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.
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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
Lamp Number of Pin or Contacts
- 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
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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.
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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.
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"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.
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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.
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.
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
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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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