I have a fixture that came with T-12 lights. Is it safe to use T-8 bulbs instead?

  • A T8 in a T12 fixture will not light, don't try! – user28331 Nov 28 '14 at 21:12

You are meant to replace the ballast on old T12s, reusing the old G13 base and enclosure. There are numerous retrofit and upgrade kits available.

The official word for the USA is:

Based on [Department of Energy] analysis... most T12 lamps are too inefficient to meet the [current] standards and can no longer be manufactured for distribution in commerce after July 14, 2012. In addition, if a consumer replaces an existing T12 system with a more-efficient T8 or T5 diameter system, he or she may need to purchase a new ballast or fixture. Even with this additional cost, however, DOE has determined that increasing the energy conservation standards will save consumers money overall. http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/general_service_fluorescent_factsheet.pdf

Mixing the two bulbs types is chancy: it might work out great for you, but probably not. Many of the old ballasts are pretty bad anyway: the new ones in this case really are better. Old ballasts are likely magnetic and might hum. They may have PCB chemicals, and they have a reputation for getting hot and even igniting.

Note: retrofit kits make sense in commercial applications. For your one fixture, it may be cheaper to buy an entirely new one.

2017 Update: The cool kids are all using LED instead. As before the hookup options are still bewildering (single ended, far side terminated, ballast bypass, ballast compatible). As before, the same G13 base and fixture can be preserved.

  • 1
    I would elaborate, new ballasts are most likely electronic, while the old ones are magnetic. The new style ones are significantly more efficient, won't leak tar/PCBs and won't hum annoyingly. – K.A.Monica Jan 9 '14 at 1:26
  • @KA added that. – Bryce Jan 9 '14 at 4:29
  • Why waste time and $ on old technology, swap out to linear LED tubes , I like the ballast bypass to eliminate the ballast but some of the t8/t12 retrofits can use the ballast or run off of 110- 277vac. – Ed Beal Jan 26 '18 at 6:11

As far as safety is concerned, you can exchange them. If you place T12 tubes in a fixture with a T8 ballast, you will wear out the ballast and have to replace it. If you place T8 tubes in a fixture with a T12 ballast, then the tubes will have a shorter life due to a higher current through the tube.


Personal experience, with T8 tubes in a T12 fixture with old ballasts, supports what Nathan L posted. T8 tubes rated to last 36,000 hours may last less than 2,000 hours.

  • Given how far prices have dropped for LED, it can be more effective to dispense with ballasts altogether and go with direct-wired (DW) LED tubes. Doing such a conversion requires some familiarity with how wiring of the ballast and 'tombstones' (what the tubes plug in to) can vary. How DW tubes must be wired can also vary. – NanooVisotor Nov 19 '16 at 3:19
  • For more depth in other ways that T12 and T8 differ, see: lrc.rpi.edu/nlpip/publicationDetails.asp?id=906&type=2 – NanooVisotor Nov 19 '16 at 3:50
  • It would be a better answer if you pointed out the ballast are rated for specific type of lamp(s). A ballast that is two large will shorten the lamp life dramatically as you state. A ballast that is two small the ballast wont light. In both cases the ballast life may be shortened by attempting constant re-strikes or providing more current than the normal. – Ed Beal Nov 21 '16 at 14:22

Apparently, at least one ballast model is rated for F32T8, F40T12 lamps, the Keystone KTEB-240-1-TP /B. Also, the Fulham Pony NPY-120-240-T8 is or was made to operate either T8 or T12 lamps. T12 lamps with a ballast for T8 lamps might cause the ballast to go bad sooner, if T12 lamps (rated at 430 mA) draw more current than what a T8 lamp is rated at (265 mA) from your ballast rated for T8 lamps. If your T* ballast is designed so as to keep lamp current close to 265 mA, you should not have that sort of worry.


Probably not without also changing the ballast. You could turn off the circuit breaker for the fixture, open it up, and read the label on the ballast to check - some can be used with either type of bulb; but most of the fixtures supplied new with T-12's at this point are bottom of the line and the ballast is T-12 only.

  • 4
    Both T8 and T12 lamps use a G13 base; however, a T12 ballast will shorten the life a T8 lamp. Due to the US putting a ban on standard (40 watt) T12 lamps -- high efficiency ones are hard to come by but still exist -- some ballasts were made T8 & T12 compliant. Also, shopping tip, it is sometimes cheaper to buy a new light fixture with a ballast then a new ballast alone. – Jason Jan 8 '14 at 15:47
  • I don't think I have seen a electronic ballast that will support both t8 and t12 with the same number of lamps, the strike voltages and wattages are different and the ballast controlls this I think some mag ballast did , but it has been a decade since I installed my last mag ballast. – Ed Beal Jan 26 '18 at 6:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.