P21/5W Bulbs Explained: The 380 BAY15d Stop and Tail Bulb, Its Offset Pins, and How to Replace It
A P21/5W bulb is the 380: a 12V 21/5W twin-filament BAY15d stop and tail bulb with offset pins. 24V 294, fitting mistakes, LED options and how to fit.
In this guide
A P21/5W bulb is a 12-volt twin-filament bulb with a 21-watt filament for the brake light and a 5-watt filament for the tail light, on a BAY15d bayonet base with offset pins. In the UK it is sold as the 380. The offset pins make it fit only one way round, so each filament lines up with the right circuit. The 24-volt version is the 294.
This guide explains what the P21/5W code means, how the BAY15d base and its offset pins work, the UK numbers for 12V and 24V, where the bulb is used, the fitting mistakes that cause dim brake lights and MOT failures, the LED options, and how to replace a 380 on a car, van, tractor or trailer.
What Is a P21/5W Bulb?
A P21/5W bulb is a twin-filament lamp category defined in UNECE Regulation 37. The figures either side of the slash give the wattage of each filament: 21 watts for the bright filament and 5 watts for the dim one. The W stands for watts. Both filaments sit inside one round clear glass envelope on a 15 mm metal bayonet cap.
Manufacturer data for a standard 12V P21/5W gives about 440 lumens from the 21-watt filament and about 35 lumens from the 5-watt filament at the 13.5V test voltage. That big step in output is the point of the bulb: one lamp shows a steady low red light as a tail lamp, then a much brighter red light when the brakes go on.
| Specification | P21/5W value |
|---|---|
| ECE category | P21/5W (UNECE R37) |
| UK number, 12V | 380 |
| UK number, 24V | 294 |
| Wattage | 21W main filament, 5W secondary filament |
| Base (cap) | BAY15d, 2 contacts, staggered pins |
| Filaments | 2 |
| Light output, 12V | About 440 lumens (21W) and 35 lumens (5W) |
| Common functions | Stop and tail, also indicator and sidelight on some front lamps |
The BAY15d Base and Why the Pins Are Offset
The BAY15d is a 15 mm bayonet cap with 2 contacts on the bottom and 2 locating pins set at different heights. BA marks a bayonet cap, Y marks the staggered pins, 15 is the diameter in millimetres and d means double contact. The metal shell is the shared earth, and each bottom contact feeds one filament.
The offset pins are there to stop the bulb going in the wrong way round. If a twin-filament bulb could turn 180 degrees in its holder, the 21-watt filament would sit on the tail circuit and the 5-watt filament on the brake circuit. The lamp would glow brightly all the time and barely change when the brakes went on. With one pin higher than the other, the bulb only locks in one position, so the brake feed always reaches the bright filament.
There is a rarer twin-contact base with level pins, the BA15d, and a P21/4W bulb on a BAZ15d base (566). Neither is a 380. If the pins on your old bulb sit level, or the box says 21/4W, match that bulb rather than fitting a 380.
380 and 294: The UK Numbers
The 380 is the UK trade number for the 12V P21/5W, used by most UK bulb brands and factors, sometimes with a brand prefix such as R380. The 24V version is the 294. The two look the same and fit the same holder, so check the voltage printed on the cap.
- 380 is the 12V 21/5W P21/5W on a BAY15d cap.
- 294 is the 24V 21/5W P21/5W on the same BAY15d cap, used on lorries, larger tractors and 24V trailers.
- 334 is a 24V 24/6W bulb on a BAY15d cap. It fits the same holder but is not a P21/5W, so only use it where the lamp maker lists it.
- 382 is the 12V single-filament P21W on a BA15s cap. It does not replace a 380.
A 12V 380 fitted to a 24V tractor or trailer blows on first use. A 24V 294 fitted to a 12V system gives a weak, orange light. If you run a 12V tractor with a 24V lorry or the other way round, keep the spares separate. The differences are explained in 12V vs 24V lighting systems.
What a P21/5W Bulb Is Used For
A P21/5W is used where one lamp compartment does two jobs at two brightness levels.
- Combined stop and tail lamps. The most common use. The 5-watt filament runs as the tail lamp with the sidelights, and the 21-watt filament lights with the brake switch.
- Tractor and trailer rear combination lamps. Many older and current agricultural rear lamps use a 380 for stop and tail, with a 382 or 581 in the indicator section.
- Front indicator and sidelight on some vehicles. Some front lamps use the 5-watt filament as a sidelight or daytime light and the 21-watt filament as the indicator.
