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LED Light Bulb Sizing

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  • Nov 12th, 2017 10:13 am
Deal Addict
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Mar 28, 2005
4363 posts
Ontario / Quebec

LED Light Bulb Sizing

I'm pretty familiar with all the parameters one has to consider now when buying an ordinary household light bulb.

With incandescent it used to be wattage and base, but now we need to look at:

Equivalent wattage (and maybe real wattage if one wants to pick the most energy efficient one)
Base (which used to be just called medium for the normal lightbulb, now we have E- numbers)
but we also need to check
Colour temperature
Beam Spread
Colour Redering Index (CRI)
Dimmable or non-dimmable - and there is a wide variety with which dimmers LED bulbs will work
Usable outdoors at low temperatures
...and finally physical size

Physical size is what I want to address.

A typical standard light bulb is considered to have a physical size known as A19
A"A"describes the shape - the basic incandescent bulb shape and the number after the "A" gives the maximum diameter in 1/8 inch increments.

What doesn't seem to be defined in this method is the length of the bulb.
I just bought a few 100 watt equivalents, siz A21, so 1/4 inch longer than the norm which was fine for my fixtures, but I didn't realize that that LED bulb was also a full inch longer than the norm - which was not fine.
It just won't fit into the fixtures it was meant for.

Something to watch out for when going LED light bulb shopping.
6 replies
Deal Fanatic
Feb 7, 2017
5592 posts
Eastern Ontario
Lol, must be a fairly common mistake.

I now have a swanky new modern lightbulb that fits a socket but hangs below the open end of the glass fixture... :rolleyes:

Too lazy / cheap to fix / replace. Good thing it's in the basement man cave. :twisted:

But, hey man, it certainly is brighter and uses less electricity. Now I only have to live with it for like a gazillion hours. :facepalm:
Deal Addict
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Dec 28, 2007
4619 posts
I’m trying to replace some G16.5 bulbs in 4 light fixtures and everything that’s available is either too big or too long by 1/4” and the cover won’t go on. I’ve looked everywhere and there is nothing made in the same size as the G16.5. Odd because it’s a fairly popular bulb.
Deal Addict
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Mar 28, 2005
4363 posts
Ontario / Quebec
To illustrate my point a bit better

The photo below are two LED bulbs - both 100 watt equivalents, both sold as A21 size both on the package and on the bulb itself
The only functional difference is that the left one (Globe) bulb is non-dimmable and the right one (Noma from Canadian Tire) is dimmable.

Boggles the mind that both are sold as A21 size and are supposed to be the same size.
  • A21 - Globe vs Noma.jpg
Deal Guru
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Jun 12, 2007
12901 posts
The other thing you need to watch is how thick the led gets right after the screw socket .

Some 100w equivalent LED bulbs won't screw all the way into some sockets. They almost go in but are 1/2 a turn too short to make full contact
Deal Addict
Feb 29, 2012
2654 posts
Not to mention the problem that a lot of LED bulbs have weird interactions with electronic switches and dimmers. I just tried a pair of Noma BR30 dimmable LED floodlights replacing 2 of the 4 LED lights in my track lighting system, only to find that now the entire system will only turn on at full brightness. If the dimmer is left set to a lower brightness, the lights won't turn on until I crank up the brightness to maximum. Aaargh! Back to the previous LED bulbs. Makes you miss the simplicity of incandescent bulbs. The industry really needs to get their act together on this stuff.
Deal Expert
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Dec 12, 2009
16482 posts
Searching for 100 watt equivalent LED bulbs reminds me of the early days searching for CFL bulbs with the same equivalency. Over time, availability improved. Right now only 60 watt equivalent or less is viable.

Dimmable bulbs are a bit all over the map too. Sometimes the dimming is very non linear and extremely iffy at the low output end. I have old leviton dimmers that date to the incandescent era and work fine with noma Andrew feit bulbs. I installed new feit and noma LED compatible dimmers that don't always work well at low output. Results are not repeatable. However, I noticed that with the bulbs running more hours at full output, dimming performance seems to improve.

Over time, all these quirks will be ironed out with next generation products.