How to Identify and Replace a Bulb by Base Type (G4, GZ6.35, G10Q, G24d)
Why Base Type Matters
When a lamp fails, the most common mistake is buying a replacement by wattage alone — only to find it doesn't physically fit. The base type (also called the cap type or fitting) determines whether a lamp will connect to your fixture at all. Getting it right the first time saves time, return freight, and frustration.
This guide covers the most common base types found in halogen, CFL, and LED lamps stocked at NZ Lights, with practical tips for identifying what you have and finding the correct replacement.
How to Read a Lamp Base Code
Lamp base codes follow an international IEC standard. The letter prefix describes the base shape, and the number indicates the pin spacing in millimetres:
- G = bi-pin (two pins)
- GX = bi-pin with a specific pin diameter
- GZ = bi-pin with a recessed contact
- GU = bi-pin with a twist-lock mechanism
- G10Q = four-pin circular base (the number refers to the ring diameter)
- G24d / G24q = double or quad-pin CFL base (d = 2 pins, q = 4 pins)
The number after the letter is the pin centre-to-centre distance in mm. A G4 lamp has two pins spaced 4 mm apart. A GZ6.35 has two pins spaced 6.35 mm apart.
Common Base Types at NZ Lights
G4 — Miniature Bi-Pin (4 mm)
One of the smallest and most common halogen bases. Found in under-cabinet lights, display cases, caravan and marine lighting, and low-voltage landscape fittings. Typically runs at 12V, 5W–35W.
How to identify: Two thin parallel pins, 4 mm apart. Very small overall lamp body.
LED equivalent available: Yes — G4 LED capsules are a direct drop-in for most 12V AC/DC fittings, though check your driver compatibility before switching.
→ Browse Halogen Lamps | Browse Specialised Lamps
GZ6.35 — Bi-Pin with Recessed Contact (6.35 mm)
Common in professional and theatrical halogen lamps, projectors, and high-intensity task lighting. Typically runs at 12V or 24V, 20W–75W. The recessed contact design prevents accidental short-circuits.
How to identify: Two pins spaced 6.35 mm apart, with a slightly recessed base collar. Larger pin diameter than G4.
LED equivalent: Limited — check the specific lamp code before assuming an LED replacement exists at the same voltage and output.
→ Browse Halogen Lamps | Browse Photo & Projection Lamps
G10Q — Four-Pin Circular Base
Used in circular fluorescent (circline) lamps and some UV germicidal lamps. The four pins are arranged in a square pattern on a circular base. Common in bathroom vanity mirrors, kitchen ceiling fittings, and some commercial downlights.
How to identify: Four pins in a square arrangement on a round base. The lamp itself is circular (ring-shaped).
LED equivalent: Circular LED panels are available as replacements for some circline fittings, but the driver/ballast must be bypassed or replaced.
→ Browse 2D & Circline Lamps | Browse Fluorescent Lamps
G24d / G24q — Double & Quad-Pin CFL Base
Standard base for plug-in compact fluorescent lamps (CFLs). Found in commercial downlights, office troffers, and hospitality lighting. The G24d has 2 pins (for magnetic ballast fittings) and the G24q has 4 pins (for electronic ballast fittings). These are not interchangeable.
How to identify: Check the lamp base — d = 2 pins, q = 4 pins. Also check the wattage printed on the lamp body (typically 9W, 13W, 18W, or 26W).
LED equivalent: G24 LED PL lamps are available as direct replacements for electronic ballast fittings, or as ballast-bypass versions for retrofit.
→ Browse T & Te (PL-T 2 & 4 Pin Lamps) | Browse Fluorescent Lamps
Step-by-Step: How to Identify Your Lamp Base
- Remove the failed lamp safely — switch off power and allow the lamp to cool completely before handling.
- Look for a code printed on the lamp — most lamps have the base type, wattage, and voltage printed on the base or envelope (e.g., "GZ6.35 35W 12V").
- Count the pins — 2 pins = bi-pin family (G4, GZ6.35, G24d); 4 pins = quad-pin family (G24q, G10Q).
- Measure pin spacing — use a ruler or vernier caliper to measure centre-to-centre distance in mm. This gives you the number in the base code.
- Check the voltage — low-voltage lamps (12V, 24V) require a compatible transformer or driver. Mains voltage lamps (230V) connect directly.
- Match the full lamp code — search NZ Lights by the full code (e.g., "GZ6.35 50W 12V") for an exact replacement.
Quick Reference: Base Type Comparison
| Base Type | Pins | Spacing | Typical Voltage | Common Application |
|---|---|---|---|---|
| G4 | 2 | 4 mm | 12V | Capsule, display, marine |
| GZ6.35 | 2 | 6.35 mm | 12V / 24V | Projector, theatrical, task |
| G10Q | 4 (circular) | 10 mm ring | 230V | Circline, bathroom vanity |
| G24d | 2 | 24 mm | 230V | CFL downlight (magnetic ballast) |
| G24q | 4 | 24 mm | 230V | CFL downlight (electronic ballast) |
Still Not Sure?
If the code has worn off or you're working with an unlabelled fitting, bring the old lamp in or send us a photo. Our team can identify most lamp types from a clear image of the base and envelope.
You can also browse by lamp family:
About NZ Lights
NZ Lights is a New Zealand-owned specialist lighting supplier with over 5 years of experience supplying replacement lamps for healthcare, dental, commercial, industrial and specialist applications.
From medical examination lights and dental equipment to specialist halogen, fluorescent and LED lamps, we help customers identify difficult-to-find replacements using model numbers, lamp specifications and equipment requirements.
Whether you're replacing a single bulb or sourcing lamps for a hospital, clinic, business or facility, our team is here to help you find the correct solution.
Need help identifying your lamp? Contact NZ Lights or send us a photo of your existing lamp, and we'll help you find the correct replacement.
