Edison screw (ES) is a standard socket for light bulbs in North
America. It was developed by
Thomas Edison and was licensed in 1909
under the Mazda trademark. Normally, the bulbs have right-hand
threaded metal bases (caps) which screw into matching threaded sockets
(lamp holders). For bulbs powered by AC current, the thread is
connected to neutral and the contact on the bottom tip of the base is
connected to live.
North America and continental Europe, Edison screws displaced other
socket types for general lighting. In the early days of
electrification, Edison screws were the only standard connector, and
appliances other than bulbs were connected to AC power via light
Edison screw sockets comply with international
3 Other uses
5 See also
Early US lamp manufacturers used different and incompatible bases. The
Thomson-Houston Electric Company
Thomson-Houston Electric Company used a threaded stud at the bottom of
the socket, and a flat contact ring. The Sawyer-Mann or Westinghouse
base used a spring clip acting on grooves in the bulb base, and a
contact stud at the bottom of the lamp. By about 1908, the Edison base
was most common in the US, with the others falling out of use.
In response to Edison's patent,
Reginald Fessenden invented the bi-pin
connector for the 1893 World's Fair. Other lamp bases include the
bayonet mount and wedge base.
Three-way E26d light socket
Edison screw to NEMA 1-15 adapter
Specifications for all lamp mount types are defined in the following
American National Standards Institute
American National Standards Institute (ANSI) and International
Electrotechnical Commission (IEC) publications:
Lamp Caps — ANSI C81.61 and IEC 60061-1
Lamp Holders — ANSI C81.62 and IEC 60061-2
Gauges (to ensure interchangeability) — ANSI C81.63 and IEC
Guidelines for Electrical Lamp Bases, Lampholders and Gauges —
ANSI C81.64 and IEC 60061-4
Generally, the two standards are harmonized, although several types of
screw mount are still defined in only one standard.
In the designation "Exx", "E" stands for "Edison" and "xx" indicates
the diameter in millimeters as measured across the peaks of the thread
on the base (male), e.g., E12 has a diameter of 12 mm. This is
distinct from the bulb glass diameter, which in the U.S. is given in
eighths of an inch, e.g., A19, MR16, T12.
There are four commonly used thread size groups for mains supply
Candelabra: E12 North America, E11 in Europe
Intermediate: E17 North America, E14 (Small ES, SES) in Europe
Medium or standard: E26 (MES) in North America, E27 (ES) in Europe
Mogul: E39 North America, E40 (Goliath ES) in Europe.
The E26 and E27 are usually interchangeable, as are the E39 and E40,
because there is only a 1 mm difference in thread outside
diameter. E11 and E12 are not interchangeable. Other semi-standard
screw thread sizes are available for certain specific applications.
The large E39 "Mogul" and E40 "Goliath" base are used on street
lights, and high-wattage lamps (such as a 100-/200-/300-watt
three-way) and many high-intensity discharge bulbs. In areas following
the U.S. National Electrical Code, general-use lamps over 300 W
cannot use an E26 base and must instead use the E39 base, 300 W
lamps may use either base. Medium
Edison screw (MES)
bulbs for 12 V are also produced for recreational vehicles. Large
Christmas lights use Intermediate base,as do some desk lamps
and many microwave ovens. Previously, emergency exit
signs also tended to use the intermediate base, but
U.S. and Canadian rules now require long-life and energy-efficient LED
lamps, which can be purchased inside a bulb as a retrofit. A medium
screw base should not carry more than 25 amperes current; this may
limit the practical rating of low voltage lamps.
E29 "Admedium" bases are used for special applications, for example UV
spotlight bulbs in magnetic crack detection machines.
In countries that use 220–240 volts AC domestic power, standard-size
E27 and small E14 are the most common screw-mount sizes and are
prevalent throughout continental Europe[a] and China.
In 120-volt North America, 100-volt
Japan and Taiwan, the standard
size for general-purpose lamps is E26.
E12 is typically used for candelabra fixtures. E17 is also sometimes
used, especially in small table lamps and novelty lighting, and
occasionally the lights on newer ceiling fans.
Christmas lights use
various base sizes E17 for C9 bulbs, E12 for C7 bulbs, E10 for
decades-old series-wired C6 bulb sets in the U.S., and an entirely
different wedge base for T1¾ mini lights. For a short time early on,
these mini lights were manufactured using E5 screw bases.
