast label, as far as I know these fixtures have been working fine and no failed ballasts as of yet. Also the more important question I have is would it be safe to wire a 4-lamp F32T8 ballast for overdrive to power two F54T5HO lamps?? The reason I ask is I prefer T5 lamps over T8 for most applications and here T5 ballasts (both NO and HO) are 3-4X price of a T8 ballast. The only issue I see with this setup would be shorter lamp life from instant starting the lamps, as most T5HO ballasts are programmed start. But other than that what other real concerns are there?

Comparing "length and wattage" makes no sense at all, neither of that is really relevant for the ballast compatibility.
For the ballast compatibility it has to be similar current and arc/ignition voltages.
The current has to be in the -30..+15% tolerance (lamp current rating ~30% below till ~15% above what the ballast delivers). And here we are speaking about lamp arc current (that is usually difficult to find, it is normally not published, however it is goiverned by the related standards), the mains current (usually printed on the ballast cases) is not relevant at all (unless the ballast is of a series choke type, there it is obviously the same).

The used lamp arc/ignition voltages shal be generally equal or below the arc voltage the ballast is designed for, for the given ignition method (hot vs cold). Minimum is about 50% of the voltages the ballast is rated for.
Be aware, higher arc voltage means higher power delivered to the lamp, so higher power handled by the ballast. So if the used lamp has 20% higher arc voltage, the ballast electronic would be ~20% overloaded.
With programmed start ballasts the use of lower ignition voltage lamp may lead to premature lamp ignition (cold starts).
With protected electronic ballasts there come into play the settings of the EOL protection: If the lamp arc voltage exceed given range, the ballast shuts down, so that may further restrict the range of usable lamps.

So all combined, the lamp rated wattage could be anywhere from about less than half till more than double the ballast "wattage rating" when combined well (F20T12 on a F40T12 ballast or F40T12 on a F21T5 ballast). But other combination may as well lead to situations, when both lamp and ballasts are heavily overloaded, even when both wattages and length are similar (F32T8 on a F30T8 ballast).

Ive found the operating voltage specs of the F32T8 and F28T5 and theyre very close, so close in fact that the difference between lamps of most brands is 5% or less. However I also read that the operating frequency of T5 lamps needs to exceed 20 Khz, whereas most T8 ballasts only provide 18-20 Khz. How would this affect operation though? Slightly less efficiency? The F54T5HO is essentially just twice the current requirement of the F28T5. So wiring all the outputs of a 2-lamp F32T8 ballast to single F54T5HO lamp would be the same as running two F28T5s on that same ballast right?

Ive found the operating voltage specs of the F32T8 and F28T5 and theyre very close, so close in fact that the difference between lamps of most brands is 5% or less. However I also read that the operating frequency of T5 lamps needs to exceed 20 Khz, whereas most T8 ballasts only provide 18-20 Khz. How would this affect operation though? Slightly less efficiency?

I don't think that would be even noticeable, the effec was present mainly with the thick lamps, where the high frequency pushed the current towards the surface, so closer to the phosphor. And that mean less reabsorbtion. But with thin tubes this effect would be negligible, the tubes are so thin, the phosphor just can not be far from the discharge. That is in fact the main idea, why they are thin in the first place.

The problem would be just with the lamp testing: As they were designed assuming only the high frequency drive, the lamp design was tested (lifetime,...) just with the HF drive. So as the mains drive was not tested, the "play safe" mentality means that just the HF drive is allowed.

But there could be some problems, not with 15 vs 20kHz, but on the mains frequency: The ionization level decay could be too fast, so the consequent reignition spike would become impractically high (so require too high OCV to maintain the arc stable) on the mains frequency.
Same, the thin lamps may require impractivcally high ignition voltage at the mains frequency, as the capacitive coupling won't be as efficient as with the high frequency supply.
But all these problems would make effect on 50/60Hz, when you stay in 15kHz or above, it will work the same way.

The F54T5HO is essentially just twice the current requirement of the F28T5. So wiring all the outputs of a 2-lamp F32T8 ballast to single F54T5HO lamp would be the same as running two F28T5s on that same ballast right?

That will work only, if the lamps are arrnged in equivalent parallel circuit operating in phase.
But that is frequently not the case: Simpler HW is sufficient, when the lamps are operated in series (with PFC rectifier you may easily get ~420V on the DC bus and that could easily feed 300V of the total arc voltage) - it means just single resonant circuit. But that mean you may operate single lamp of twice the arc voltage...

Sometimes even two inverters are used (to make sure one deffective lamp does not cause the other to shut down as well, as it is the only option when the inverter is common for both), then you can not combine them at all.
So to do that, you have to be sure, how the ballast is internally connected.

Ive found the operating voltage specs of the F32T8 and F28T5 and theyre very close

They're not close at all. The F28T5 is 166V by 0.17A, and the F32T8 is 130V by 0.25A. Unless I've made an error in my research? I got the former data from Philips datasheets, and the latter data is well known and backed up by a lot of sources.

Hmmm, then I wonder why I see this somewhat often in commercial installations? Ive at least seen half a dozen different occasions of T5 lamps running on T8 ballasts. The info on the voltage specs were from Plusrite, so who knows how accurate that is. Thanks for the info, so now I know this is NOT a good idea long term.

Actually I would expect the voltages to be about 10% higher, for both lamps (with sinewave current drive the power factor of nearly all discharges is ~0.9)...

@Medved:
As far I know this is valid only for main frequency (50-60Hz).
At HF the discharge should be pure resistive (power factor ~1).

that level of overdrive is mild.
i have some f28t5 running at 250ma on 24v dc ballasts at a solar installation i did 3 years ago.no end darkening yet and they get used every day as they are in a normal household.i thought running them i.s would hurt them but so far ok.there are stock ac powered ones there too and the dc ones are a bit brighter.so using a f32t8 ballast would be close to this.i doubt anyone would care about the slightly shorter life.call em mid output?

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Ive found the operating voltage specs of the F32T8 and F28T5 and theyre very close

They're not close at all. The F28T5 is 166V by 0.17A, and the F32T8 is 130V by 0.25A. Unless I've made an error in my research? I got the former data from Philips datasheets, and the latter data is well known and backed up by a lot of sources.

The main problem with this overdrive is, it does consume 50% higher power, but the light output is almost the same. The reason is, the higher temperature causing lower efficacy.
And mainly with battery powered installation thishurt at most, as the cost of the energy is there way higher (you have co calculate in the cost and lifetime of the extra equipment it means) than when working from the public grid.

the overdrive in this case is quite noticable compared to the normal.
these fixtures are open so heat is much less an issue.
if it were enclosed the gain from overdrive probably would be lost as the lamps would go outside optimum temp.
i dont think this guy cares about a bit of inefficiency as he has more panels than needed.has to plan usage on sunny days to avoid excess going to the water tank dump load.system was at float around noon last week when i stopped in.

Back to the original question what would be the first to fail in the setup of operating T5 lamps on T8 ballast? Lamp or ballast?

Throw a penny...

http://www.voltimum.com.au/content/fluorescent-tubes
good reference with voltages/currents of common tubes.

That PDF looks like an excellent source of information. Unfortunately I can't seem to be able to download it, for some reason. Download button does nothing. And it's too small to read on the site.