I have a 70 W 2 bulb fluorescent light fixture which I want to put a couple of 40 W bulbs into. I know it's generally a bad idea to exceed the wattage rating of a light fixture, but there are 2 things that make me think it may be OK in this case

1) I asked an ex-electrician at my local home improvement store told me that 35 and 40 watt tubes are interchangeable and not to worry (he said the 35 is just an energy saving 40, and the manufacturer put 70 instead of 80 to make the product sound more energy efficient)

2) People on the internet have vaguely suggested that any tube of a certain pin type (in this case T12) is designed to be safely used in a fixture that fits said pin type

Can I use the 40W tubes in the fixture without shortening their life or the life of the ballast?

EDIT: I see a lot of you are suggesting LEDs. As much as I love LEDs, they're not practical for this particular application

  • Please give us the actual tube identification numbers of the tubes in question. An example would be F40 T12. Commented Jul 11, 2018 at 4:47
  • if they turn on you're probably fine. if they "blow the ballast", you're better off anyway as it will prompt LEDs.
    – dandavis
    Commented Jul 11, 2018 at 17:44
  • The tubes are F40 T12
    – Shane
    Commented Jul 11, 2018 at 21:22
  • They're plant/aquarium bulbs, which don't typically come in 35w. Otherwise I'd just use a 35 and be done with it
    – Shane
    Commented Jul 11, 2018 at 21:54

2 Answers 2


First, the internet is wrong, bulbs will fit that won't work. Especially with T12 sockets which are the same for T12, T8 and several wirings of LED conversion. Some LED conversion wiring is violently incompatible with real fluorescent tubes or even other LEDs.

For instance a T12 on a T8 ballast will be very dim and flicker.

Now the way you find out what works is:

  1. Pop the cover off the ballast compartment.
  2. Look at the sticker and Read the list of tube sizes permitted. Most likely either F40T12 or F32T8 will be at the front of the list.
  3. Also get the ballast model number and search the web for the ballast's data sheet. It will have some additional lamp sizes and combos.
  4. Buy a compatible tube.
  • The electrician insisted that T8 tubes would not fit in T12. Is this not true? (note we're talking tubes here, not LED replacements)
    – Shane
    Commented Jul 11, 2018 at 22:52
  • I do T8 conversions all the time. He's wrong. What do you expect from a "former" electrician who's taking $10/hr to work at HD? Real electrical supply houses regularly visit the big box stores looking for staff that knows anything, and hire them away. That leaves... Commented Jul 11, 2018 at 23:04
  • And yes, we're talking real fluorescents not LED. Commented Jul 11, 2018 at 23:22

It's not an issue (heat increase is neglible), just check that the tubes are compatible with your fixture.

  • 2
    The tubes need to be compatible with the ballast heat is not so much the issue. The wrong type of lamp can damage the ballast and or drasticly reduce the lamp life. I would suggest led retrofit T8 lamps, I like the direct wire style with a minimum of 18w (I use 20 & 22w lamps) t12 & t8 lamps have the same length and pin separation, a quality LED will outlast several fluorescent lamps.
    – Ed Beal
    Commented Jul 11, 2018 at 15:43
  • Ballasts? Those bulky pieces of iron that are "current reducers". It's impossible for a lamp to demage those, at worst it get short-circuited, but because of their nature (of current limiters) they can bear also that.
    – DDS
    Commented Jul 11, 2018 at 17:17
  • 2
    Ballast have been electronic since 2004 they were supposed go earlier , give looking up ballast a try as both the problems are listed on several web sites
    – Ed Beal
    Commented Jul 11, 2018 at 17:58

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