- Rear fog and tail on some lamps. A few lamp designs use one filament for the tail and the other for the rear fog.
The lens gives the colour, so a 380 behind a red lens is a stop and tail lamp. For the wider picture of which bulb sits where in a rear cluster, see rear brake light bulbs and tractor brake lights.
Common Swapping Mistakes
Most 380 faults at the parts counter come from the wrong bulb or a bulb forced into place. These are the ones to avoid.
Forcing the bulb in the wrong way round
A worn holder or a bit of force can let the bulb lock the wrong way round. The tail lamp then glows at brake brightness and the brake light hardly shows. The MOT fails a stop lamp that is not a steady red or has significantly reduced intensity, and a stop lamp affected by the operation of another lamp. If the tail looks too bright, take the bulb out and refit it with the high pin in the high slot.
Fitting a 382 in a 380 holder
A single-contact 382 only touches one of the two contacts in a 380 holder. You get either a tail lamp with no brake, or a brake with no tail. It can look fine on a quick check, so test both functions.
Ignoring a bad earth
A twin-filament bulb shares one earth for both filaments. If the earth is poor, current finds another way back through the other filament or a neighbouring bulb. The signs are a tail lamp that dims when the brakes go on, or an indicator that makes the tail lamp flicker. A new bulb will not cure it. Clean the holder and check the earth wire to the chassis or trailer plug. Trailer earth faults are covered in trailer lights not working.
Mixing 12V and 24V spares
The 380 and 294 look identical. Read the cap before fitting, particularly on a farm with both 12V tractors and 24V lorries.
LED P21/5W Options
LED bulbs with a BAY15d cap are sold as P21/5W or 380 replacements. They draw less current, start instantly and cope with vibration better than a filament. Before you fit one in a road lamp, check these points.
- Two clear brightness levels. The tail and brake must look clearly different. Some cheap LED 380s give a weak step between the two, which defeats the point of the brake light.
- Light through the lens. A lamp built for a filament spreads light from a point. An LED bulb with poor side output can look dim or patchy from an angle.
- Bulb monitoring. Vehicles that check bulb current may show a warning. A CANbus-compatible LED adds a load to avoid it.
- Polarity. A filament does not care which way current flows, but an LED does. Most LED bulbs expect a negative earth on the metal cap, so an old positive-earth classic tractor needs a bulb made for it.
The lamp holds its approval with the bulb it was designed for. If you want LED stop and tail lights on a tractor or trailer, the most dependable road option is a complete E-marked LED rear lamp. Browse our LED rear lamps.
How to Replace a P21/5W (380) Bulb
- Switch off the lights and ignition, or turn off the battery isolator on a tractor.
- Remove the lens screws or unclip the bulb holder from the back of the lamp.
- Push the bulb in, turn it anticlockwise about a quarter turn and pull it out.
- Check the old bulb: 2 contacts on the bottom and staggered pins means a 380 (12V) or 294 (24V). Read the voltage on the cap.
- Clean any corrosion from the holder contacts and spring.
- Line up the higher pin with the higher slot, push in and turn clockwise until it locks.
- Test with the sidelights on, then with someone pressing the brake. The lamp should go from a low glow to a clearly brighter red.
- Refit the lens and check the seal is not split.
Shop standard miniature bulbs for 380 and 294 replacements. The single-filament bulb used in many of the same clusters is covered in P21W (382) bulbs explained.
Frequently asked questions
Is a 380 bulb the same as a P21/5W?
Yes. P21/5W is the ECE category and 380 is the UK trade number for the 12V version. Both describe a twin-filament 21/5-watt bulb on a BAY15d cap. The 24V version is the 294.
What is a BAY15d bulb?
BAY15d describes the base: a 15 mm bayonet cap with 2 bottom contacts and 2 pins at different heights. The most common BAY15d vehicle bulb is the P21/5W (380), but other bulbs share the base, so check the wattage too.
Why is my brake light dim but the tail light bright?
The bulb is most likely in the wrong way round, so the bright filament is on the tail circuit. Refit it with the high pin in the high slot. If the holder is worn and lets it turn, replace the holder.
Can I fit a 382 instead of a 380?
No. A 382 has one filament and one contact, so in a 380 holder you lose either the tail or the brake light. Always replace a twin-filament bulb with a twin-filament bulb.
Are LED 380 bulbs legal?
LED 380 bulbs are widely sold, but a road lamp is approved with the bulb it was designed for, and the light must stay red, steady and clearly brighter on the brake. For a guaranteed road-legal LED result, fit a complete E-marked LED rear lamp.