Light Bulb Specs, Lighting 101

All About Light Bulb Caps and Bases

light bulbs caps bases

The cap or base of a light bulb joins the bulb to the lamp socket in order to provide an electrical connection. Your lamp or fixture’s socket will dictate which cap or base a light bulb needs in order to fit, but there are a number of different base types and sizes to choose from, and it’s not always easy to determine which type is right for your fixture. To add to the confusion, different fixtures often feature different types of sockets.

Purchasing a bulb with the wrong cap or base is a common mistake. There are a wide variety of caps and bases available, and it can be difficult to know which one is right for your fixture, especially if its socket is not a standard size. To help you find the right cap or base for your fixture, we’ve created a guide to the most common light bulb caps and bases. If you are still unsure after reviewing our guide, we are happy to assist you to find the right bulb.

When you’re shopping for new bulbs or upgrading to energy efficient lamps, these are some of the caps and bases you’re most likely to encounter:

BC Bayonet Cap B22SBC Small Bayonet Cap B15ES Edison Screw E27SES Small Edison Screw E14GU10/Twist Lock
Bayonet Cap - BC- B22

Fit: Push and twist

Commonly found on: GLS/Regular, Candle and Golfball shaped lightbulbs

Key feature: 22mm in diameter
Small Bayonet Cap - B15 - SBC

Fit: Push and twist

Commonly found on: Candle, Golfball and other small decorative shaped lightbulbs

Key feature: 15mm in diameter
Edison Screw - ES - E27

Fit: Screw in

Commonly found on: GLS/Regular, Candle and Golfball shaped lightbulbs

Key feature: 27mm in diameter
Small Edison Screw - SES - E14

Fit: Screw in

Commonly found on: Candle, Golfball and other small decorative shaped lightbulbs (Please note, 12mm CES, 10mm MES and 5mm LES screw caps are also available, please measure carefully)

Key feature: 14mm in diameter
GU10

Fit: Twist and lock

Commonly found on: 50mm 240volt spotlights (Please note 35mm, 63mm and 111mm lamps are also available with this fitting)

Key feature: 2 thick prongs with 10mm between the 2 prongs
GU4Gu5.3G4GY6.35G9
GU4

Fit: Push in

Commonly found on: 25mm and 35mm 12volt spotlights

Key feature: 2 thin pins with 4mm between the 2 pins
GU5.3

Fit: Push in

Commonly found on: 50mm 12volt spotlights

Key feature: 2 thin pins with 5.3mm between the 2 pins
G4

Fit: Push in

Commonly found on: 12volt capsules

Key feature: 2 thin pins with 4mm between the 2 pins
GY6.35

Fit: Push in

Commonly found on: 12volt capsules

Key feature: 2 thin pins with 6.35mm between the 2 pins
G9

Fit: Push in

Commonly found on: 240volt capsules

Key feature: 2 looped (paperclip style) pins with 9mm between the 2 loops
S15G5G13
S15

Fit: Push in

Commonly found on: 221mm and 284mm Striplights for picture lights and shaving mirrors

Key feature: Bayonet style cap withouth the lugs on either end of the lamp. 15mm in diameter
G5

Fit: Push in

Commonly found on: 12mm and 16mm diameter tubes.

Key feature: 5mm between the 2 pins. 2 pins on either end of the tube.
G13

Fit: Push in

Commonly found on: 26mm diameter tubes

Key feature: 13mm between the 2 pins. 2 pins on either end of the tube.

To view a more extensive range please of go to www.thelightbulb.co.uk/cap-base-samples-part-1/ .

Building a Solid Knowledge Base

It’s disappointing to get home after buying a light bulb only to discover it’s the wrong size or type of base. Nobody wants the hassle of going back to the shop and making a return. Understanding what to look for with regards to terms and codes on bulb packaging can help save you time, effort, and even money in the long run.

If you are still unable to locate the cap type for your particular Light Bulb then please call us on: 01869 362222, email us or for instant expert advice use our “Live Support Online” facility.

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About Sarah Levison

Sarah is a lighting expert at The Lightbulb Company with a background in hospitality and a keen awareness of the important role lighting plays in our everyday lives. She has worn many hats with us over the years, developing an extensive knowledge of lighting and the lighting industry. She is passionate about working with customers, understanding their needs, and providing them with the best solutions.

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