An R5W bulb is a 5-watt single-filament bulb on a large single-contact BA15s bayonet base, used for sidelights, tail lights, number plate lamps, and interior lamps. The R5W carries the UK trade code 207, so a box marked 207 holds the same bulb as a box marked R5W. The bulb twists into a standard 15-millimetre bayonet holder on older cars, tractors, trailers, and commercial cabs where the marker lamp needs a low, steady output. Knowing the R5W by its 5-watt rating and its large single-contact base answers the question behind most R5W searches.
This guide explains what an R5W bulb is, how its BA15s base fits, why R5W and 207 name the same bulb, what the bulb is used for, how it differs from the 10-watt R10W and the twin-filament 380, and how to replace one with halogen or LED.
What an R5W Bulb Is
An R5W bulb is a low-output tungsten bulb with one filament on a single-contact bayonet base. The letter R marks the round glass, the number 5 gives the 5-watt rating, the W marks the wedge-free bayonet fitting, and the single filament gives one output level, so the R5W suits marker and tail duties rather than main lights. The trade number 207 is the figure most British factors print on the 12-volt box for this bulb.
The R5W produces a low, steady glow from a standard bayonet bulb. The 12V R5W runs 5 watts, the large bayonet base twists into a matching holder, and the single filament switches full on or off, so the R5W lights a sidelight, a tail lamp, or a number plate lamp that needs a small, reliable output. The lamp lens, not the bulb, spreads the final light on an R5W.
The BA15s Large Bayonet Base
The R5W uses a BA15s base, a large single-contact bayonet base 15 millimetres across. BA marks the bayonet family, the number 15 gives the base diameter in millimetres, the letter s marks a single contact for one filament, and the two bayonet pins sit level so the bulb drops in either way round. The large bayonet twists a quarter turn to lock, which holds the bulb against vibration on a working vehicle.
The single-contact base is the detail that sets the R5W apart. An R5W (BA15s) carries one central contact for its single filament and two level side pins, so it locks into a standard bayonet holder without a keyed orientation. The BA15s base is larger than the BA9s base used by the 4-watt T4W, so an R5W will not fit a small panel lamp holder. The BA15s base and its twin-contact sibling are set out in BA15s vs BA15d bulb bases. The compact 4-watt bayonet bulb sits in T4W (233) bulbs explained.
R5W and 207: One Bulb, Two Names
The R5W name and the 207 trade code describe the same 12-volt bulb. R5W is the descriptive category, a 5-watt round bulb on a BA15s bayonet base, and 207 is the UK trade number that British factors print on the box, so a buyer asks for either name and receives the same bulb. The code and the description point to one item.
- R5W is the descriptive category, a 5-watt bulb on a large single-contact bayonet base.
- 207 is the UK trade number for the 12-volt R5W, used on cars, tractors, and trailers.
- R207 is a common retailer variant of the same number, printed by brands such as Ring.
The name and the number match one bulb. A factor stocking 207 stocks the R5W, and a farmer asking for an R5W receives a 207, so the two labels never disagree. The R5W sits alongside the other single-filament marker bulbs set out in vehicle bulb types explained.
What an R5W Bulb Is Used For
An R5W bulb fills a low-output marker, tail, or plate lamp. The 5-watt filament suits five common roles, listed below, which is why the R5W appears across older and commercial vehicles.
- Front sidelights and rear tail lights on older cars, tractors, and trailers.
- Number plate lamps with a standard bayonet holder.
- Interior and cab lamps that use the large bayonet base.
- Marker and parking lamps on commercial vehicles and trailers.
- Panel and tell-tale lamps that need a 5-watt output.
The sidelight and tail role defines the bulb. An older tractor or a trailer runs an R5W in each sidelight, tail lamp, and number plate lamp, so a working older machine carries R5W bulbs where the standard bayonet holders sit. The sidelight role across bulb types is covered in position lamps explained, and the reasons a sidelight fails in sidelights not working.