A tiny E5 or E5.5 size is used only for extra-low voltages, such as in
interior illumination for model buildings, and model vehicles such as
model trains. These are often called "pea bulbs" if they are
globe-shaped, but they commonly look like mini Christmas bulbs, or
large "grain-of-wheat" bulbs. E10 bulbs are common on battery-powered
flashlights, as are bayonet mounts (although those are usually held in
with a circular flange located where the base meets the bulb). The E11
base is sometimes used for 50/75/100-watt halogen lights in North
America, where it is called the "mini-can", and tighter threads are
used to keep them out of E12-base nightlights and other places where
they could start a fire.
There are also adapters between screw sizes, and for adapting to or
from bayonet caps. A socket extender makes the bulb stick out further,
such as to accommodate a compact fluorescent lamp with a self-ballast
that doesn't fit in a recessed lighting fixture.
Most Edison screws have right-hand threads (lamp is turned clockwise
to tighten), but left-hand threaded screws are sometimes used, usually
for a non-standard voltage or wattage bulb. This prevents the use of
an incorrect bulb, which could cause damage. Public locations such
as railway trains and the
New York City Subway
New York City Subway have used light bulbs
with left-hand threads to discourage theft of the bulbs for use in
regular light fixtures.
A 1909 toaster with Edison plug
Edison screw socket was used as an outlet (such as for toasters)
when mains electricity was still mainly used for lighting, and before
wall outlets became common.
In North America, fuses were used in buildings wired before 1960.
These Edison base fuses would screw into a fuse socket similar to
Edison-base incandescent lamps.
Some adapters for wall outlets use an Edison screw, allowing a light
socket to become an ungrounded electrical outlet (such as to install
Christmas lights temporarily via a porch light), or to make a
pull-chain switch with two outlets, or to split it for two lamps.
Another adapter can make a wall outlet into a lamp holder (lamp
Various other accessories have been made, including a smoke detector
that recharges over a few hours and lasts for a few days or weeks
thereafter, and still allows the attached lamp to operate normally.
There have also been electronics that stick onto the end of the screw
base and allow the attached lamp to flash, for example, to attract the
attention of arriving guests or emergency vehicles; others function as
a dimmer or timer, or dim gradually in a child's bedroom in the
Some thermionic valves, such as certain rectifiers, use an Edison
From left to right: E27, E14, and E10 bulbs
Base major diameter (thread external)
IEC 60061-1 standard sheet
Lilliput Edison Screw (LES)
Indicator lights, decorative lights
Miniature Edison Screw (MES)
Flashlights, bicycle lights
Mini-Candelabra Edison Screw (mini-can)
120 V halogen mini-candelabra
Candelabra Edison Screw (CES), C7
120 V candelabra/night lamp
Small Edison Screw (SES)
230 V candelabra/chandelier, night lamps, and some pendant
Intermediate Edison Screw (IES), C9
120 V appliance / Outdoor Christmas
[Medium] (one-inch) Edison Screw (ES or MES)
Standard 120 V lamps
[Medium] Edison Screw (ES)
Standard 230 V lamps
[Admedium] Edison Screw (ES)
Single-contact (Mogul- in America) Goliath Edison Screw (GES)
120 V 250+ W industrial
(Mogul) Goliath Edison Screw (GES)
230 V 250+ W industrial
Three-way lamps have a d suffix to indicate double contacts, usually
E26d or E27d, or rarely E39d. The second contact is used for the
lower-wattage filament of the two inside the lamp. This extra contact
is a ring located around the main contact. Unlike bayonet sockets,
three-way and regular lamps are interchangeable, although the low
filament or low setting doesn’t work if mismatched.
Edison screw has nine threads per inch, or about 2.8mm per
Edison screw has seven threads per inch, or about 3.6mm per
thread. In the U.S., the Energy Independence and Security Act of 2007
requirement for greater energy efficiency only applies to the medium
Edison screw, all other being considered "specialty" lamps.
Diazed fuses DII uses the same E27 thread as standard 230 V
lamps, but have a longer body and cannot be screwed into a lamp holder
(socket). A lamp base is too short to contact the bottom terminal of a
fuse holder. However it's possible (but not useful) to screw a DII
fuse holder without a fuse in an E27 lamp holder.
Screw bases have a number of disadvantages compared to the bayonet fit
type:[original research?]
The metal screw itself forms one of the contacts for the circuit. If
the lighting system is not correctly wired, or a lamp is plugged into
a non-polarized outlet, the metal screw can become live, presenting an
electric shock hazard to anyone attempting to change the lamp.
