Different Light Bulb Bases and Your Guide to Using Them Properly

Light bulbs are constantly evolving, becoming more and more energy-efficient. Changing the light bulbs in your home is one of the easiest ways to earn extra green points, but where to start? There are dozens of types of light bulbs and literally hundreds of unique light bulb bases. While the Edison screw base may be the most familiar, there are others to consider, as well. Whether you use incandescent bulbs, halogen, compact fluorescent, HID or LED bulbs, you might need a comprehensive guide to use them properly.

When picking out the right light bulb, you need to make sure you choose the correct base type and size. Thankfully, the light bulb base is probably the simples variable when it comes to buying the right light bulb. Read on to find out more about the most common base types. We hope this short guide will provide you with the necessary understanding for a greener lifestyle.

Modern Light Bulb Bases

These are the most common light bulb bases you’ll find in the home:

  • Medium Screw Base (E26 are common in the U.S., E27 in Europe)
  • Candelabra
  • Bi-Pin
  • GU

The following image will help you get a better idea of what these base types:

types of light bulb bases

Left to right: Medium Screw Base, Candelabra, GU and Bi-Pin / Credit

As you may have already guessed, some types of light bulb shapes work better if paired with a certain type of light bulb base. This is because the design of some shapes and styles of light bulb will require the use of a certain type of light bulb base. If paired correctly, the light bulb will illuminate at its maximum capacity.

For instance, track lighting features halogen style light bulbs (MR shaped) quite frequently. In order for these bulbs to fit into the fixture’s socket, they come with a GU or a Bi-Pin base style.

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Light bulb bases also come in a wide variety of sizes. All base types – regardless of shapes or style – follow the same measurement type. The size of a bulb base is determined by measuring the width of the base in millimeters at its widest point. The size of the bulb base appears right next to the base style on the packaging in the form of a number. Therefore, a light bulb described as a “GU-10 base” will feature a 10 millimeter-wide base.

Light Bulb Fittings: Caps & Bases

The part of the light bulb that fits into the light fixture bears the name of “base” or “cap,” in some cases. This element of the bulb is essential as it provides the electrical contact which enables the bulb to emit light. To ensure that any given fitting uses only the correct type of light bulb, manufacturers have designed a vast variety of caps and bases. The following sections describe the most popular fittings.

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Edison Screw Cap

What exactly is an E27 light bulb? Named, evidently, after the pioneering innovator Thomas Edison, the Edison Screw – or “ES” lamp fitting – is probably the most common type of light bulb base used worldwide. Fit for a wide range of applications, screw bases have been paired with both incandescent or halogen screw base light bulbs.

How does it work? Two contact wires, soldered into the base metal, connect the filament to the base of the light bulb. This contact point is where the electrical voltage flows into the light bulb. It’s also permanently glued to the base metal, allowing electrical current to continuously flow through the wires, thus producing light.

The most popular ES (E27 fitting) is 27mm diameter. It enjoys a wide popularity in both the United States and Europe. The SES is common among chandeliers, smaller decorative fittings, and appliance bulbs, especially in the UK and Europe. The U.S. and Canada, on the other hand, uses CES predominantly.

Most Edison base bulbs are “single contact.” This name points out the fact that the center of the base features just one contact button. However, there’s a handful called “double contact” because they have two buttons. In this case, the classification becomes E26d or E39d. We have some good news, too. When it comes to Edison base designations, the number is relatively low. We cannot say the same about pin-based bulbs! There are more than 75 different types!

Wedge Base

One of the newer light bulb bases, the wedge base is usually found in incandescent T3.25 bulbs. Instead of using a metal base or soldering, these bulbs are made by tapering the glass in a way that seals the light bulb. The two wires located at the base of the glass provide the required electrical voltage contact points for the wedge base bulb.

Bayonet Cap

With its recognizable “push and twist” action, the bayonet cap also bears the designation of BC or B22d. It’s the light bulb base you’re most likely to encounter in regular light bulbs. It features two locating lugs and a 22mm diameter. The small bayonet cap – code SBC or B15d – is very similar to the B22, but only 15mm in width. Usually used for mains voltage lamps, the SBC fitting can also pair with a small number of specialist low voltage halogen lamps.

Compared to the Edison screw, the bayonet base is a smaller light bulb base. Quite similar to the mogul pre-focused bulb base, this fail-safe fitting has a smooth base metal which fits into the socket with a push. This way, the user ensures the light bulb is properly positioned in the socket.

On the other hand, the double contact bayonet – code DCB – features two contact points at the bottom of the base. Protruding from the base, they ensure the continuous electrical contact. If you live in the U.S., you’re probably not familiar with bayonet caps, because they are very rare here. In Europe, Australia, and New Zealand, however, they are the primary option for incandescent and CFL light bulbs.

Also, there are more “BC” variants than Edison screw types. The 3-pin BC, B22d-3 is among them. While sometimes used on Fireglow lamps, this variants is perhaps more predominant in high pressure mercury lamps (used for industrial applications). The BY22d is most commonly paired with low-pressure sodium (SOX) lamps.

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William E. Eubanks

I'm one of the main writers on the site; mostly dealing with environmental news and ways to live green. My goal is to educate others about this great planet, and the ways we can help to protect it.

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