Technical Articles

Bulb Bases and Sockets Explained

Bulb Bases and Sockets Explained
In this lesson we discuss the naming conventions of LED light bulb bases and light bulb sockets. Details of several types of bases and light sockets are shown in diagrams with dimensions and names. 

Naming Conventions of Light Bulb Sockets and Base Types

Light bulb bases and sockets are normally defined by a letter-number-letter format, with the last letter optional. The first letter designates the shape or form of the base, the numbers represent either the width of the base or the distance between the pins. The second letter designates the number of pins or contacts on the lamp. The numbers are normally in millimeters. LED light bulb sockets and bases are manufactured to the same standards as halogen, incandescent and other traditional lights. 
Click on the image to enlarge. 
By way of example a standard USA screw-in incandescent bulb has a base type of E26. E stands for Edison screw-in and the 26 means that the base is 26 millimeters. E27 is the European standard screw-in size. 
LED light bulbs and LED lamps use the same socket designations as traditional light bulbs. Retrofit Solid State Lights (SSL) that use LEDs or OLEDs are normally designed to be compatible with existing sockets. 
Light Bulb Base and Socket Shape Types
  • B Bayonet Collar Base
  • E Edison Screw Light Base
  • F Single Pin Type Base
  • G Multiple Pin Type Light Bulb Base
  • K Cable Connections
  • P Pre-focused Light Base
  • R Recessed Contact(s) Base
  • S Shell-type Light Bulb Base
  • T Telephone Slide Base
  • W Wedge Base
  • X Special Type Lightbulb Base

Lamp Number of Pin or Contacts
  • s Single Pin
  • d Double or Bi-Pin Base Type
  • t Triple Pin Base or Tri-Pin Base or 3 Pin Base
  • q Quadruple Pin Base or 4 Pin Base

Bulb Type Details

B or Bayonet light bulb base and sockets

LED light bulb bayonet base types. 
Examples of these base types include: BA75, BA95, BA150, BAY150, BA155, BA220
Click on image to enlarge.

Bi-pin and G sockets and bases

LED light bulb bi-pin base types. 
Examples of LED bipin base types include: G4, G5, GU5.3, GU10, G13, T-1/2, T-3/4, T-1, T-1 1/4, T-1 3/4. 
For more information on G type bases see G below. 
Click on image to enlarge.

E or Edison screw-in sockets and bases

Screw-in bases, also known as Edison bases and light bulb sockets are used through out the world. 
The naming conventions include terms like medium base and candelabra base. Often they are referred to with a letter-number designation that starts with an "E" followed by the number of millimeters in diameter of the base. If your base ends in a "d" this indicates a double contact base, this is typically used for 3-way bulbs. 
Typically North American 120VAC bases and 240VAC bases differ by only a millimeter and carry the same full name in each country. 
Click on image to enlarge. 
"E" or Edison base types, sizes and names 
Base Number Diameter Base or Socket Name
E5 5mm Lilliput Edison Screw Base
E10 10mm Miniature Edison Screw Base
E11 11mm Mini-Candelabra Edison Screw Base
E12 12mm Candelabra Edison Screw Base
E14 14mm Small Edison Screw Base (supports 240V)
E17 17mm Intermediate Edison Screw Base
E26 26mm Medium Edison Screw Base
E27 27mm Medium Edison Screw Base (supports 240VAC)
E39 39mm Mogul or Giant Edison Screw Base
E40 40mm Mogul or Giant Edison Screw Base (supports 240VAC)

F or Flange light bulb base and sockets

LED light bulb flange base types. 
LED flanged bases include base types like: micro-midget flanged base, midget flanged, miniature midget flanged, sub-midget flanged, and special-midget flanged base types. These LED light bulb types are used in automotive and electronic device lighting applications. 
Click on image to enlarge.

G or Bi-pin light bulb base and sockets

The "G" base type of lamp and light bulb is used to designate a "pinned" base. This can include several pin types and different numbers of pins. 

Designation is by letter(s)-numbers-optional letter: The first letters include "G" followed by an optional U, X, Y or Z. Next is a number that designates the number of millimeters between the center of each pin. Finally the optional final letter designates the number of pins, no letter implies 2 pins as does a d for double. Pin number designations include: s : Single, d : Double, t : Triple for 3 pins and q : Quadruple for 4 pins. 
Examples include: GU10 base type, G24q base type, or for T8 tubes G13 bi-pin. 

Click on image to enlarge. 

Type Pin to pin distance Pin Diameter Typical Bulbs that use this base
G4 4mm 0.65-0.75mm MR11 and other small halogens of 5/10/20 watt and 6/12 volt
GU4 4mm 0.95-1.05mm
GY4 4mm 0.65-0.75mm
GZ4 4mm 0.95-1.05mm
G5 5mm T4 and T5 fluorescent tubes
G5.3 GU5.3 GX5.3 GY5.3 5.33 mm 1.47-1.65mm MR16 and other small bulbs typically using 12/24 volts
G6.35 GX6.35 GY6.35 6.35 mm 0.95-1.3mm
G8 8mm
G9 9mm
GU10 10mm bulbs using twist-lock bi-pin base
G13 12.7mm T8,T10 & T12 fluorescent tubes
G23 23mm 2mm
GU24 24mm 2 and 4 pin bases with center key
GX53 53mm used with puck shaped lamps typically twist-lock

K or Cable connected lights and lamps

So where you see a K used in a base type location in a part number. You can assume that the product is a wired connection to power.

P or Pre-focused lamp base and sockets

LED light bulb pre-focus base types: S.C. Prefocus and D.C. Prefocus 
Click on image to enlarge.

R or Recessed lamp base and sockets

S or Slide light bulb base and sockets

LED light bulb slide base types. 
Examples of LED slide base types like Slide #1. 
Click on image to enlarge.

W or Wedge light bulb base and sockets

LED light bulb wedge base types. 
Examples of LED Wedge base types include: D.F Wedge and S.F. Wedge sockets. 
Click on image to enlarge.


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