Everything You Need to Know About A21 Bulbs
The term A21 is used to describe the overall shape and dimensions of a light bulb. A21 light bulbs are lesser known than their extremely popular counterpart - the A19 bulb - and are more commonly used for higher light output applications.
The name A21 consists of two parts - the first letter and the two digit number.
The "A" refers to the overall shape of the bulb. An "A" bulb has the classic light bulb shape, commonly compared to an upside-down pear.
The two digits after the letter refer to the bulb's diameter at its widest point, and is measured in eighths of an inch. An A21 bulb, therefore, has a diameter of 21 divided by 8 inches, or approximately 2.6 inches. Compare this to an A19 bulb, which has a diameter of 2.4 inches.
Dimensions of an A21 bulb
By definition, an A21 bulb has a diameter of approximately 2.6 inches. In countries that use the metric system, A21 bulbs are also referred to as A67 bulbs, where 67 refers to the diameter in millimeters.
The length of an A21 bulb is typically 4.1 - 5.4 inches (105-140 mm) in length, slightly longer than A19 bulbs.
The standard dimensions of an A21 bulb are governed in North America by the ANSI C79.1 standards. For different bulb wattages, different bulb size maximum dimensions are given:
100W (Version 215):
112.7 mm max length
75.5 mm max diameter
100W (Version 216):
134.9 mm max length
78.0 mm max diameter
150W (Version 262):
139.7 mm max length
78.5 mm max diameter
For the most part, an A21 bulb can be considered to be within its dimensional requirements if it is within 139.7 mm in length, and 78.5 mm in diameter.
A21 incandescent lamps are essentially higher power versions of A19 incandescent lamps. Because they have a higher wattage level, the larger bulb shape is necessary to effectively contain and dissipate the heat generated.
A21 bulbs always use E26, but not vice versa
According to ANSI standards, A21 bulbs must always use an E26 base for mounting into a light fixture.
A21 and E26 refer to different things, but in this case, you will find that all A21 bulbs use E26 lamp bases.
Be aware, however, that the opposite does not hold true: not all E26 bulbs will be of A21 shape. Some E26 bulbs come in shapes completely different from a standard A21 or A-style bulb altogether.
What about A21 CFL and A21 LED bulbs?
Most energy efficient bulbs are designed around the A19 form factor due the larger market opportunity of these lamps.
However, LED lamps are much more susceptible to heat than incandescent bulbs. A larger bulb body contributes to faster heat dissipation into its environment, so a larger LED bulb size can accommodate more brightness.
Therefore, you will find that some brighter A-style LED bulbs (e.g. 100W equivalent) will have a larger form factor than A19, and may be categorized as A21.
See below regarding compatibility.
Are A19 lamps compatible in A21 lampholders?
Yes, because A19 lamps are smaller in all dimensions than A21 lamps, they will fit in virtually all fixtures and lampholders designed for A21 lamps. Since they utilize the same E26 base, the A19 lamps will fit into the socket just fine.
Are A21 lamps compatible in A19 lampholders?
This one is a bit trickier, because some A19 lampholders are not designed to accommodate a larger bulb.
Additionally, A19 lampholders are likely not to be rated for use with higher wattage bulbs such as A21 incandescent bulbs.
The reason for this is twofold - first, the wiring and socket may not be rated for a higher current draw, and the lamp fixture may not be designed around the additional heat generated. Second, a larger A21 light bulb may result in the heat being placed closer to the lampshade or other apparatus, increasing the risk of fire.
If you are using an A21 LED bulb with a lower overall power consumption, your safety risks are likely much lower, since the power consumption is lower and heat generation is also lower.
Read more: using LED lamps in a fixture that has a MAX WATTAGE warning
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