Light Bulb Specifications


A correspondent writes:
I have mini-track lights under the kitchen cabinets.
Instructions say that replacement bulbs should be
R14 (E17). What kind of code is this? How do I
interpret it when buying new light bulbs online?
Also, on another subject, what are the names of the flood
light bulbs that have curved glass surfaces? I have indoor
ceiling track lights which take bulbs slightly under 4 inches
wide, and outdoor flood lights which take bulbs about 5 inches

Suppose you are reading the labeling on a package of light bulbs, or the description given by an online store. Something that you need to know, in addition to the power rating (measured in watts), is how to be sure that you are buying the size and shape that will physically fit into the fixture.

In the case of your mini-track lighting, if a package of light bulbs, or the specifications listed on a web site, says R14, you know that a bulb will fit properly into the housing. R14 is sufficient information for you to be sure that you have the right size.

The designation E17 would not be sufficient information for you to purchase the exact product that you need. The E17 describes only the size of the threaded base that screws into the socket. As you have probably noticed, the E17 threaded base, which is sometimes called an "intermediate base", is a lot smaller than the screw base of an ordinary light bulb.

The threaded base of an ordinary light bulb, a base about an inch in diameter, commonly used in living room lamps, is called the E26 base. It is also known as the "medium base" or "standard base." Any bulbs which have this size threaded base will be labeled E26. For example, the smaller "appliance bulbs" that many people use in refrigerators, and which also fit into lava lamps, are called the A15 size. They also have E26 bases. The decorative three-inch-wide spherical bulbs, which many people use for lighting around bathroom vanities and mirrors, are called the G26 size. They also have E26 bases. These various kinds of bulbs are noticably different in sizes and shapes, but their bases -- the brass part that screws into the socket -- are the same size as that of a common light bulb, namely, E26.

Now I will cover the subject of your flood lights. What you are calling a curved glass surface is called a "bulged reflector" or BR design. They are named with the letters BR, followed by a measurement of the width (circular diameter) in units of multiples of 1/8 inches. Your indoor ceiling track lights are using BR30 bulbs, which have a width of 30/8 inches, that is, 3.75 inches. Your outdoor flood lights are using BR40 bulbs, which have a width of 40/8 inches, that is, 5 inches. Both the BR30 and BR40 bulbs have the standard size screw base that we are familiar with from ordinary light bulbs, namely, the E26 size screw base.

Now that you know what size to buy, also remember never to exceed the power rating, measured in watts, that the manufacturer of the fixture recommends. The power is the rate of energy consumption, which includes both light and heat added together. For example, a 40 watt bulb consumes 40 watts of power purchased from the electric company (a rate of 40 joules of energy per second). Some fraction of that 40 watts is converted to heat instead of light. The manufacturer has probably determined, on the basis of lab tests, that exceeding the specified power level can produce a fire hazard.

What about LED bulbs, instead of the traditional incandescent (hot filament) bulbs?

Suppose that the power rating for a particular fixture is, for example, 40 watts. That manufacturer is telling you that you may safely use an incandescent bulb of 40 watts or less. In that case, it would be safe to install LED (light-emitting diode) bulbs having labels similar to "5 watts, equivalent to 40 watts incandescent." Such a label means that, through the use of LED technology, the total power dissipation, light and heat added together, has been reduced to 5 watts, while giving you as much light as an older-style incandescent bulb dissipating 40 watts of light and heat. Switching to bulbs that generate LESS heat than the manufacturer's recommendation is economical as well as safe. Switching to bulbs that generate MORE heat than the amount approved by the manufacturer would be a major safety hazard and must be avoided.

(About the author of this article -- Mike Lepore has a master's degree in electrical engineering from the University of Vermont.)