It is possible to over-tighten the screw, risking breaking the bulb or
separation of the glass from the metal base and leaving the base in
the socket, especially when subsequently attempting to unscrew it.
If the lamp becomes loose in the socket due to vibration or
under-tightening, it can lose contact with the center contact and stop
working until it is tightened. The bayonet type is resistant to
vibration and much less likely to become loose.
As the metal thread carries current, any arcing can jam the thread.
Corrosion is more likely to jam a screw thread than a bayonet fixing.
Screwing in and unscrewing the bulb places more force on the glass
When fixing anything with a cable attached, the cable will twist as
the screw is turned.
Screw bases have a number of advantages compared to the bayonet fit
type:[original research?]
Screw bases are more suitable for small size bulbs
A bulb fully screwed home is more secure than a bayonet fit bulb
Moisture and debris are less likely to contaminate the contacts of a
screw base bulb
Spring contacts are not necessary on screw base bulbs (the spring
tension must not only ensure a good contact but it must also hold the
bulb securely in the bayonet so there is a compromise)
Wikimedia Commons has media related to Edison screw.
A-series light bulb
Screw thread diameters
^ The BC or bayonet mount fitting is the commonest light bulb fitting
in the UK and many British Commonwealth countries, and is found in
older installations in some other countries,
including France and Greece.
^ I.C.S. Reference Library volume 4B, International Textbook Company,
Scranton PA 1908, page 43-41
^ "Lamp Size Reference". lightopedia.com. Retrieved 2017-04-23.
^ "Light Bulb Shape and Size Chart Reference Charts Bulbs.com".
Bulbs.com. Retrieved 2018-02-05.
General Electric Incandescent Lamps manual, publication no. TP 110,
1976 page 12
^ "灯泡螺口规格、灯泡的选购技巧". 搜房网家居装修
(in Chinese). Retrieved 28 February 2017. 最长[常]
^ "The E26 is the standard 120
Volt American base." LED waves, FAQ,
retrieved 5 February 2018
^ "E26: 一般電球、ボール電球の多くがこのサイズ。"
Sharp Japan, support pages, retrieved on 30 January 2015
^ Eisenbraun, Blair (Mar 24, 2011). "Left Handed Incandescent Light
Bulbs?". eLightBulbs. Retrieved 27 July 2016.
^ McManus, Chris (2004). Right Hand, Left Hand: The Origins of
Asymmetry in Brains, Bodies, Atoms and Cultures. Harvard University
Press. p. 46. Retrieved 2015-01-25.
^ Schneier, Bruce (2003). Beyond Fear: Thinking Sensibly About
Security in an Uncertain World. Springer Science+Business Media.
p. 221. Retrieved 2015-01-25.
^ "IEC 60061 INTERNATIONAL STANDARD - Lamp caps and holders" (PDF) (in
French and English) (3.51 ed.). Geneva: IEC. December 2014. Archived
from the original (PDF) on 16 April 2013. Retrieved 22 March
2015. (subscription required)
^ Formisano, Bob. "2007 Energy Bill - Are They Phasing Out or Making
Incandescent Bulbs Illegal? Incandescent Exemptions: Where You Can
Still Use Existing Bulbs". About.com. Archived from the original on 27
April 2009. Retrieved 22 March 2015.
ANSI C81.61-2007 American National Standard for electrical lamp
bases—Specifications for Bases (Caps) for Electric Lamps, available
at www.nema.org, retrieved 2009-01-20
List of Edison patents
Incandescent light bulb
Thomas A. Edison, Inc.
Edison and Swan Electric Light Company
Edison Gower-Bell Telephone Company of Europe, Ltd.
Edison Illuminating Company
Edison Machine Works
Edison Manufacturing Company
Edison Ore-Milling Company
Edison Portland Cement Company
Edison Storage Battery Company
Motion Picture Patents Company
Mine Safety Appliances
Oriental Telephone Company
Memorial Tower and Museum
National Historical Park
Storage Battery Company Building
General Electric Research Laboratory
Charles Edison (son)
Theodore Miller Edison (son)
Young Tom Edison
Young Tom Edison (1940)
Edison, the Man
Edison, the Man (1940)
"The Wizard of Evergreen Terrace" (1998)
The Future Eve (1886)
Edison's Conquest of Mars
Edison's Conquest of Mars (1898)
Tales from the Bully Pulpit (2004)
The Execution of Mary Stuart
The Execution of Mary Stuart (1895)
The Kiss (1896)
A Night of Terror (1911)
Thomas Edison in popular culture
War of Currents
Pearl Street Station
Thomas Edison House