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

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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.

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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 Jan 9 '14 at 1:26
@KA added that. – Bryce Jan 9 '14 at 4:29

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.

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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

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.

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