Lamp Screw Base Sizes
April 1999, last change August 2010
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A reader inquired "I have an old household light fixture with a socket that is larger than that used for a standard light bulb. What type of light bulb was used back then?"
Here is a description of the screw base sizes most commonly used for electric lamps in the past 50 years together with typical U.S.A. applications. All are right hand threaded. There may have been other sizes that are less common and for which we don't have any specimens.
The dimension given is the approcimate outside diameter of the lamp base, or the size of a hole required (in a piece of cardboard) so the lamp base would fit through without being turned.
A photograph cannot be used to depict the size since the size as seen would vary depending on the size of your computer monitor screen.
We have seen only medium and mogul screw bases used for street lighting.
Midget Screw Base, 7/32 inch, E5
Used for miniature series Christmas tree lights in the early 1960's. The bulbs were about the same size as those in use today. (Shortly thereafter the plastic push-in or snap-in lamp bases we see today were introduced to reduce the costs of fabricating lamp bases and sockets.) Also used for hobby applications such as model railroads using less than 25 volts.
Miniature Screw Base, 3/8 (12/32) inch, E10
Used today for flashlight lamps and instrument panel lamps, typically under 30 volts. Common in the 1940's and 1950's for "C6" 15 volt series Christmas tree lights.
Candelabra Screw Base, 15/32 inch, E12
Used today for decorative lighting, chandeliers, 120 volt multiple Christmas lights (C7-1/2 size), night lights.
Intermediate Screw Base, 21/32 inch, E17
Used for decorative lighting, chandeliers, 120 volt multiple Christmas lights (C9-1/2 size).
Medium aka Edison Screw Base, 1-1/16 (34/32) inch, E26/E27
Standard household lamp base for lamps up to about 250 watts, also used for a few mercury and sodium lamps under 150 watts. Used for older household circuit plug style fuses without the no-tamper feature.
Mogul Screw Base, 1-9/16 (50/32) inch, E39
Used for most mercury, metal halide, sodium lamps, also incandescent lamps over 300 watts. Lamps requiring more than 200 volts are more likely to have a mogul (or larger) base rather than a medium base.
(one larger size, name and dimensions not known)
Used for some incandescent lamps over about 1000 watts and over 200 volts, for example searchlight and lighthouse lamps. With the advent of sodium and mercury lamps, and quartz-halogen incandescent lamps, the large lamps using this size socket are becoming much rarer.
The "E" designations represent the diameter in millimeters. See, also, Harrington Lights.
There are also E11 (minican for "between miniature and candelabra) and E14 sizes, which are common in Europe. Due to the small differences in size, these can be confused with miniature and candelabra bases.
We have been told that the medium screw base was also made in a left handed thread version, used for lamps mounted in public places including subway cars. This was intended to deter pilfering of the lamps, which would not fit in household sockets.
Lamps are not meant to be constantly moved from one socket to another. The glue holding the bulb to the base can fail, and then the two wires inside can twist together and cause a short circuit.
We are not sure whether the mogul screw base was more common in Europe for household lamps.
Household fuses (for circuit overcurrent protection) were made with medium screw bases and fuse boxes used standard lamp sockets for the fuses. Starting inthe 1960's, "tamper resistant" fuses were introduced with a smaller diameter base and with different pitched threads for each current rating so a fuse of the wrong current rating could not be substituted. Adapters are available to screw into a medium screw base fuse holder and accept the tamper resistant fuse.
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All parts (c) Copyright 1999-2010, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.