We are talking about lamp caps. ‘Lamp caps’ are the part of a light bulb which makes the electrical connection with your light fitting.
These can often be described as BC (bayonet) or ES (Edison Screw developed by Thomas Edison), but in the case of screw-in bulbs there are quite a few similar names or codes for bulbs which are very similar – but get the wrong lamp cap and the bulb simply won’t fit!
A ‘standard’ ES (Edison screw) bulb is also known as E27, which you might see on some light bulb boxes. This is the diameter of the screw thread, i.e. 27mm in diameter.
The only larger screw fitting that you are likely to see is in some high-bay fittings (e.g. a barn or warehouse). They use E40/GES (Giant Edison Screw) bulbs, which are 40mm in diameter, but they aren’t normally used in the home.
The other most common screw-in bulb for the home is the E14 or SES (Small Edison Screw). This, as you will have correctly guessed has a 14mm diameter screw cap.
Ok, the easiest way to tell an E14/SES bulb from an E27/ES bulb is:
Other screw capped bulbs that you might be looking for, especially if you need a replacement pygmy bulb are:
| Designation | Base Diameter (Crest of thread) |
Name | Usage |
|---|---|---|---|
| E5 | 5mm | Lilliput Edison Screw (LES) | Very uncommon |
| E10 | 10mm | Miniature Edison Screw (MES) | e.g. torch bulbs |
| E11 | 11mm | Mini-Candelabra Edison Screw (mini-can) | Very uncommon |
| E12 | 12mm | Candelabra Edison Screw (CES) | Candelabras, boats and some Christmas decorations |
| E14 | 14mm | Small Edison Screw (SES) | Common household bulb, pygmy version used in inflatable Santas |
| E17 | 17mm | Intermediate Edison Screw (IES) | e.g. microwaves |
| E26 | 26mm | [Medium] (one-inch) Edison Screw (ES or MES) | Very uncommon |
| E27 | 27mm | [Medium] Edison Screw (ES) | Common household screw-in bulb |
| E29 | 29mm | [Admedium] Edison Screw (ES) | Very uncommon |
| E39 | 39mm | Single-contact (Mogul) Giant Edison Screw (GES) | Very uncommon |
| E40 | 40mm | (Mogul) Giant Edison Screw (GES) | High-bay barns and warehouses |
We recommend measuring the screw thread of the bulb (preferably with calipers) to work out which type of bulb you need.
For much more information about Edison screw bulbs, try this Wikipedia article: http://en.wikipedia.org/wiki/Edison_screw
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