W5W Bulbs Explained: The 501 Capless T10 Sidelight Bulb, Its Base, and How to Replace It
A W5W bulb is the 501: a 12V 5W capless T10 wedge bulb on a W2.1x9.5d base for sidelights and number plates. 24V 507, LED T10, CANbus and MOT rules.
In this guide
A W5W bulb is a 12-volt, 5-watt capless wedge bulb with a W2.1×9.5d glass base, sold in the UK as the 501 and often called a T10. It has no metal cap: the glass wedge pushes straight into the holder. It is used in sidelights, number plate lamps, interior lamps and side repeaters. The 24-volt version is the 507.
This guide explains what the W5W code means, how the capless base works, the 501, 507 and T10 names, where the bulb is used, how to fit an LED T10 without warning lights or MOT trouble, and how to replace one.
What Is a W5W Bulb?
A W5W bulb is a filament lamp category defined in UNECE Regulation 37. The first W marks a glass wedge base, 5 is the rated wattage and the final W stands for watts. The bulb has one filament inside a small glass envelope about 10 mm across, and 2 fine wires that fold down the sides of the flat glass base to form the contacts.
A standard 12V W5W gives about 50 lumens at its 13.5V test voltage, according to manufacturer data. That is a gentle marker light, about a ninth of a 21-watt indicator bulb, which is all a sidelight or number plate lamp needs.
| Specification | W5W value |
|---|---|
| ECE category | W5W (UNECE R37) |
| UK number, 12V | 501 |
| UK number, 24V | 507 |
| Common name | T10, capless, wedge |
| Wattage | 5W (12V and 24V) |
| Base | W2.1×9.5d glass wedge |
| Filaments | 1 |
| Light output, 12V | About 50 lumens |
| Coloured versions | WY5W amber (501A), WR5W red |
The W2.1×9.5d Capless Wedge Base
The W2.1×9.5d code describes the base. W means a wedge, 2.1 is the thickness of the glass wedge in millimetres, 9.5 is its width in millimetres and d means 2 contacts. The 2 contact wires sit one on each face of the wedge, and the holder grips the glass by friction.
A capless bulb has no pins to line up, so it pushes straight in and pulls straight out. It can go in either way round, which does not matter for a filament but does matter for an LED (see below). The same base is used by the 3-watt W3W (504) and by the 16-watt W16W reversing bulb, which has a larger glass. The W16W is covered in W16W (921) bulbs explained.
501, 507 and T10: Three Names for the Same Family
- 501 is the UK trade number for the 12V 5W W5W. Most UK brands print it on the box, sometimes with a prefix such as R501.
- 507 is the 24V 5W W5W on the same base, used on lorries, larger tractors, plant and 24V trailers.
- T10 is an informal name that refers to the roughly 10 mm glass. It is used mostly for LED replacements. A box marked T10 is not an ECE category, so check it is the W2.1×9.5d base and the right voltage.
- 501A is the amber WY5W, used in some side repeaters with a clear lens.
The 501 and 507 look identical, so read the voltage on the glass. A 12V 501 in a 24V lamp blows straight away. A 24V 507 in a 12V lamp gives a weak glow that can fail an MOT for low intensity.
What a W5W Bulb Is Used For
- Front sidelights (front position lamps) on cars, vans and many tractors, often in the same housing as the headlamp.
- Number plate lamps on cars, vans, tractors and trailers.
- Rear position lamps on some vehicles, behind a red lens.
- Side repeaters and side marker lamps, using an amber WY5W or a clear W5W behind an amber lens.
- Interior, cab, boot and glovebox lamps, and some dashboard and switch lighting.
Because the bulb is cheap and the holder is simple, the W5W is one of the most common bulbs on any vehicle. What each of these lamps must do on the road is set out in position lamps explained.
W5W and the MOT
A 501 failure is one of the easiest MOT faults to avoid, and the test is strict about colour. The MOT inspection manual for cars and light vans says front position lamps should show white light and rear position lamps a steady red. A lamp showing red to the front or white to the rear is a major defect, and so is a missing or inoperative position lamp. A colour, position or intensity that is not as required, or a product on the lens or bulb that changes the colour, is a minor defect.
