Light bulb socket types define how bulbs connect to fixtures—sizes determine the physical fit, standards ensure safety and consistency, and compatibility decides which bulbs work with which sockets.
What Does the Socket Code Mean?
Every socket code combines letters and numbers that describe its structure and size.
● E = Edison screw (the number indicates the outer thread diameter in millimeters)
● B = Bayonet mount (twist-and-lock style)
● G = Two-pin or multi-pin base (number indicates pin spacing)
● GU = Two-pin with locking lugs (“U” means twist-lock)
● R7s = Double-ended linear base
● S = Festoon or special single-contact style
Example:
● E26 means an Edison screw base with 26 mm diameter (standard in North America).
● G9 means two pins spaced 9 mm apart.
Overview of Common Light Bulb Socket Families
| Family Type | Code Examples | Typical Diameter / Spacing | Common Bulb Shapes | Typical Applications | Regional Standard |
| Edison Screw | E12, E14, E17, E26, E27, E39 | 12–39 mm | A19, BR30, PAR38 | Lamps, ceiling fixtures, floodlights | E26 (US), E27 (EU) |
| Bayonet Mount | B15, B22 | 15 mm / 22 mm | Candle, GLS | UK, India, Australia | BS 161 |
| Bi-pin | G4, G5.3, G6.35, G9 | 4–9 mm | MR16, capsule, LED | Track, under-cabinet, decorative | Global |
| Twist-lock GU Series | GU5.3, GU10, GU24 | 5.3–24 mm spacing | MR16, PAR16 | Spotlights, recessed, energy-saving | US / EU |
| Linear Double-ended | R7s | Lengths: 78–189 mm | T3, halogen | Work lights, floodlights | Global |
| Fluorescent / Plug-in | G13, G24, 2G11 | 13–11 mm pin spacing | T8, CFL | Offices, commercial | Global |
| Wedge Base | T10, T15 | — | Indicator, automotive | Low-voltage lighting | US / JP |
The Edison Screw Family in Detail
Edison Screw sockets are the most common type of light bulb base, identified by the “E” prefix and size in millimeters, such as E12, E26, or E27.
E12 (Candelabra Base)
● Diameter: 12 mm
● Voltage: 120 V (commonly used in North America)
● Use: Chandeliers, night lights, decorative lamps
● Standard: UL/CSA (North America)
E14 (Small Edison Screw)
● Diameter: 14 mm
● Voltage: 220–240 V (Europe and Asia)
● Use: European chandeliers, ovens, appliances
● Standard: IEC (Europe)
E17 (Intermediate Base)
● Diameter: 17 mm
● Voltage: 120 V
● Use: Ceiling fans, sign lamps, C9 holiday lights
● Standard: UL/CSA (North America)
E26 (Standard Edison Screw)
● Diameter: 26 mm
● Voltage: 120 V
● Use: Most household bulbs (A19, BR30, PAR38)
● Standard: UL/CSA (North America)
E27 (European Standard Edison Screw)
● Diameter: 27 mm
● Voltage: 220–240 V
● Use: EU domestic bulbs, table lamps, ceiling fixtures
● Standard: IEC (Europe)
● Note: Similar to E26 but not always interchangeable due to voltage differences.
E39 / E40 (Mogul Base)
● Diameter: 39–40 mm
● Voltage: 120–277 V (E39, North America) / 220–240 V (E40, Europe)
● Use: High-wattage industrial and outdoor fixtures
● Standard: UL/CSA (E39), IEC (E40)
Bayonet and Twist-Lock Bases
While Edison Screw sockets dominate in North America and Europe, bayonet and twist-lock bases are also widely used in the UK, Australia, and certain energy-efficient systems. These designs secure bulbs with a push-and-twist mechanism rather than a screw, offering strong contact and vibration resistance.
B22 (Bayonet Cap)
● Mount Type: Push and twist
● Diameter: 22 mm
● Voltage: 220–240 V
● Use: Ceiling and pendant lights
● Region: UK and Commonwealth countries
● Note: Incompatible with Edison fixtures
GU10 (Twist-Lock Mains Voltage)
● Pin Spacing: 10 mm
● Voltage: 120–240 V
● Use: Recessed and track lighting
● Standard: IEC (Europe), UL/CSA (North America)
● Note: Do not mix GU10 (mains voltage) with GU5.3 (low voltage).
GU24 (Energy-Efficient Twist-Lock)
● Feature: Twist-lock design prevents use of non-efficient bulbs
● Voltage: 120 V
● Use: Compact fluorescent (CFL) and LED fixtures
● Standard: ENERGY STAR / UL (North America)
Bi-pin and Specialty Sockets
In addition to screw and bayonet types, bi-pin and specialty sockets are commonly used for low-voltage, decorative, and high-intensity lighting. These sockets rely on pins or linear contacts rather than screws or twist-locks, making them compact and ideal for focused or decorative applications.
