Mixing ballasts and tubes
 

Do at your own risk.
Lethal voltage and current present in on energized systems.
But it seems to work. It all started where I work, had some T5 high output lights with burned out ballasts.
But our supply department was out of T5 ballasts and hadn't ordered more but they had standard 34w T8 ballasts (non rapid start) and my boss who had been an electrician for a long time told me to put T8 ballasts in them and rewirw th like a T8 so I did.
This started 2 or 3 years ago.
The 34w T8 ballasts powered T5 lamps are still working just fine to this day. So I know they work and work for a long time.
They are just not as bright, which is to be expected.
So ballasts are expensive, I'm cheap, plus I consider reduce, reuse and recycling hobbies.
I figure that even if the T8 ballast powered T5 lamps aren't as efficient as T5 ballast powered T5 tubes it will take a long time for an occasional use light to use up the $30 to $40 difference in KWHs.

So I have 2 types of ballast left over from my t8 LED upgrade.
I have some old looking and heavy T12 rapid start ballasts for 4 foot 40w tubes, this tube ballast combo sucked the tubes wouldn't even come on at or below freezing.
These may just be junk.
But it would be nice to have some free 40w rapid start ballasts for my dead T5 fixtures.
The other standard T8 ballasts are non rapid starting but will work fine for lights that are kept inside.

I found that pin to pin on the rapid start T8 tubes is about 2 ohms, my German made T5 pin to pin are 1.1 ohm, the cheap Chinese GE T5 tubes are 1.6 ohms pin to pin.
So the GE tubes are going on the T12 rapid start ballasts, so the end filament doesn't get burned up as fast.

Note: all T5, T8 and T12 light fixtures are 4 foot and all ballasts used are listed for use on 4 foot tubes.

So I tried the T12 ballasts, I think they were warn out, the ends got hot but the tube wouldn't come on.

So I pulled the T12 ballast out tossed it in the recycle bin.

Then wired up a T8 ballast for 3 tubes to one ballast. Came right on.
It struggles lighting up 3 tubes.
Need more testing.
I may take this 4 tube fixture, use it to power 3 tubes, then put in my 120v powered LED.

I put the T8 ballast powered T5 fixture outside with it around 25°F.
The tubes came right on, a little weak at first but warmed up after a few minutes, coming to full brightness much slower than a real T5 HO ballast, not as bright either, but it works good enough considering the price.
These fixtures are fully enclosed, so eventually they will warm up, exposed tubes may not get warm enough to achieve full brightness when it's around 20°F.
Definitely use LED for cold climates if the lights will not be enclosed and if used in unheated areas.

I tested it some more and found the T8 ballast works pretty good in a fully enclosed fixture.
The ballast draws right about 1 amp, or 120 volt-amps, obviously sending up to 40w to each tube.
This cuts the cost of making 2 of my multi function fixtures in half.
I'm liking the multi function fixtures since four full power 54w tubes as your only option is a bit much most of the time.

I got some pulled T8 fixtures with dead tubes, ballasts are good.
I took 2 new T8 tubes and stuck them in the fixture and they lit up with about 25 watts each. They look pretty dull, guess I'm just used to T5 HO.
I may just pull the ballasts out and use them to repower a dead T5 fixture, since these industrial T8 ballasts work great with T5 HO tubes.

I retried my T12 ballast to T8 tube experiment again.
Got a free damaged 40w T12 high output rapid start fixture, with no tubes, only about 2 years old so I Figured the ballast was probably good.
I decided to replace my very lame 25w producing T8 ballast with the T12 ballast just to see what would happen.
This time it worked, because the ballast wasn't dead.
Only problem is the fixture is pumping out 60 watts per tube.
They put off a lot of light, they look like 1 inch thick T5 high output tubes, so bright you don't want to lock your eyes on them.
I'm sure that this will shorten their life a bit.
Then tried some 40w high output T12 tubes left over from my T8 LED upgrade.
Even the T12 tubes are running 55w. The T12 will probably last a little longer than the T8 tubes.
Both the T8 and T12 tubes have the same bi-pin base.

It appears that I am getting a significant improvement in light output taking a 34w T8 ballast that I was using on T8 tubes and using that ballast on T5 tubes.
The ballast was using the same amount of power with the different tubes.

Got another commercial 120-277v F40T12 rapid start ballast.
This time I put it in a 4 foot T5 fixture.
It works. The tube are doing about 44w each.
The old ballasts we only doing about 21w each. The old ballast was intended for 3 or 4 foot T5 Tubes.
They are not horrible ballasts aside from low output, they run about a 0.8pf which is really good for 120v only ballasts.

I picked up some electronic ballasts for my aquarium - got tired of listening to the magnetic ones buzz. they were $7ea, instant on (like a good CFL), and no buzzing!

Problem is that the cheap ballasts tend to run a low power factor, which means that they typically use more than 30% 50% more power than the good ones.