B22 bayonet vs E27 screw bases
Push and twist, or screw in: the two household bases of the UK, Ireland, Australia and much of the world
A bayonet cap is a lamp base that pushes into its holder and locks with a short twist, using two pins on the side of the cap. The household size, B22d, is 22 mm across with two contacts on its end; it is standard in the UK, Ireland, Australia, New Zealand and India, where it is called BC.
Most of the world screws its light bulbs in. The UK and many countries that followed British practice push and twist them instead. Both designs work well and both carry the same LED bulbs, so the choice is usually made for you by the fittings in the house. This page explains the differences and how to cope with a mix of both.
B22 vs E27: what is the difference?
A B22 bayonet cap pushes in and locks with a twist on two side pins, and its two end contacts press against spring-loaded plungers in the holder. An E27 screws in, with the thread as one contact and the center button as the other. B22 is standard in the UK, Ireland, Australia and India; E27 in most other countries.
Common lamp bases, drawn to the same scale
10 mm
The plungers keep steady pressure on the contacts, and the pins stop the bulb from working loose with vibration, which is why bayonets also suit vehicles, boats and trains. The metal skirt of a B22d cap is not a contact, so nothing live is exposed while the lamp is fitted. On an E27 the thread itself is a contact, and in countries where plugs fit either way round it can be the live one.
Can you use an E27 bulb in a B22 fitting?
Yes, with a B22-to-E27 adapter, which converts a bayonet holder to a screw socket. It works because both run on the same 220–240 V mains. Adapters add height and a second joint, so check that the bulb still fits inside the shade and that the adapter is rated for the bulb's wattage.
With LED bulbs the wattage is rarely a problem, but a heavy bulb on an adapter in a pendant can work loose. When you are replacing many bulbs, buying B22 LED bulbs for B22 fittings is simpler: they are sold everywhere B22 is standard.
BC, ES, SBC and SES: the British names
UK packaging uses letters rather than IEC codes. BC, bayonet cap, means B22d and SBC, small bayonet cap, means B15d. ES, Edison screw, means E27, SES, small Edison screw, means E14, and MES means the tiny E10. GES, giant Edison screw, is the E40 used for large commercial lamps.
| UK name | Meaning | IEC code | Diameter | Typical lamps |
|---|---|---|---|---|
| BC | Bayonet cap | B22d | 22 mm | Household bulbs |
| SBC | Small bayonet cap | B15d | 15 mm | Candle, golf-ball and appliance lamps |
| ES | Edison screw | E27 | 27 mm | Household bulbs |
| SES | Small Edison screw | E14 | 14 mm | Candle, golf-ball and appliance lamps |
| MES | Miniature Edison screw | E10 | 10 mm | Torches, fairy lights |
| GES | Giant Edison screw | E40 | 40 mm | Streetlights, high-bays |
The screw sizes are compared in E26 vs E27 and E12 vs E14.
Bayonet bulbs in cars and boats
Vehicles use small bayonet bulbs because they resist vibration. BA15s has one end contact and level pins, as in a single-filament reverse or indicator lamp. BAY15d has two contacts and offset pins, so a two-filament stop and tail lamp can only fit one way round. They run on 12 or 24 V, never mains.
LED replacements for vehicle bulbs must suit the vehicle: brake and indicator circuits may monitor current, so a low-power LED can trigger bulb-failure warnings or fast flashing, and in many countries only approved lamps are legal on the road. See LED replacement bulbs for the rules.
Frequently asked questions
Is B22 the same as BC?
Yes. BC, bayonet cap, is the UK name for the B22d base. The d means it has two contacts.
Can I use a B22 bulb in an E27 socket?
Not directly. An E27-to-B22 adapter makes it possible, but buying an E27 bulb is simpler and avoids the extra height.
Why does the UK use bayonet bulbs?
Mainly history: the bayonet cap became Britain's standard early in the age of electric light, and the country never switched. It also holds firm against vibration and keeps its contacts out of reach.
What is the difference between B22 and B15?
Size: B22d is 22 mm across, for household bulbs, and B15d (SBC) is 15 mm across, for candle, golf-ball and appliance lamps.
Are BA15s and B15d the same?
No. They share the 15 mm size but not the contacts: BA15s, used in vehicles, has a single end contact, while B15d has two. Check the full code before buying.
Sources and further reading
- IEC 60061-1, Lamp caps and holders together with gauges for the control of interchangeability and safety: Lamp caps (B22d, B15d, BA15s, BAY15d and E27 caps).
- IEC 61184, Bayonet lampholders.
- IEC 60238, Edison screw lampholders.
- UN ECE Regulation No. 37, filament light sources for motor vehicles (P21W, P21/5W).
Image credits
- Bayonet cap and holder: Jnestorius, CC BY-SA 3.0, via Wikimedia Commons. Base chart: LEDlights.org.