Light Bulb Fitting Guide: Light Bulb Bases and Bulb Socket Types

It's time for you to replace or get a new light bulb for your light fixture. It should be pretty simple! All you have to do is get the same type of bulb that you previously had. But what happens when you go to the store to find that there are dozens of different types of bulbs? And even more confusing -- many different bulb socket types. Don’t panic you are going to find all of the answers to all of your questions including: How do I know what bulb type I need and why should the light bulb base matter?

Which Bulb Socket Type Do I Need?

Bulb Base:
4 pin T94 pin T9 base
  • Has 4 pins
  • Generally used with T9 circular light bulbs
  • Ceiling fans
  • Ceiling fixtures
  • 'BA' stands for bayonet base
  • 'd' means that it has double or two points of contact on the bottom
  • The number following 'BA' means that it has a 15 millimeter wide base
  • Replacing lighting for automobiles. Most commonly for their headlights
  • Lighting for school buses
  • 'BA' stand for bayonet base
  • 's' refers to its single point of contact
  • The number following 'BA' means that it's 15 millimeters in diameter
  • Replacing lighting for automobiles. Most commonly for their headlights
  • Candelabra Base
  • Usually supports a blunt tipped, flame tipped, or globe shaped light fixture
  • E-11 and E-14 candelabra bases are not interchangeable with this base
  • The 'E' stands for Edison
  • Diameter of 12 millimeters
  • Screws into a light socket
  • Chandeliers
  • Wall sconces
  • Some bath bars
  • Also known as the intermediate Edison screw
  • The 'E' stands for Edison
  • A base between a candelabra and medium base
  • 17 millimeters in diameter
  • Screws into a light socket
  • Inidcators for toys and holiday decorations
  • Exit Signs
  • Occasionally used in ceiling fans and other appliances
  • A medium base
  • Most common type of base in the US
  • Can be used with many different shaped bulbs
  • Standard for the household
  • 'E' stands for Edison
  • Has a 26 millimeter diameter
  • Screws into a light socket
  • Table lamps 
  • Most light fixtures around the house
  • Desk lamps
  • Also known as the mogul base
  • Rarely used except for commercial lighting
  • 'E' stands for Edison
  • A diameter of 39 millimeters
  • It's so large in order to handle the heat of a 300 watt bulb
  • Screws intop a light socket
  • A bi pin connector
  • Can be used with CFL or LED lamps
  • Has a twist lock design
  • Interchangeable between manufacturers with standard GU24 bulbs
  • Household lighting
  • Construction lights
  • A replacement for GU-110 halogen bases
  • Has 2 pins that are 10 millimeters apart
  • Operates at the same voltage as GU-110 so you won't have a problem with the transformer.
  • Twist and lock type base
  • Bath bars
  • Ceiling fans
  • Track lighting
  • MR-16 lamp base
  • With an electronic transformer you are going to have to purchase an LED MR-16 to confirm proper functionality
  • Track lighting heads
  • Bath bars
  • Ceiling fans
  • The pins diameter and spacing are the same as G4, 4 millimeters
  • Usually has a pair of grooves on the sides for a clip to hold the lamp
  • Mostly used in industry and commerce
  • Compatible with 2 and 4 pin G24d, GX24d, G24q or GGX24q base sockets
  • Usually used in ceiling reflectors
  • Industry lighting
  • Commerce lighting
  • Ceiling reflectors
  • A medium bi pin base
  • 13 millimeters between the two pins
  • Used for most of the T8, T10, and T12 linear fluorescent bulbs
  • Warehouses
  • Gyms 
  • Garages
  • Factories
G4 G4 bulb base
  • A bi pin base
  • 4 millimeters between the two pins 
  • Halogen version releases a lot of heat and can use only 20 watts
  • Under cabinet lighting 
  • Decorative lighting
  • Task lighting
  • RV/camper
  • Landscape lighting
  • Has 4 pins
  • 24 refers to the millimeters between the pins
  • 'Q' means quad or that there are 4 bulbs
  • There are G24Q1, G24Q2, G24Q3
  • G24Q1 is usually used for 10 and 13 watt bulbs
  • G24Q2 are used with 18 watt bulbs
  • G24Q3 can use 26 watt bulbs
  • A common type base for fluorescent tubes
  • Usually used in LED linear high bays
  • Bi pin refers to the two pins located on the base
  • Warehouses
  • Gyms 
  • Garages
  • Factories
Fitting Guide
Find the LED bulb you need at HomElectrical!

Recommended Reading

Making the switch to LED lighting can seem daunting at first. Read our guide to learn about the different bulb types and bases, as well as their common applications.

Shakir Williams
Shakir Williams

A true creative with a penchant for the spiritual and natural order. She loves the Earth, almost as much as she loves writing about it.

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