Wednesday, May 31, 2017

Socket Sizes and Bulb Shapes

Working on repairs, restorations and lighting creations, it is important to use products designed to fit the right application. A socket is not just a socket. It could be an E39 Keyless Porcelain Socket. And a bulb is not just a bulb. For the E39 socket you would need the right bulb to fit.

Fortunately, there are standards in the lighting industry where parts are built to spec. Pieces of an older lamp that fail can be replaced with an almost exact replacement part. Worn out sockets and switches can be replaced. Bulbs are manufactured to meet these same standards to fit the socket.

Bulb Shapes Explained

Bulb shapes are difficult to explain. The standards for bulb shape relate to the width, height, and curvature. For an A-19 bulb the A parts describes the shape and the 19 is the size of the bulb. For the A series bulbs, the width is measured in 1/8 inch and an A19 is 2 3/8 inch wide ( 19 times 1/8 inch). 

A-19, Standard base, all purpose house and appliance bulb

Other bulb shapes like the G, PS, CT, or T all describe the shape and the corresponding number is the size. These standards are part of the ANSI C79.1-2002 Nomenclature For Glass Bulbs Intended For Use With Electric Lamps. If you want to learn more, good luck with that journey. 

Socket Sizes Explained

Socket sizes follow a simpler rule of Type-Size. E type sockets are the most common. The E stands for Edison and designates a screw type socket. The size is a simple measurement of width in millimeters. An E26 socket (most common type) has an Edison thread interior and is 26 millimeters wide. This socket is also referred to as a Medium socket.

48226 - 3" Medium Base Keyless Candle Socket
The 3 most common sockets in the USA are the E12 (candelabra), E26 (medium) and the  E39 (mogul). As this table shows, there are some sockets in between these sizes. For traditional lighting, those three are the most commonly used.

DesignationBase major diameter (thread external)NameApplicationIEC 60061-1 standard sheet
E55 mmLilliput Edison Screw (LES)Indicator lights, decorative lights7004-25[12]
E1010 mmMiniature Edison Screw (MES)Flashlights, bicycle lights7004-22
E1111 mmMini-Candelabra Edison Screw (mini-can)120 V halogen mini-candelabra(7004-6-1)
E1212 mmCandelabra Edison Screw (CES), C7120 V candelabra/night lamp7004-28
E1414 mmSmall Edison Screw (SES)230 V candelabra/chandelier, night lamps, and some pendant lights.7004-23
E1717 mmIntermediate Edison Screw (IES), C9120 V appliance7004-26
E2626 mm[Medium] (one-inch) Edison Screw (ES or MES)Standard 120 V lamps7004-21A-2
E2727 mm[Medium] Edison Screw (ES)Standard 230 V lamps7004-21
E2929 mm[Admedium] Edison Screw (ES)
E3939 mmSingle-contact (Mogul- in America) Goliath Edison Screw (GES)120 V 250+ W industrial7004-24-A1
E4040 mm(Mogul) Goliath Edison Screw (GES)230 V 250+ W industrial7004-24
Source: Wikipedia: Edison Screw 5-31-2017

California requires remodels and new construction use a GU24 (bi-pin) socket. These types of sockets require matching bulbs and maybe limited to CFL or LED bulbs. Traditional sockets can be adapted to GU24 sockets with a simple adapter.

48707 - Medium Base E26 Socket to GU-24 Socket Adapter

Some Standards Don't Apply

As always, there are some exceptions to the rule. One exception might be a failed swivel on an old lamp. No matter how hard you try, the replacement piece does not exist. This is true, but swivels, arms, bodies, and bases are decorative features and not electrical parts. The standards for these parts are part of the thread sizes. A different topic.

Another complaint might be related to a European fixture. Maybe it has bayonet sockets or a strange size thread socket. Standards are still in play but are subject to regional preferences. Some older European fixtures have E17 Intermediate sockets. If you are in the process of a repair for a US market, convert the sockets to E12 Candelabra since these bulbs are easier to find.

47624 - Single Contact Brass Bayonet Socket With 1/8IPS Male Thread


This article only scratches the surface of bulb and socket standards. We didn't touch Wattage, the measurement of energy consumption for bulbs. We also didn't cover specialty antique style or decorative bulbs shapes and styles. 

What you can do with this new found knowledge? We suggest using it for light conversation at your next dinner party. 


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