GU10 vs MR16 (GU5.3) spotlight bulbs
Two spotlights the same size, one on mains power and one on 12 V
GU10 is a twist-and-lock lamp base with two thick pins 10 mm apart, used for mains-voltage spotlights. The similar-looking MR16 lamp normally has a GU5.3 base, two thin straight pins 5.33 mm apart, and runs on 12 V from a transformer or LED driver.
Both lamps are about 50 mm across, both throw a spotlight beam and both sit in the same kinds of recessed downlights and track heads. The base decides everything else: the voltage the fixture supplies, whether the lamp twists or pushes in, and why an LED replacement might flicker. A ten-second look at the pins tells you which one you have.
How to tell GU10 and MR16 apart
Look at the pins. A GU10 has two thick pins with flared ends, 10 mm apart: push it in and twist to lock. An MR16 has two thin straight pins 5.33 mm apart, its GU5.3 base, and simply pushes in. GU10 lamps run on mains voltage; MR16 lamps run on 12 V.
Common lamp bases, drawn to the same scale
10 mm
Other clues help while the lamp is still in the ceiling. An MR16 fixture has a transformer or driver somewhere behind it, often a small box above the ceiling, while a GU10 fixture connects straight to the mains. Halogen MR16 lamps have a faceted, often open-backed glass reflector, which is where the name comes from: multi-faceted reflector, 16 eighths of an inch across. GU10 lamps usually have a solid body and a thicker neck.
Replacing halogen MR16 lamps with LED
An LED MR16 draws about 5 W instead of 35–50 W, which can confuse the transformer behind it. Electronic halogen transformers often need a minimum load of 20 W or more, so LED lamps flicker, buzz or stay dark. Choose lamps listed as compatible with your transformer, or replace it with a constant-voltage LED driver.
- Magnetic transformers are heavy, with an iron core. Many LED MR16 lamps work on them, but check the lamp maker's list.
- Electronic transformers are light and compact, with a load range printed on the label, such as 20–60 W. Several LED lamps on one transformer may reach the minimum; a single lamp rarely does.
- LED drivers: a 12 V constant-voltage LED driver has no minimum load and gives the steadiest light. Dimmable versions need a matching dimmer.
- Dimming: the dimmer, the transformer and the lamps must all be compatible. If one is not, expect flicker at low levels (see dimming controls).
Converting MR16 fittings to GU10
Many people convert 12 V MR16 downlights to GU10 to drop the transformers. It means removing each transformer and fitting a mains GU10 lamp holder, and often a new fire-rated or insulation-contact downlight. It is fixed-wiring work: have an electrician do it where rules require, and never wire a 12 V MR16 holder straight to the mains.
The change pays off when the transformers are failing or will not work with LED lamps: GU10 LED lamps are inexpensive, need no separate driver and dim on LED-compatible mains dimmers. Keep MR16 where low voltage is the point, such as landscape lighting, display cases and places where a 12 V supply is safer.
Beam angles, brightness and MR11
Both lamps come in beam angles from about 10° to 60°. Use 36–40° for general downlighting and 15–25° to pick out pictures or objects, and compare candelas rather than watts for spot brightness. The smaller MR11, 35 mm across on a GU4 base, is the same idea for compact fittings.
| Code | Halogen lamp | Beam | Typical LED replacement |
|---|---|---|---|
| BAB | 20 W MR16 | Flood, about 38° | 3–4 W |
| FMW | 35 W MR16 | Flood, about 38° | 4–6 W |
| EXN | 50 W MR16 | Flood, about 38° | 6–8 W |
| EXT | 50 W MR16 | Narrow spot, about 12° | 6–8 W, narrow beam |
The spread and brightness of a beam are explained in beam angles and spacing, and the reflector shapes in the bulb shapes guide.
Frequently asked questions
Can I put a GU10 bulb in an MR16 socket?
No. The pins and the voltage differ: a GU10 will not fit a GU5.3 socket, and it is built for mains voltage, not 12 V.
Do LED MR16 bulbs need a transformer?
Yes. MR16 lamps run on 12 V, so the fixture needs a transformer or LED driver. Some fixtures have one built in.
Why do my LED MR16 bulbs flicker?
Usually because the electronic transformer's minimum load is higher than the LED lamps draw, or the transformer and dimmer are not LED-compatible. Use compatible lamps or a 12 V LED driver.
Is GU10 the same as PAR16?
Often, but not always. Many GU10 lamps have a PAR16 shape, 2 inches across, but PAR16 lamps also come with E26 and E27 screw bases.
What is the difference between MR16 and MR11?
Size and base: an MR16 is about 50 mm across on GU5.3 pins, an MR11 about 35 mm across on GU4 pins. Both normally run on 12 V.
Sources and further reading
- IEC 60061-1, Lamp caps and holders together with gauges for the control of interchangeability and safety: Lamp caps (GU10, GU5.3 and GU4 caps).
- IEC 61347-2-2, Lamp controlgear: Particular requirements for d.c. or a.c. supplied electronic step-down convertors for filament lamps.
- ANSI C78.379, Electric Lamps: Reflector Lamps, Classification of Beam Patterns.
- NEMA SSL 7A-2015, Phase-Cut Dimming for Solid State Lighting: Basic Compatibility.
Image credits
- GU10 spotlight: Matt Kieffer, CC BY-SA 2.0. MR16 lamp: Dmitry G, CC BY-SA 3.0. Both via Wikimedia Commons. Base chart: LEDlights.org.