The base is the part of a lamp that makes the mechanical and electrical connection with its socket. It holds the lamp in position and carries power to it. Correctly identifying the base is therefore one of the most important steps in selecting a replacement.
A matching base does not automatically make two lamps interchangeable. Voltage, wattage, dimensions, technology, control gear and fixture ratings must also agree. Think of the base as one part of the identification—not the complete answer.
Edison screw bases
Edison screw bases use the letter E, followed by a number that broadly identifies the base diameter in millimetres. Common examples include E12 candelabra, E26 medium, E27 medium, E39 mogul and E40 mogul bases.
E26 and E27 look very similar and are sometimes discussed together, but regional voltage, product design and fixture requirements still need to be checked. The same applies to E39 and E40. Never assume that a close physical fit confirms safe electrical compatibility.
When identifying a screw base, check:
- The stated base code
- Approximate diameter
- Fixture voltage
- Maximum permitted wattage
- Lamp length and width
- Whether the fixture is enclosed or exposed
Bayonet bases
Bayonet lamps are inserted and turned so that side pins lock into the socket. Codes may include B15 or B22, with additional letters describing the contact and pin arrangement.
Two bayonet bases of the same general diameter can still have different contact configurations or offset pins. Compare the original lamp directly with the replacement rather than relying on the first part of the code alone.
Pin and twist-lock bases
Pin-base lamps include miniature capsules, reflector lamps, compact fluorescent lamps and linear tubes. Common examples include G4, G9, GU5.3, GU10, G13 and G5.
The letters and numbers communicate details about the base family and spacing. The prefix matters. A GU10 is not the same as a GU5.3, and a G4 is not interchangeable with a G9. Some lamps push straight into the socket, while others insert and twist to lock.
Check:
- Number of pins
- Pin shape and thickness
- Distance between pins
- Whether the connection pushes in or twists
- Lamp voltage
- Required transformer, ballast or driver
Do not bend pins to make a lamp fit. A forced connection can damage the lamp or socket and may create an unsafe contact.
Fluorescent and linear-tube bases
Linear lamps are commonly identified by both tube family and base. A T5 tube typically uses a smaller bi-pin base than a T8 or T12. T8 and T12 tubes may share a G13-style connection in common lengths, but that does not make the lamps or their LED replacements universally interchangeable. The ballast, wiring method, lamp type and fixture configuration must be checked.
Circline, U-bend and compact fluorescent lamps have their own base arrangements. Count the pins and record the complete code printed on the lamp.
Why shape and base should be stored separately
A lamp name such as PAR30 describes a general shape and diameter, not one finished product. PAR30 lamps can be made with different technologies and bases. Separating bulb shape from compatible bases makes it possible to describe the full lamp family without pretending that every version is identical.
For example:
- Bulb shape: PAR30
- Possible technologies: LED, halogen or incandescent
- Common available bases: E26 or E27, depending on the product and market
The precise product must still be confirmed before purchase.
A reliable identification process
- Read the complete base code from the lamp or package.
- Compare the original and replacement bases visually.
- Measure the base or pin spacing if the code is missing.
- Confirm voltage and wattage.
- Check lamp dimensions and fixture clearance.
- Review any ballast, driver, transformer or dimmer requirement.
- Confirm environmental ratings for damp, wet, hot or enclosed locations.
If the base is damaged, corroded or missing, bring the lamp and fixture information to a lighting specialist. Bulb World Lighting can help identify both common and hard-to-find base configurations, but installation and electrical modifications should be completed by an appropriately qualified professional.