R5W vs R10W: 207 vs 245
The R5W and the R10W share the same BA15s base but carry different wattages. The R5W runs 5 watts as the 207 and the R10W runs 10 watts as the 245, so the two bulbs fit the same holder but throw different amounts of light. The table below sets out the split.
| Bulb | Code | Base | Wattage | Typical role |
|---|---|---|---|---|
| R5W | 207 | BA15s single-contact | 5W | Sidelight, tail, number plate |
| R10W | 245 | BA15s single-contact | 10W | Brighter marker, interior, load lamp |
The wattage is the difference. An R5W gives a dimmer, cooler marker light suited to sidelights and plates, while an R10W gives a brighter output suited to interior and load lamps, so a buyer matches the wattage to the lamp’s job. Fitting a 10-watt R10W where a 5-watt R5W belongs draws more current and can overheat a small lens, so the original code is the safe choice. The 12V and 24V split across all bulb types is explained in 12V vs 24V bulbs.
R5W vs 380: Single Filament vs Twin Filament
The R5W and the 380 look similar but differ in filaments and base. The R5W carries one filament on a BA15s single-contact base, while the 380 (P21/5W) carries two filaments on a BAY15d offset-pin twin-contact base, so the two are not interchangeable even though both are round bayonet bulbs. The R5W lights one function and the 380 lights two.
An R5W handles a single lamp function, such as a sidelight on its own circuit. A 380 handles a combined tail and brake lamp, running a dim 5-watt filament for the tail and a bright 21-watt filament for the brake, so a lamp with one wire takes an R5W and a lamp with a stepped output takes a 380. The offset pins on the 380 keep the two filaments the right way round, a feature the level-pin R5W does not need. The twin-filament bulb is covered in 380 (P21/5W) bulbs explained.
LED R5W Conversions
An LED R5W replaces a halogen R5W in the same BA15s holder with no rewiring, and the low-output role makes it a straightforward swap. The LED bulb keeps the large bayonet base, twists into the existing holder, draws a fraction of the 5-watt current, and gives a brighter, whiter sidelight or tail light, so it appeals as an upgrade on an older cab. The convenience is real, and the marker role carries less legal risk than a headlamp.
An LED R5W in a sidelight, tail, or plate lamp is a common upgrade because the marker role does not need an approved beam pattern. The sidelight marks the vehicle’s presence rather than lighting the road, so an LED R5W that keeps the correct colour and a steady output suits the job, though a road vehicle uses an E-marked LED to stay clear of an MOT query. A classic tractor or a trailer marker lamp takes an LED R5W freely for a brighter, cooler-running light. The wider retrofit rules sit in LED headlamp conversions.
How to Replace an R5W Bulb
Replace an R5W bulb by reaching the lamp, pressing and twisting the bayonet bulb out, and fitting the new one. Five steps cover the job on most lamps, so work through them in order.
- Reach the sidelight, tail, plate, or interior lamp and remove the lens or holder if it clips off.
- Pull the bayonet holder from the lamp, or reach the bulb directly in a fixed lamp.
- Press the old R5W in, twist it a quarter turn anticlockwise, and lift it out of the bayonet holder.
- Choose a replacement of the correct code (207 for the 5-watt R5W), in halogen or LED, and confirm it is not a 10-watt R10W (245).
- Line the bayonet pins up with the slots, press and twist the bulb clockwise to lock, refit the holder, and test the lamp.
Handle the new bulb by its base to keep the glass clean. An R5W runs cool at 5 watts, so fingerprints matter less than on a headlamp bulb, but a clean glass still gives the clearest light. For R5W bulbs in halogen and LED, see the bulb range. The 10-watt R10W, the twin-filament 380, and the compact T4W sit close by, covered in 380 (P21/5W) bulbs explained and T4W (233) bulbs explained.
Summary
An R5W bulb is a 5-watt single-filament bulb on a large single-contact BA15s bayonet base. The R5W runs as the 207 in 12 volts, used for sidelights, tail lights, number plate lamps, and interior lamps on older and commercial vehicles. It differs from the 10-watt R10W (245) in wattage, from the twin-filament 380 in filaments and base, and from the compact T4W in base size. Match the code to the lamp’s job, fit an E-marked LED for road markers, and twist the bayonet base fully home.