I was going to replace the fluorescent T12 fixtures around my house with T8 fixtures, but I noticed that the T8 bulbs seem to fit just fine into the T12 fixtures. It seems like I could save myself a few bucks and maybe some hassle if I just replace the ballasts instead of replacing the entire fixtures. Plus, I could make sure to buy higher-efficiency or higher-quality ballasts instead of just using whatever mystery ballasts come with the fluorescent fixtures at the home improvement store. Is there anything wrong with this plan?
Replace ballast and lamps, and for extra savings, on a four tube per bay fixture, get a 3-lamp ballast and run only three tubes.
The extra efficiency of the smaller tube plus high-frequency electronic ballast means that three T-8 tubes can put out nearly the same light as four T-12 tubes.
The ballast tends to be a lot smaller, and weigh much less than the t-12 ballasts.
Wiring varies -- your ballast will have a wiring diagram. But to give you an idea, here is an example of wiring for an instant-start electronic ballast (which are easier to wire, but tend to fail bulbs faster); and a rapid-start or programmed-start electronic ballast (which often matches the old magnetic ballasts):
For the smaller 18AWG solid-core wires in ballasts, don't use the common yellow/red wire nuts. Use blue wire-nuts, or for instance, push-in wiring connectors such as Gardner Bender PushGard, others are available... make sure the size is correct for 18 AWG wire.
You can reduce number of bulbs, since T8s are brighter. A complete building rewire from four T12 to three T8 tubes per fixture eliminated 38 tubes and at 32W each, was a savings of 1216 Watts in power reduction if you only count tube elimination and not also ballast efficiency over mangetic (est. 10-15w per ballast replaced). PLUS, NO FLICKER! For someone with fast vision, not seeing 60Hz flicker in the peripheral vision was almost worth more than the power savings.
you have to remove the ballast- as most t8 tubes have an electronic one inside. If you put them in t12- you will burn the bulb up and they will work sporadically before they die. Also the fixture gets blackend. You must use non shunted lamp holders. you cant just "change the light".
You put 115 volts to ONE END only- and use the black and white wire- both to each lamp holder- but look for wiring diagram and get the right lampholders before you go rewiring things.