Traditional C6 Bulbs Were Typically 15 Volts and Used in Series Strings of Eight Bulbs

Traditional C6 bulbs were typically 15 volts, and used in series strings of eight bulbs, or multiples of 8. The use of eight bulbs (120 volts for 8 lamps equals 15 volts per lamp) gives each lamp the rated voltage for proper brightness. Later sets used nine bulbs on a string to increase the life of the bulbs by reducing the voltage each lamp received (120 volts divided by 9 lamps = 13 volts per bulb) but not significantly reducing the light output of the bulbs. Large C7½ and C9¼ bulbs typically come in sets of 25, though bubble lights come in sets of seven, and some non-holiday sets come in ten or twelve. Sockets are usually spaced about one foot or 30 cm apart, and are clamped to the wire with an integrated insulation-piercing connector. Some older parallel sets had 15 bulbs, as do some of the newer globe sets manufactured today. Both of these bulbs are designed to run on 120 volts and the light sets that use them are parallel wired. Mini lights first came in sets of 35 (3.5 volts per bulb), and sometimes smaller sets of 20 (6 volts per bulb). For a short time, these early mini lights were manufactured using E5 screw bases rather than the current wedge base and were called "italian lights". Sets of ten (12 volts per bulbs) were made for very small trees, but are quite hot, and are now usually used for tree toppers only. This number is convenient for stars, which have a total of ten points (five outward and five inward), and often have another light in the middle, occasionally on both sides. Incandescent miniatures now usually come in sets of 50 or 100 (which contains two circuits of 50) 2.5 V 170 mA bulbs, though decorative sets with larger bulbs (C6 or pearl style) typically come in sets of 35 or 70. Several "extra-bright" sets also use 70 or 105 bulbs, keeping the per-bulb voltage at 3.5 instead of 2.5. LED sets can vary greatly. Common is a set of 60 (2 volts per bulb), but white LED sets use two circuits of 30 (4 volts per bulb). Multicolor sets may have special wiring, because red and yellow require less voltage than the newer blue-based ones (blue, emerald green and fluorescent white), but typically come in sets with a multiple of 35. Battery-powered sets typically come in 10 or 12, and can use standard 2.5 to 3.5-volt bulbs because they run two batteries, totaling three volts or less. LEDs are becoming increasingly common as they greatly prolong battery life, but because they also last longer they are often soldered directly to the wires, making up for some of the increased cost of the newer LEDs. 'Rice lights" are often made this way as well, and likewise may also have more bulbs per set as they draw somewhat less power per bulb than other incandescents. Source: en.wikipedia.org