P21W Bulbs Explained: The 382 BA15s Single-Filament Bulb, Its Uses, and How to Replace It
A P21W bulb is the 382: a 12V 21W single-filament BA15s bulb for indicators, reversing and fog lamps. 24V 241, amber PY21W 581, LED tips and fitting.
In this guide
A P21W bulb is a 12-volt, 21-watt single-filament bulb on a BA15s bayonet base, sold in the UK as the 382. It lights one function at one brightness, so it is used for indicators behind an amber lens, reversing lamps, rear fog lamps and some brake lamps. The 24-volt version is the 241, and the amber PY21W indicator bulb is the 581.
This guide explains what the P21W code means, how the BA15s base fits, the UK numbers for the 12V and 24V versions, where the bulb is used on cars, vans, tractors and trailers, how it differs from the twin-filament P21/5W (380) and the amber PY21W (581), what to know before fitting an LED version, and how to replace one.
What Is a P21W Bulb?
A P21W bulb is a filament lamp category defined in UNECE Regulation 37, the standard that sets the dimensions, wattage and light output of approved vehicle bulbs. The 21 is the rated wattage and the W stands for watts. The bulb has one filament, a round clear glass envelope, and a 15 mm metal bayonet cap with one contact on the bottom.
A standard 12V P21W is rated at 21 watts and gives about 460 lumens at its 13.5V test voltage, according to manufacturer data sheets. That is roughly 9 times the light of a 5-watt sidelight bulb, which is why the P21W sits in lamps that must be seen clearly in daylight, such as indicators and reversing lamps.
| Specification | P21W value |
|---|---|
| ECE category | P21W (UNECE R37) |
| UK number, 12V | 382 |
| UK number, 24V | 241 |
| Wattage | 21W (12V and 24V) |
| Base (cap) | BA15s, single contact, level pins |
| Filaments | 1 |
| Light output, 12V | About 460 lumens |
| Glass colour | Clear (amber version is PY21W, red version is PR21W) |
The BA15s Base: Why a 382 Fits the Way It Does
The BA15s base is a 15 mm bayonet cap with a single contact and 2 locating pins set level and directly opposite each other. BA marks a bayonet cap, 15 is the cap diameter in millimetres, and the s means single contact. The metal shell of the cap is the earth and the solder blob on the bottom feeds the filament.
Because the 2 pins sit at the same height and 180 degrees apart, a 382 drops into its holder either way round. Push it in against the spring and turn it about a quarter turn clockwise to lock it. The single contact is the quick check at the counter: one solder pad on the bottom means a single-filament bulb such as the 382, while 2 pads mean a twin-filament bulb such as the 380.
The same BA15s cap is used by the 5-watt R5W (207) and the 10-watt R10W (245). Those bulbs fit the same holder but give far less light, so a 382 holder must always get a 382. The R5W is covered in R5W (207) bulbs explained.
382, 241 and 581: The UK Numbers
The 382 is the UK trade number for the 12V P21W, printed on the box by most UK bulb brands and parts factors. A box marked P21W, 382 or a brand prefix such as R382 holds the same bulb. The trade number matters most when you buy for a 24V machine, because the 24V bulb has a different number.
- 382 is the 12V 21W P21W on a BA15s cap, the common clear indicator, reversing and fog bulb.
- 241 is the 24V 21W P21W on the same BA15s cap, used on lorries, larger tractors, plant and 24V trailers.
- 581 is the 12V 21W PY21W amber bulb on a BAU15s cap with offset pins.
- 380 is the 12V P21/5W twin-filament stop and tail bulb on a BAY15d cap. It is a different bulb.
A 241 and a 382 look identical and fit the same holder, so read the voltage printed on the cap before fitting. A 12V bulb on a 24V system blows almost at once, and a 24V bulb on a 12V system glows dim and orange. The reasons machines run 12V or 24V are set out in 12V vs 24V lighting systems.
What a P21W Bulb Is Used For
A P21W is used wherever a lamp needs one bright, single output. The filament is either fully on or off, so the bulb suits signal lamps that switch on a single circuit.
- Indicators. Older lamps with an amber lens use a clear 382 behind the lens. The lens supplies the colour.
- Reversing lamps. A clear 382 behind a clear lens gives the white light the law requires.
- Rear fog lamps. A clear 382 behind a red lens gives the bright red fog signal.
- Brake lamps on some vehicles. Where the stop lamp is a separate compartment from the tail lamp, a 382 can serve as the stop bulb behind a red lens.
- Tractor and trailer rear clusters. Many combination rear lamps use a 382 for the indicator section, alongside a 380 for stop and tail.
The lamp, not the bulb, decides the colour. A clear P21W behind an amber, red or clear lens does three different jobs, which is why the 382 is one of the most common bulbs on a farm vehicle. For the indicator side of the job, see tractor indicator lights.
P21W vs PY21W: Clear 382 vs Amber 581
The PY21W is the amber version of the P21W, sold as the 581. The amber colour is in the glass, so it is used in indicator lamps with a clear or smoked lens. Its light output is lower than the clear bulb at about 280 lumens, because the amber coating absorbs some light.
