incandescent
household light bulbs

Bases

Most light bulbs screw into a socket. Almost all household bulbs have a medium screw base, sometimes called an edison screw, 1 ¹⁄₁₆ inches in diameter with 7 threads per inch. Bulbs with a base that looks like a medium base but isn't are made to defeat bulb thieves in public places. Some have a left-hand thread; others are the size called “admedium,” used in signs, just a bit bigger (1 ⁵⁄₃₂ inches) so they won't fit a residential socket. Other common screw bases, in order of decreasing diameter, are:

• Mogul, 1¹⁹⁄₃₂ inch in diameter. High-wattage bulbs: some overhead fixtures, modern outdoor security lights, and old-fashioned sun lamps. Screw-in adapters that convert mogul sockets to medium screw are readily available.
• Intermediate, ⅝ inch in diameter. Bulbs in refrigerators and some other appliances. Intermediate base light bulbs found in the home are almost always tubular.
• Candelabra, ½ inch in diameter. Seven-and-a-half-watt Christmas tree lights, the kind often used outdoors.
• Min-can, ⁷⁄₁₆ inch in diameter. In the home this base is only found on some special high-intensity lights.
• Miniature, ⅜ inch in diameter. Night lights, Christmas tree lights and some flashlights.
• Midget, still smaller. Signal lights.

Size

Householders usually describe bulb size by wattage, but the industry has a system that reveals the actual dimensions of the bulb.

Bulb shapes are identified by letters: “A” is the shape of a typical household bulb; “B” is the shape of a candelabra base Christmas tree bulb; “C” is the shape of a miniature screw night light bulb, and so on.

The bulb size is then given by a letter for shape followed by the bulb's maximum diameter in eighths of an inch. For example, a T-8 bulb would be a tubular bulb 1 inch in diameter. Ordinary 40-watt and 75-watt bulbs are A-19, with a few A-21.

The size of the base or the shape and size of the bulb are not sure signs of the bulb's wattage or voltage. For example, A-19, medium base bulbs are made for 12 volts, 100 volts, 115 volts, 130 volts and 250 volts.

resources

Collecting old incandescent bulbs is a thriving hobby. A valuable compendium of information on early, pre-standardization sizes can be found in Edward J. Covington's Early Incandescent Lamps at home.frognet.net/~ejcov/index40.html. Also, KiloKat's Antique Light Bulb Site at www.bulbcollector.com

standards

Copyright © 2000 Sizes, Inc.  All rights reserved.
Last revised: 9 November 2002.