What is the size designation of this fluorescent light bulb?

Question

I have a fluorescent tube that I need to replace. The label on it says:

The bulb measures 47" end-to-end, including the metal endcaps but not including the pins.

The diameter appears to be 1-1/2".

The pins look like this:

I'm extremely confused by fluorescent light bulb sizes. What is the size designation of this bulb? USA if it matters.

Answer 1

In almost all cases you can replace an F40T12 (what you have) with an F32T8 bulb. Which will be a lot easier to find (in 2023), in general, and save 20% on wattage, often with no decrease in light out. T12 bulbs are nearly as obsolete as incandescents.

The number after the F is the nominal wattage, and goes with length in a mysterious manner. That manner varies with diameter. Beware of circular or U tubes with the same wattage when shopping blindly, though.

The number after the T is diameter of the tube in eighths of an inch.

T12 and T8 use the same lengths and pin spacing. T5 is different on both and incompatible.

A fixture with a T12 is likely to be old. If a new tube fails to fix it, the ballast may have expired, and need to be replaced as well, which might call into question the value of repair .vs. replace. Or it may be so ancient it has a separate "starter" that needs to be replaced (look for a small round "can-like" thing sticking up from the fixture behind the tubes - it twists and pulls out.) Ballasts typically last longer than bulbs, but not forever. A new ballast is likely to be more efficient than the old ballast, and won't need a starter.

Answer 2

You want F40T12 tube size

The T-number is the diameter in 1/8'ths of an inch.

• T5 = 5/8" diam.
• T8 = 1" diam. (8/8")
• T12 = 1-1/2" diameter

The 48" long tubes are the most common, and are called F40T12. The "40" is the nominal watts.

Note that T8 tubes will physically fit, but will not work properly with a T12 ballast. If in doubt, pop the ballast cover off and read the labeling on the ballast as to whether it supports F40T12 or F32T8. A bulb/ballast mismatch will perform badly.

And 5000K (kelvin) color temperature

The tubes come in a variety of color temperatures. These decide how "yellow-ish" or "blue-ish" your light looks. You generally want to match all your tubes in the room or premises to the same color temperature so it doesn't look bad.

• 2700K is "hearth and home" traditional incandescent
• 3500K is a warm white by fluorescent standards
• 4100K is normal fluorescent standard
• 5000K is "daylight" fluorescent, which is what you have.

CRI is how nice the stuff looks.

That is Color Rendering Index, a recent phenomenon. Old tubes have pretty bad CRI. All modern tubes are 80-90 CRI and there are even 98 CRI tubes. 100 is perfect light. You can't tell CRI by looking at the tube, but you can sure compare fixtures of 2 different CRIs by looking at the stuff they're lighting up. Good CRI makes things look natural.

90 CRI is readily available in real fluorescent, which is why I'm staying with it rather than going LED (at least for 48" tubes). LED conversions are also an option; some keep the ballast in place, others require you rewire to bypass the ballast. (and others work either way).

Answer 3

That is a 48" or T-5 bulb. It is noted on the far right ( E7T5)