The PY21W sits on a BAU15s cap, where the 2 pins are about 150 degrees apart instead of 180 degrees. That offset stops an amber bulb going into a clear-bulb holder, and stops a clear bulb going into a holder designed for amber. If a lamp has a clear lens and an offset-pin holder, it needs a 581. Fitting a clear bulb by force would show white light from an indicator, which fails an MOT. The 24V PY21W is listed by Ring as the 588.
P21W vs P21/5W: Single Filament vs Twin Filament
The P21W (382) has one 21-watt filament on a BA15s single-contact cap. The P21/5W (380) has a 21-watt filament and a 5-watt filament on a BAY15d twin-contact cap with offset pins. The 380 lights a combined stop and tail lamp: 5 watts for the tail, 21 watts when the brakes are applied.
| Bulb | UK number (12V / 24V) | Base | Filaments | Typical use |
|---|---|---|---|---|
| P21W | 382 / 241 | BA15s, level pins, 1 contact | 1 (21W) | Indicator, reversing, rear fog, separate brake |
| PY21W | 581 / 588 | BAU15s, offset pins, 1 contact | 1 (21W amber) | Indicator with clear lens |
| P21/5W | 380 / 294 | BAY15d, staggered pins, 2 contacts | 2 (21W and 5W) | Combined stop and tail |
The two are not interchangeable. A 382 forced into a stop and tail holder will only touch one of the 2 contacts, so you lose either the tail lamp or the brake lamp. Full details of the twin-filament bulb are in P21/5W (380) bulbs explained.
LED P21W Bulbs: What to Check
LED bulbs with a BA15s cap are sold as P21W or 382 replacements. They draw much less current and run cooler, and they suit work and yard lamps well. Before fitting one in a road lamp, check 3 things.
Indicator flash rate
A traditional thermal flasher unit relies on the current of a filament bulb, so a low-draw LED can make it flash slowly or not at all. An electronic flasher or a bulb monitoring system reads the low current as a blown bulb and flashes fast (hyperflash). The MOT requires indicators to flash between 60 and 120 times a minute. The fix is an LED-compatible flasher relay or a load resistor across the circuit. Browse our flasher units if you are converting indicators.
Bulb failure warnings
Vehicles that monitor bulb current can show a bulb warning with an LED fitted. A CANbus-compatible LED includes a load to stop the warning.
Colour and output
The MOT checks that reversing lamps show white, rear fog and stop lamps show steady red and indicators show amber. An LED must keep the right colour and give enough light through the lens. A single-filament bulb rarely has a light pattern problem in a signal lamp, but a cheap LED with poor side output can look dim through a lens built for a filament. A road lamp keeps its approval with the bulb it was designed for, so the safest road option is a halogen 382 or a complete E-marked LED lamp.
How to Replace a P21W (382) Bulb
- Switch off the lights and ignition. On a tractor, turn the battery isolator off if the lamp is close to other wiring.
- Remove the lens screws or unclip the bulb holder from the back of the lamp.
- Push the old bulb in gently, turn it a quarter turn anticlockwise and pull it out.
- Check the cap: one bottom contact and level pins means a 382 (12V) or 241 (24V). Check the voltage printed on the cap.
- Look at the holder. Green or white corrosion on the contact or the spring stops a new bulb working, so clean it with a small wire brush or contact cleaner.
- Line up the pins with the slots, push in and turn clockwise until it locks.
- Test the lamp before refitting the lens, then check the lens seal is intact so water does not get in.
If a new 382 blows quickly, look for a charging fault or a water-damaged holder rather than fitting another bulb. The common causes are covered in why tractor bulbs keep blowing. Shop standard miniature bulbs for 382, 241 and 581 replacements, or keep a spare set in the cab with a bulb assortment box.
Frequently asked questions
Is a 382 bulb the same as a P21W?
Yes. P21W is the ECE category and 382 is the UK trade number for the 12V version. Both describe a 21-watt single-filament bulb on a BA15s bayonet cap. The 24V version is the 241.
Can I use a 382 instead of a 380?
No. The 380 (P21/5W) has 2 filaments, 2 bottom contacts and offset pins for a combined stop and tail lamp. A 382 has one filament and one contact, so in a 380 holder it would only light one of the two functions.
What is the difference between P21W and PY21W?
The PY21W (581) has amber glass and a BAU15s cap with pins about 150 degrees apart. It goes in indicators with a clear lens. The P21W (382) has clear glass and level pins, and goes behind an amber, red or clear lens.
Is a BA15s 21W bulb the same as a P21W?
Usually, yes, if it is a vehicle bulb marked P21W with an E mark. Some BA15s 21W bulbs are sold for marine or general use without ECE approval, so for a road lamp choose one marked P21W.
Why does my indicator flash fast with an LED 382?
The LED draws too little current, so a load-sensing flasher unit or the vehicle’s bulb monitoring reads it as a blown bulb and flashes fast. Fit an LED-compatible flasher relay or a load resistor to bring the rate back to between 60 and 120 flashes a minute.