E26 vs E27 bulb bases
The two Edison screws that light most of the world's homes, and when the 1 mm difference matters
An Edison screw is a threaded lamp base: the bulb screws into the socket, the metal thread carries the neutral and the button at the bottom carries the live supply. Its code is E plus the thread diameter in millimeters, so E26 is the North American household base and E27 the standard in Europe and most other 230 V countries.
Look at the base of almost any household bulb outside the UK, Ireland and Australia and you will find an Edison screw, a design that goes back to Thomas Edison's company in the early 1880s. North America settled on a 26 mm version for its 120 V grid, Europe on 27 mm for 230 V. The two are close enough to fit each other's sockets, which is convenient and occasionally dangerous.
What is the difference between E26 and E27?
The thread diameter: an E26 base measures 26 mm across and an E27 base 27 mm. E26 is the standard base in North America, where homes run on 120 V, and in 100 V Japan. E27 is standard in Europe and most 220–240 V countries. Threads are otherwise so alike that each bulb fits the other's socket.
Common lamp bases, drawn to the same scale
10 mm
In North America the E26 is called the medium or standard base, and in the UK the E27 is called ES, for Edison screw. Both carry the same bulb shapes: A19 or A60 household bulbs, globes, filament bulbs and BR and PAR reflectors. The base code says nothing about the shape or the brightness, which have codes of their own, explained in the bulb shapes guide.
Can you use an E27 bulb in an E26 socket?
Physically, yes: an E27 bulb screws into an E26 socket and the reverse also works. What matters is voltage. A bulb made only for 230 V may stay dark or flicker on 120 V, and a 120 V bulb on 230 V can fail at once. Some LED bulbs accept 100–240 V; check the label.
- Voltage: the label states it, for example "120V", "220–240V" or "AC 100–240V". Use only a bulb rated for your mains voltage.
- Safety marks: in North America look for a UL or ETL listing; in Europe the CE mark, and in the UK the UKCA or CE mark. Bulbs bought abroad may lack the mark your fixtures expect.
- Fit: an E27 bulb sits a little tighter in an E26 socket and an E26 bulb a little looser in an E27 one. Screw it in until it is snug, without forcing it.
- Dimmers: the base says nothing about dimming. Use bulbs marked dimmable, ideally from the dimmer maker's compatibility list.
Edison screw sizes from E10 to E40
Edison screws come in about ten sizes, each named for its diameter in millimeters. The common ones are E10 for flashlights and fairy lights, E12 and E14 for candle bulbs, E17 for appliance lamps, E26 and E27 for household bulbs, and E39 or E40 mogul bases for streetlights and high-bay lamps.
| Base | Diameter | Common name | Where | Typical lamps |
|---|---|---|---|---|
| E10 | 10 mm | Miniature (MES in the UK) | Worldwide | Flashlights, fairy lights, models |
| E11 | 11 mm | Mini-candelabra | North America | Halogen display and accent lamps |
| E12 | 12 mm | Candelabra | North America | Candle bulbs, chandeliers, night lights |
| E14 | 14 mm | Small Edison screw (SES) | Europe and 230 V countries | Candle, golf-ball, fridge and oven lamps |
| E17 | 17 mm | Intermediate | North America | Appliance, ceiling-fan and desk lamps |
| E26 | 26 mm | Medium, standard | North America (120 V), Japan (100 V) | Household bulbs and reflectors |
| E27 | 27 mm | Edison screw (ES) | Europe and most 220–240 V countries | Household bulbs and reflectors |
| E39 | 39 mm | Mogul | North America | Streetlights, high-bays, corn lamps |
| E40 | 40 mm | Goliath, giant Edison screw (GES) | Europe and worldwide | Streetlights, high-bays, corn lamps |
A letter can follow the number. E26d is the three-way base: an extra ring contact lets a three-way lamp switch between two filaments or two LED circuits to give three brightness levels from a three-way socket. The smaller sizes have their own guide, E12 vs E14 candelabra bases.
Live, neutral and safety in an Edison socket
In a correctly wired socket the center contact is live and the screw shell is neutral. In North America the electrical code requires the shell to connect to the neutral conductor for this reason. Switch off before changing a bulb, and do not touch the metal shell while screwing one in.
The threaded shell is the part your fingers can reach, so it should be the neutral side; Article 200.10(C) of the U.S. National Electrical Code requires exactly that. In a fixture wired with live and neutral swapped, the shell itself can be live. In much of Europe plugs can go into the wall either way round, so either contact of a lamp on a plug-in lamp may be live: treat both as live, switch off before changing a bulb, and switch off at the breaker before working on the holder itself.
Frequently asked questions
Are E26 and E27 bulbs interchangeable?
Mechanically, yes: each fits the other's socket. Use a bulb rated for your mains voltage, such as 120 V in North America or 220–240 V in Europe, or one marked 100–240 V.
Is E26 the same as a standard or medium base?
Yes. In North America "standard" and "medium" both mean E26, the base on most household bulbs and reflector lamps.
What is an E26d base?
The three-way version of E26, with an extra ring contact, so a three-way bulb can give three brightness levels from a three-way socket.
Can I use a European E27 LED bulb in the United States?
Only if its label covers 120 V, for example 100–240 V. A 220–240 V-only bulb may not light properly, and it may lack the UL or ETL listing that U.S. fixtures expect.
What does ES mean on a bulb?
Edison screw, the UK name for E27. SES means small Edison screw (E14), MES miniature Edison screw (E10) and GES giant Edison screw (E40).
Sources and further reading
- IEC 60061-1, Lamp caps and holders together with gauges for the control of interchangeability and safety: Lamp caps, International Electrotechnical Commission (IEC).
- ANSI C81.61, Electrical Lamp Bases: Specifications for Bases (Caps) for Electric Lamps.
- IEC 60238, Edison screw lampholders.
- NFPA 70, National Electrical Code, Article 200.10(C), screw shells.
- UL 496, Lampholders.
Image credits
- Edison screw bases: Medvedev, CC0, via Wikimedia Commons. Base chart: LEDlights.org.