The number plate lamp has its own rules. It must light the rear registration plate and come on with the position lamps, or the vehicle fails. A number plate lamp that throws direct white light to the rear is a minor defect, so a bright LED that shines out past the plate rather than onto it can cause an advisory or fail.
Most tractors do not take an MOT, but the lighting regulations still apply on the road, and a dead sidelight or number plate lamp is an easy roadside stop. If a sidelight has gone and a new bulb does not cure it, work through position lamp not working.
LED T10 Retrofits and CANbus
LED T10 bulbs with a W2.1×9.5d base are the most popular LED swap on any vehicle. They draw well under a watt, give a whiter light and last much longer than a filament. A few checks make the difference between a neat job and a dashboard warning or an MOT fail.
Polarity
An LED only works with current flowing one way. If a new LED T10 stays dark, pull it out, turn it round 180 degrees and push it back in. The capless base allows this.
CANbus and bulb warnings
Many cars and vans, and some newer tractors, check bulb circuits by measuring current or sending short test pulses. A low-draw LED can trigger a bulb failure warning, flicker or glow faintly when switched off. A CANbus-compatible T10 includes a load or driver to stop this. On a simple tractor or trailer circuit a standard LED T10 normally works without one.
Colour and size
Choose a white LED for sidelights and number plate lamps. A strongly blue LED can be failed for colour, and any LED that changes the colour the lamp is meant to show is a defect. Check the length too: some LED T10s are longer or wider than a 501 and will not fit a tight sidelight holder or will sit against the lens.
Light pattern
A filament gives light in all directions. An LED with chips only on the tip can leave a sidelight or number plate looking patchy. A design with chips on the sides as well spreads light more like the original.
For a tidier result on a tractor or trailer plate, a complete LED lamp avoids the bulb question altogether. Browse our LED number plate lights.
How to Replace a W5W (501) Bulb
- Switch off the lights and ignition.
- Reach the holder. On a headlamp sidelight it usually twists out of the back of the headlamp. On a number plate lamp, remove the lens screws or unclip the lens.
- Pull the old bulb straight out of the holder. Do not twist it, or the glass can break.
- Check the glass for the code and voltage: 501 or W5W 12V, or 507 for 24V.
- Check the 2 contact strips in the holder are clean and springy. Bend them in slightly with a small screwdriver if the bulb feels loose.
- Push the new bulb straight in until the glass base is fully seated.
- Switch on the sidelights to test, and turn an LED round if it does not light.
- Refit the holder or lens and check the seal.
Shop standard miniature bulbs for 501 and 507 replacements, or keep a spare set in the cab or van with a bulb assortment box.
Frequently asked questions
Is a 501 bulb the same as a W5W?
Yes. W5W is the ECE category and 501 is the UK trade number for the 12V version. Both describe a 5-watt capless wedge bulb on a W2.1×9.5d base. The 24V version is the 507.
Is a T10 bulb the same as a W5W?
A T10 is the common name for a bulb with a roughly 10 mm glass on a W2.1×9.5d wedge base, which is the W5W size. Most T10s sold in the UK are LED replacements for the 501. Check the voltage and wattage before buying.
What does W2.1×9.5d mean?
It describes the capless base: a glass wedge 2.1 mm thick and 9.5 mm wide with 2 contacts. Several bulbs share it, including the W3W, W5W and W16W, so match the wattage as well as the base.
Will an LED 501 pass the MOT?
An LED 501 that lights properly, shows white at the front and on the number plate, and does not shine white light directly to the rear is unlikely to cause a problem. A blue or coloured LED, or one that is too dim, can be failed for colour or intensity.
Why does my LED sidelight not work?
Turn it round 180 degrees in the holder, because LEDs only work one way. If it still stays dark or a warning light comes on, the vehicle may need a CANbus-compatible LED.