Socket Types

Lighting Guide | Socket Types

What are socket types?

A socket (also called a lamp holder or fitting) is the part of a luminaire that holds the light source and connects it to the power supply. Every bulb has a matching base, or "cap," that slots into it. The socket and the cap have to be the same type for the bulb to fit and work — much like a plug and an outlet.

Knowing your socket type is essential when replacing a bulb or switching to LED. The code stamped on most caps (for example E27 or GU10) tells you both the connection style and a key dimension, so you can buy the right replacement with confidence and avoid bulbs that simply won't fit.

Common socket types

E27 (Edison Screw):

The most widely used fitting in Europe, with a 27 mm screw thread. Mains voltage. Found in most pendants, table lamps and floor lamps.

E14 (Small Edison Screw):

The smaller 14 mm screw version of the E27. Mains voltage. Common in chandeliers, wall lights and compact fixtures.

GU10 (Twist-Lock):

Two flared pins 10 mm apart — push in and turn a quarter-turn to lock. Runs directly on mains voltage with no transformer. The standard for recessed spots and track lighting.

G5.3 / GU5.3 (MR16 Spotlight):

A bi-pin base with 5.3 mm spacing, used for MR16 reflector spotlights. Low voltage (12 V), so it needs a transformer or LED driver. Not to be confused with the smaller 4 mm G4 capsule base.

G9 (Looped Pin):

A compact base with two looped wire contacts. Mains voltage, push-fit. Often used in small spotlights, wall lights and decorative fixtures.

G53 / GX53 (Flat Puck LED):

A twist-lock base with pins 53 mm apart, carrying a flat, disc-shaped lamp. Mains voltage. Designed for slim downlights and cabinet lighting. Despite the name, unrelated to G5.3.

T5 / T8 (Linear Tubes):

Tube formats named by their diameter (T5 ≈ 16 mm, T8 ≈ 26 mm). The end caps are bi-pin — G5 for T5, G13 for T8. Used for linear fluorescent or LED tubes in kitchens, garages and workspaces.

R7S (Double-Ended):

A linear lamp with a contact at each end. Mains voltage, available in fixed lengths (most often 78 mm or 118 mm). Used in floor uplighters and floodlights.

S14d / S14s (Tubular Festoon / Linestra):

Long tube-shaped lamps with an S14 disc cap — S14d has a contact at each end (double-ended), S14s groups both contacts at one end (single-ended). Mains voltage. A signature look for linear and mirror-style fittings.

How to read a socket code

Socket codes follow a simple logic. The letter describes the type of connection, and the number gives a key measurement in millimetres:

The letter indicates the base style — E for Edison (screw), B for bayonet, G for pin-based, R for recessed/double-ended contacts.
The number gives a dimension in millimetres: the thread diameter for screw caps (E27 = 27 mm) or the distance between the pins for pin bases (G4 = 4 mm).

When in doubt, unscrew or unplug the old bulb and check the cap — the code is usually printed on the metal or plastic base, or on the original packaging.

Choosing the right replacement bulb

Match the socket type exactly. A bulb with the wrong cap will not fit, and adapters are best avoided as a permanent solution.

Check the voltage. Most screw and GU10 fittings run on mains voltage, while G4 and MR16 bases are typically low voltage (12 V) and need a compatible transformer or LED driver.
Mind the maximum wattage of the fixture, and choose a color temperature and lumen output suited to the room. Switching to LED usually lets you keep the same socket while using far less energy.

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