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Thread: LED conversion for 2x2 U-bend fixture ?

  1. #1

    Default LED conversion for 2x2 U-bend fixture ?

    I have some 2x2 fluorescent fixtures Id like to convert to LED. Each ceiling mount fixture uses a pair of 40W T12 tubes.

    Ive seen LED U-tubes, but they require a T8 type ballast. That means I would need to replace the ballasts.

    I thought why not use four 24 LED direct wire tubes. A Google search actually came up with a video showing this, but they only used three LED 24 T8 tubes.

    Some more searching found comments saying since the LEDs faced down, all their light was projected downward & therefore you didnt need as many lumens. However Im not sure if that pertained to both LED strip light conversions and also LED tubes which may help diffuse the light because they are enclosed in a tube.

    Home Depot sells 24 direct wire LED tubes for $10 which come with non-shorting tombstone holders. The original holders would be swapped with non-shorting ones, and I would have to add holders to the opposite side of the fixture since there are none there. These would not be powered but needed to hold up the other end of the tube.

    Depending on where I look, a 40W U-bend fluorescent is about 2,200 lumens. The Toggled brand 24 LED tube is 1,120 lumens. A pair of them would be 2,240 which should match the original tubes.

    My concerns -
    Will the LED tubes produce too much light, or more than Im used to? I could use 3 tubes, but would probably have to replace the reflector in the fixture in order to maintain an even light distribution.

    How will the ≥ 80 CRI of the LED look compared to the 90 CRI of the fluorescent? Would most people notice a difference?

    Any comments & suggestions would be appreciated.\

  2. #2
    *Flashaholic* idleprocess's Avatar
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    Feb 2004

    Default Re: LED conversion for 2x2 U-bend fixture ?

    Direct drop-in LED tubes have a spotty history - ballast compatibility is pot luck and unlike with floro tubes it's another component to fail that's superfluous. If local codes and your insurance allow it and you're comfortable doing it yourself, it's a reasonably straightforward process to update a fixture to line-voltage tombstones.

    Yes, floro tube lumens are for 360 emissions. If your fixtures are like all T12/T8 fixtures I've seen (semi-glossy white), the reflectivity of the backside of the fixture was cr_p when new and has gotten markedly worse with time, so you'll harvest perhaps a third of that back ~180 so your net output is more like 66% of faceplate.

    Yes, LED tubes are out the front lumens - often over a useful 140 downward-firing arc. Most models I've seen feature diffused housings so you can skip the fixture diffusers. You also don't need fixture reflectors to do anything other than stay out of the way.

    Back of the napkin, your pair of 40W 2200lm floro tubes are likely netting around 1825 lumens out the front (66%) for a yield of 2900 lumens. A pair of 24" LED tubes would lose nothing on the backside and push their rated 2240 lumens. LED lumens will be something like ~77% of our assumed floro output. 3 tubes will up the output to 3360 lumens or 115% of floro output. 4 tubes will push 4480 lumens for 154% of floro output. It's entirely possible that the two-tube substitute would prove acceptable since half as bright to an instrument is generally perceived as an incremental reduction as brightness to a human thanks to the logarithmic scale our sense of sight works with. All other things being equal, the 3-tube solution at 115% will have no perceptible difference. The 4-tube solution will likely be perceived as slightly brighter.

    Now in reality, an old T12 fixture will have pretty low reflectivity on its backside, the ballasts likely won't be delivering full power to the floro tubes, and the tubes themselves will have degraded - performing the two-tube retrofit will likely perform substantially similar to the fixture as-is.

    LED spectrum is different from floro spectrum, for sure. Depending on application, the 80-90 CRI spread might be a dealbreaker: medical, print shops, art studio, auto body shop are some applications where that CRI spread could be a dealbreaker. General workspace areas less of an issue.

    A greater concern is likely to be flicker - inexpensive LED lighting typically suffers it to some degree or another. It's not as bad as LED Christmas lights or typical automotive LED tail lights, but typically to the point that it can cause eye strain. I gather it's typically comparable to magnetic floro ballast flicker levels.
    I apologize that this letter is so long; I did not have time to write a short letter

  3. #3

    Default Re: LED conversion for 2x2 U-bend fixture ?

    Thank you for the reply.

    These are the LED tubes I was thinking about using. Their website says they eliminate light flickering so hopefully thats accurate. And as a bonus they come with the tombstone holders.

    My T12 fixtures are semi-glossy white & there are two in the kitchen. They are on a lot of the time which means the LED ≥ 80 CRI may be an issue.

    I dont have an issue using only 3 tubes, but my ballast cover & wiring is in the center. This makes it difficult to place the tubes equidistant from each other.

    One video showed a 2x2 conversion to 3 LED tubes, however they also replaced the reflector. So that can be done, but appears Id have to remove the fixture from the ceiling in order to swap out all the parts.

    In my simplistic view of the project, Id swap out the 4 powered sockets, eliminate the ballast, and screw in the 4 additional sockets to support the non-powered end of the tubes. All while leaving the fixture on the ceiling.

    I could then remove 1 or 2 tubes & see how that looks but Im guessing there will be large dark areas when looking at the lights, just like a burned out fluorescent tube.

    I did see LED direct wire U-bend tubes online, but they seemed like a rare commodity, which is why I thought about using 24" tubes. One of my thoughts regarding having too much light is to place a sheet of frosted plastic inside the existing lens/diffuser to reduce the light. Guess I'll order some 24" LEDs & convert just one fixture first, that way I can compare it to the other fixture.

    Thanks again for the information.

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