Light Bulb Fittings Guide | Light Fitting Types Explained | Toolstation

Light Bulb Fitting Guide

Light Bulb Fitting Guide

Replacing a light bulb should be a fairly easy job, right? Whether you’re replacing a single bulb or fitting out a kitchen with new lighting, you’ll need to make sure you know your light bulb fitting types before you begin.

The cap type or base fitting not only conducts electricity to the light bulb, but also secures it into your light fixtures. Buy a bulb with the wrong cap or fitting, and it simply won’t fit into the fixture – so picking up the right fitting type is a must.

Light bulb fitting types are often identified with a letter followed by a number. The letter generally refers to the type of fitting, and the number is the diameter in millimetres of the bottom of the cap itself (not including any pins). If there are pins on the bottom of the cap, the number usually refers to the distance between them.

Bayonet Bulbs

Bayonet cap bulbs use a twist and lock motion to secure into the fitting. They’re highly versatile and commonly used in residential properties, and are one of the most widely used types of fittings.

There are also micro bayonet caps (MBC) which are more commonly used in industrial environments, and Ba15d caps which are used in vehicles or mains voltage lamps.

B22d Bayonet Cap (BC)
B15 Small Bayonet Cap (SBC)

Edison Caps

Edison Screw caps are commonly used around the home, but the smallest and largest varieties may also be used for industrial applications. Edison caps screw into a fixture for easy installation.

In general, the larger the cap the greater the wattage and heat that the bulb can withstand.

The main types of Edison Screw caps are the E27, and the Small Edison Screw (SES) which is also known as an E14 cap. These are 27mm and 14mm wide respectively.

E27 Edison Screw (E27)
E14 Small Edison Screw
E17 Intermediate Edison Screw (IES)

Halogen and LED Bulbs

G and GU fittings are used for different types of halogen and LED lights you find in your home or in a commercial setting – such as spotlights in a kitchen or bathroom. They connect through two pins on the base of the cap and use a push and twist mechanism to fit into the socket.

In the case of GU caps, the numbers represent the distance in millimetres between the two pins.

GU10 Halogen Cap
GU5.3 Halogen Caps
GX53 LED Cap

Tube Caps

Tube caps are generally used in large commercial areas such as offices because they’re energy efficient and don’t produce much heat. They typically have two pins at either end to slot into the contacts in the light fitting to transfer electricity, unless they are called ‘mono pins’ – with just a single contact.

Tube caps also last a long time, are more cost-effective, and some have fly-killing properties.

They are referred to as T caps followed by a number, but this number doesn’t usually represent the distance between the pins or size of the cap. Instead, you may see the cap referred to as a G cap followed by the distance between the pins – such as a G13 fitting.

T8 Tube Cap
T5 Tube Caps

Architectural & Strip Light Caps

Architectural caps have either one main contact or two at either end, while strip lights always have two contacts at either end. Generally they’re used for picture lights or shaving mirrors.

You’ll find architectural and strip light caps in a small, bayonet style cap making for easy installation without the need for twisting which can be difficult in tight spaces with small bulbs.

Strip and architectural lights are referred to as S caps followed by a number which represents the diameter of the base.

S15 Strip Light Cap

Halogen Capsules

Halogen capsules are referred to as G caps followed by a number which represents the distance in millimetres between the two pins.

From September 2021, most halogen light bulbs were phased out because they are less environmentally friendly than LED alternatives – the ones that remain are generally the lower-voltage decorative bulbs which have less of a negative environment impact.

They’re mainly used in domestic environments, spotlights and lamps. They’re generally found in halogen adaptors where you have an adaptor base, the replaceable capsule, and a decorative cover. You may find halogen capsules in small, decorative lights such as Christmas lights.

G4 Halogen Capsule
GY6.35 Halogen Capsule
G9 Halogen Capsule
R7s Halogen Capsules

Compact Flourescent Caps

Compact fluorescent lights (CFL) are commonly used for commercial applications and some energy-saving fixtures in residential buildings.

Both two and four pin CFL caps are simple to install – just hold the bulb and push the pins into the fixture.

You’ll find 2-pin and 4-pin CFL caps available. 2 pin CFLs use magnetic ballasts to hold the bulb in place, whereas 4-pin CFLs use electronic ballasts. Electronic ballasts work at much higher frequency than magnetic alternatives so they’re less likely to flicker or make buzzing sounds.

2 Pin CFL Caps
4 Pin CFL Caps

Frequently Asked Questions

  • What are the different light bulb fittings?

    Lightbulb fittings are generally categorised into three main categories – Bayonet (push and twist), Edison (screw), and pins (push).

  • How are light bulb fittings measured?

    Lightbulb fittings are measured in different ways depending on the type, but generally it will either be the diameter of the cap, or the distance between pins if they’re found on the bottom of the cap.

  • What is a bayonet cap light bulb?

    A bayonet light bulb is a common type of bulb in the UK. They work using a push and twist type. Simply push the pins on the side of the cap in, then twist into the fixture until it locks into place.

  • How do you tell what light bulb cap you have?

    First find out which category the cap is in – bayonet, edison, or pin. If it has pins on the side, it is probably a bayonet. If it screws into the fitting, the bulb is probably an Edison type. If there are pins at the bottom of the cap, it’ll be a pin cap. Then you can measure the diameter of the cap or the distance between the pins to find out the specific type.

    Alternatively, the type of cap may be printed on the lightbulb for easier identification.

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