C9 are the large of these two bulbs, with a bulb size that's approx. 2" tall and 1 1/8" wide these bulbs are perfect for outlining roofs and lighting larger Christmas trees. C9 bulbs go into a Intermediate base and are most commonly found in a 7 watt variety, but they are sometimes available in a 10 watt style.
Both C9 and C7 bulbs are available in 3 styles, Transparent Steady burn, Twinkle, and Painted (Opaque). Steady Burn bulbs are most commonly Transparent colors and don't twinkle at all, Twinkle are Transparent as well and have a thermocouple that heats up and causes the bulb to randomly Flash, and Painted bulbs have an opaque painted finish that gives off a classic soft glow that is unmistakable "Christmas"
The most important thing to know about any incandescent lights when doing a decorating job is the power restriction of the wire. Wire for Christmas lights strings is usually 18 gauge, and depending on the insulation rating you can get different amounts of mileage out of the wire itself. The two types of insulation ratings are SPT-1 which is 7 Amps, and SPT-2 which is 10 Amps. Most of the common light strings you find will fall into the SPT-1 category. Below is a simple math equation we use to calculate how many amps your number of lights is pulling, you should never exceed the amount of amps the wire can handle for a continuous run.
(Bulb Wattage) x (Number of Bulbs) = Total Wattage
(Total Wattage) ÷ 110 (Volts) = Amps
Example:
5 Bulb Wattage x 100 Bulbs = 500 Watts
500 Watts ÷ 110 Volts = 4.55 Amps
We hope that this short blog helps give you some insight into C7 and C9 bulbs and how to calculate the power those light strings will use.
