This page shows some of the common lamp caps that are in use today, and provides some basic information about each one. This isn't an extensive list of all the available lamp caps, but it shows the most common ones.
G24d ('d' is for double pin) PL lamps are available with either a double or triple turn tube.
Double turn bulbs usually have a square lamp base (G24d) and triple turn usually have a hexagonal lamp base (GX24d).
Buy G24d bulbs
G24q ('q' is for quad pin) PL lamps are available with either a double or triple turn tube.
Double turn bulbs have a square lamp base (G24q) and triple turn have a hexagonal lamp base (GX24q).
Buy G24q bulbs
Rectangular-based 2G11 and 2G7 4 pin lamps with a single turn tube.
Buy 2G11 / 2G7 bulbs
Rectangular-based G23 2 pin lamps with a single turn tube.
Buy G23 bulbs
G9 bulbs are available at various wattages, and are only now available in clear glass or LED version. They are often used on small decorative light fittings where a high light output is required.
Some manufactures add a ceramic base to the lamp. The pins are 9mm apart, hence G9.
Buy G9 bulbs
G4 bulbs are the smallest type of commonly used bulb, the pins are just 4mm apart.They are low voltage bulbs usually requiring 6v, 12v or 24v.
Due to their tiny size, they are often used in intricate light fittings, where style is more important than light output. Light fittings that use G4 bulbs often use many bulbs to compensate for the relatively low light output. The pins are 4mm apart, hence G4.
Buy G4 bulbs
GU10 bulbs are often spot lights, used in recessed downlighters. They are mains voltage bulbs running at 230v. GU10 is one of the safest types of mains voltage lamp holder as all the electrical live parts are concealed even when the bulb is removed.
Although the GU10 cap is similar to the L1 lamp cap, GU10 bulbs can't be used in L1 light fittings.
Buy GU10 bulbs
Also known as MR16
GU5.3 bulbs are low voltage usually running at 12v. The pins are 5.3mm apart.
The most common use for this lamp cap is for spot lights, in recessed or decorative light fitting.
Buy MR16 bulbs
Also known as BC and B22
The bayonet cap lamp holder has exposed electrical connections inside when the light bulb is removed.
Historically the bayonet lamp cap was predominently used in the UK and Austraila. Replacement low energy lamps are available with BC caps.
Buy BC bulbs
Also known as SBC and B15
This is the same as the larger bayonet cap, only smaller. The diameter is about the same as an adult little finger.
Buy SBC bulbs
Also known as ES and E27
The diameter of the screw thread is about the same as an adult thumb.
When the light bulb is removed, there are exposed electrical connections inside.
E27 light bulbs are very common all over the world, except in the USA and Canada where a slightly smaller E26 thread is more common.
Buy ES bulbs
Also known as SES and E14
This is the same as the larger screw cap, only smaller. The diameter is about the same as an adult little finger.
Buy SES bulbs
The L1 lamp cap is used a various shapes and sizes of bulb, its based on GU10, with important difference of a hole in the middle of the lamp. The lamp holder has a corresponding pin that fits into this hole and prevents a standard GU10 bulb being used.
The L1 cap is used on self-ballasted compact fluorescent, and LED bulbs.
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