G4 LED replacement: voltage, transformer, fit

How to identify a G4 base

With power off and the old lamp cool, look for two thin, straight metal pins set very close together. The nominal spacing is 4 mm from the center of one pin to the center of the other. Measure center to center rather than outside edge to outside edge, because pin thickness would otherwise distort the result.

G4 is different from G9, which uses two wire loops with nominal 9 mm spacing. It is also different from GU5.3, whose straight pins are farther apart and often appear on MR16 reflectors. Do not spread, bend, trim, or force pins to make a near match enter the socket.

The small G4 connection appears on compact T3 and capsule lamps, MR11 reflectors, puck lights, under-cabinet lighting, landscape fixtures, marine and RV fittings, and some desk lamps. Those uses are identification clues, not proof of the base or voltage.

Why 12 V is common but not guaranteed

Many current G4 products for the U.S. market are designed for 12-volt systems. Feit Electric, for example, publishes G4 LEDs marked 12 V, including one specified as 12 V AC only and another 12 V model for landscape and accent lighting. Bulbrite also lists 12 V G4 halogen capsules. These model records show a common pattern, not a definition of G4.

Read the old lamp, fixture label, transformer, or power-supply documentation. Match not only the voltage number but also whether the replacement accepts AC, DC, or both. A product marked 12 V AC only should not be assumed suitable for an unspecified 12 V DC system, and the words G4 or LED do not settle that question.

If the fixture is connected directly to household line voltage, do not insert a low-voltage lamp. If the supply marking is inaccessible or ambiguous, identify the fixture and transformer model before replacing the lamp.

The transformer can matter after an LED conversion

A low-voltage fixture may use a magnetic transformer, an electronic transformer, or a regulated DC supply. An LED replacement draws much less power than the halogen lamp it replaces, and some older electronic transformers need a minimum load or have output characteristics that do not suit every LED driver. Possible symptoms include failure to start, flicker, pulsing, noise, or unstable dimming.

Look for a transformer compatibility statement from the lamp or fixture manufacturer. When several lamps share one transformer, consider their combined actual LED load and the transformer's permitted range. Do not add an arbitrary load merely to make an incompatible system behave; use components approved for the application.

Dimming adds another layer. The lamp must be dimmable, the transformer must support dimming, and the wall control must be compatible with that system. A dimmable G4 marking alone cannot establish all three conditions.

Measure a tight fixture before buying

G4 fixtures are often designed around a very small halogen capsule. An LED replacement may be wider, longer, or thicker because of its circuit board, emitters, driver, and protective cover. The pins can line up correctly while the body strikes a glass lens, reflector, socket collar, or retaining clip.

Measure the old lamp's maximum width, overall length, and distance from the pin tips to the widest point. Then measure the fixture opening, usable depth, and space around the socket. Compare those values with the exact product drawing; G4 itself provides only nominal pin spacing.

Official product pages illustrate the variation. Feit lists one 12 V G4 LED at 1.4 inches long and 0.8 inch wide, while other G4 capsules use different dimensions. Never generalize one model's envelope to the entire base family.

Choose lumens, color, and ratings separately

For brightness, compare lumens instead of the replacement-watt claim. Current official examples include a 1.5-watt LED around 90 lumens and a 2-watt LED around 135 lumens, while halogen G4 capsules at 10 or 20 actual watts have other output values. These are examples from specific models, not universal conversions.

Select color temperature in kelvins after compatibility is established. A warm or bright-white description affects appearance, not base fit. If a landscape or marine fixture is exposed to moisture, confirm the lamp and fixture's location ratings. A damp rating is not the same as a wet rating.

For a sealed puck light or covered fixture, require explicit enclosed-fixture approval when the application calls for it. The Department of Energy notes that trapped heat can affect some LED lamps. Enclosure approval, moisture rating, dimmability, voltage, and transformer support are distinct fields.

Installing the matched G4 lamp

Photograph the old lamp, fixture label, power supply, and control before removing anything. Search the complete model number when possible. A listing that says only G4 LED leaves voltage type, dimensions, brightness, transformer behavior, and ratings unresolved.

After confirming the replacement, align the pins without bending them and insert the lamp according to the fixture instructions. Do not touch a quartz halogen capsule with bare fingers when the manufacturer advises against it, and never work on a live low-voltage circuit merely because its voltage is lower than household mains.

• Match straight G4 pins and nominal 4 mm spacing.
• Match voltage and the AC/DC requirement.
• Confirm transformer and dimmer compatibility.
• Check body width, overall length, and cover clearance.
• Choose lumens and color, then verify enclosure and location ratings.