Triumph Bonneville T100 Tail Light Bulb Size

The Triumph Bonneville T100 tail light uses a P21/5W bulb size for this motorcycle model. The replacement chart covers vehicle production years spanning 2002 to 2015 with corresponding bulb specifications.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
2002-2015	P21/5W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Dual-filament signalling bulb with BAY15d bayonet base. 12V, 21W/5W.	P21/5W

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Additional Notes

The Triumph Bonneville T100 uses the P21/5W bulb for the tail light position across the 2002-2015 model years.
The P21/5W is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope. The higher 21W filament serves the brake light function, while the lower 5W filament handles the running light function.
The P21/5W operates on a 12V electrical system, which is standard for motorcycle applications in this category.
The bulb uses a BAY15d bayonet base. The BAY15d designation indicates an offset pin configuration, where the two retaining pins are positioned at different heights on the base. This offset is intentional, as it prevents the bulb from being inserted in the wrong orientation, which would reverse the brake and running light filaments.
The P21/5W conforms to ECE (Economic Commission for Europe) standards, a regulatory framework recognized across Europe and numerous other markets worldwide.
Because the BAY15d base uses asymmetric pin placement, a standard single-filament BAY15s bulb is not a compatible substitute, even though the physical base size is similar.