G4 and G9
● Type: Low-voltage capsule / bi-pin
● Pin Spacing: 4 mm (G4) / 9 mm (G9)
● Voltage: 12 V (G4), 120 V (G9)
● Use: Cabinet lighting, accent lights, decorative fixtures
● Region: Global
● Note: Ensure correct voltage before replacing bulbs
GU5.3 (MR16)
● Type: Bi-pin, low-voltage
● Pin Spacing: 5.3 mm
● Voltage: 12 V
● Use: Track lighting, recessed lighting, display lighting
● Region: North America / Europe
● Note: Do not mix with GU10 mains voltage sockets
R7s
● Type: Double-ended linear socket
● Voltage: 120–240 V (depends on fixture)
● Use: Halogen floodlights, work lights, LED linear lamps
● Lengths: 78 mm, 118 mm, 189 mm (non-interchangeable)
● Region: Global
● Note: Always match the exact length and wattage for safe operation
Regional Standards and Compatibility
| Region | Common Standards | Notes |
| North America | E12, E17, E26, GU10, GU24 | 120 V supply |
| Europe | E14, E27, GU10, G9 | 230 V supply |
| UK / India / Australia | B22, B15 | Bayonet standard |
| Japan | E26 (same thread as US) | Voltage 100 V |
Important:
While E26 and E27 threads are almost identical, using a mismatched bulb may cause voltage or fit issues. Always check the rated voltage and base compatibility.
How to Identify and Measure Your Bulb Socket
1. Check the code printed on the bulb or fixture (e.g., “E26” or “GU10”).
2. Measure diameter or pin spacing with a caliper or ruler:
a. Screw bases: measure thread’s outer diameter.
b. Bi-pin: measure the distance between pin centers.
3. Confirm voltage (12 V / 120 V / 230 V).
4. Compare with standard tables above to find the exact match.
Safety and Material Considerations
| Material | Use Case | Notes |
| Porcelain | High-heat applications (halogen, metal halide) | Excellent heat resistance |
| Phenolic (Bakelite) | General household | Economical, but limited temperature tolerance |
| Plastic / Nylon | LED fixtures | Lightweight, heat-sensitive |
| Metal / Ceramic Hybrid | Outdoor and industrial | High durability and heat stability |
Safety Tips:
● Never exceed the wattage or voltage rating printed on the fixture.
● Avoid adapters that mix voltage regions (e.g., E27 bulb into 120 V E26 fixture).
● Use damp- or wet-rated sockets outdoors or in bathrooms.
● Three-way sockets (E26d/E39d) require compatible bulbs with dual contacts.
Common Replacement and Conversion Scenarios
| Situation | Compatible Alternatives | Caution |
| Halogen MR16 (GU5.3, 12 V) → LED MR16 | Direct swap with same pin spacing | Verify transformer compatibility |
| E27 → E26 | Usually fits physically | Check voltage (230 V vs 120 V) |
| B22 → E26 adapter | Works with proper adapter | Not recommended for permanent use |
| G9 → G9 LED | Direct fit | Ensure fixture has ventilation |
| R7s Halogen → R7s LED | Match length exactly | Watch for heat clearance |
Key Takeaways
● Read the code carefully: Letters define type; numbers define size.
● Match voltage and region: Avoid using bulbs incompatible with local standards.
● Use heat-resistant sockets: Porcelain or ceramic bases handle high-wattage bulbs safely.
● Verify physical fit: Check thread size, pin spacing, and length to prevent poor contact.
● Follow fixture limits: Always respect the maximum wattage and safety instructions.
Conclusion: Choosing the Right Light Bulb Socket
Understanding your light bulb socket type is key to safe, efficient, and compatible lighting. From E26 household bulbs to GU10 spotlights and B22 bayonet fittings, refer to the correct size and regional voltage before replacing or upgrading bulbs.
Frequently Asked Questions About Light Bulb Socket
What Is the Most Common Light Bulb Socket Type?
E26 (Standard Edison Screw) is the most widely used socket in North America.
Are E26 and E27 Bulbs Interchangeable?
They fit physically, but voltage and certification differ. Always use the type specified for your region.
Can GU10 Bulbs Replace MR16 Bulbs?
No. GU10 operates on 120–240 V mains voltage, while MR16 (GU5.3) uses 12 V low-voltage systems.
What Is the B22 Socket Used For?
B22 is the standard bayonet fitting in the UK, India, and Australia.
How Can I Identify Low- or High-Voltage Sockets?
Check the bulb or transformer rating. GU5.3 and G4 typically indicate 12 V; E26, E27, and GU10 indicate mains voltage.
Which Socket Types Are Safe for Outdoor Use?
Sockets made of weatherproof porcelain or metal with “wet location” or IP-rated certification are suitable for outdoor use.