LED C9 and C7 bulbs explained: bases, sizes, lenses and wattage
C9 and C7 bulbs started as incandescent lamps: a glass cone, a tungsten filament and a screw base. The LED versions installers use today keep the same shape and the same bases so they fit the same socket wire, but almost everything inside is different. This guide covers how the sizes are named, which base goes with which bulb, how LED retrofit bulbs are built, what the lens options do to the light, and how LED power draw compares to the incandescent bulbs they replace.
What "C9" and "C7" mean
The C stands for cone, the candle-flame shape of the original bulbs. The number is the bulb's diameter in eighths of an inch. A C9 (more precisely C9¼) is about 1⅛ in. (29 mm) across, and a C7 (C7½) is about 15/16 in. (24 mm) across. Despite the C, both are blunt-tipped rather than pointed.
Globe bulbs use a G and the diameter in millimeters: a G40 is 40 mm across and a G50 is 50 mm. They screw into the same sockets as C9s and are used where a round, retro look is wanted on the same run.
Bases: E17 for C9, E12 for C7
The base is what decides which socket wire a bulb fits. C9 bulbs use an E17 intermediate base (17 mm Edison screw). C7 bulbs use an E12 candelabra base (12 mm), the same base as a chandelier lamp. The two are not interchangeable: a C7 bulb will not seat in a C9 socket, and a C9 bulb will not thread into a C7 socket.
That means the bulb size is set when the socket wire is chosen. C9 socket wire takes C9 and G40/G50 bulbs. C7 stringers take C7 bulbs. Mixing sizes on one job means running two kinds of socket wire.
How an LED retrofit bulb is built
An incandescent C9 is a glass envelope around a filament that glows when current heats it. An LED C9 is a plastic shell (usually polycarbonate or acrylic) over one or more small LED chips mounted at the base. The shell is shaped and textured to spread the chip's light, because an LED emits light mostly in one direction.
Because there's no filament and no glass, an LED C9 has nothing to break if it's dropped on a driveway or knocked by a ladder, and nothing to burn out the way a filament does. The plastic shell is also what holds the color on many bulbs, which is why lens type matters more on LEDs than it did on painted glass.
LED filament bulbs are a variation: thin strips of LEDs arranged to look like an incandescent filament inside a clear shell. They're used where an installer wants the look of a classic bulb with LED power draw. Christmas Light Supplier carries C9 LED filament bulbs in 25-packs and 500-count cases.
Faceted vs smooth lenses
A faceted lens is molded with cut-glass style facets. Each facet catches and redirects the LED's light, so the bulb sparkles and reads bright from the street at an angle. Faceted C9s are the common choice for rooflines.
A smooth lens gives an even glow across the whole bulb, closer to the look of a painted incandescent. Smooth bulbs suit a retro or softer look and close-range viewing, such as porch rails and entryways.
Transparent vs opaque
Transparent bulbs let the LED show through the shell, so the bulb has a visible bright point and a jewel-like color. Opaque bulbs have a frosted or ceramic-look shell that diffuses the light across the whole surface, so the bulb looks evenly lit with no hot spot.
On one roofline, keep to one lens style. A run that mixes transparent and opaque bulbs of the same color will look uneven from the street, because the two styles carry light differently even when the color matches.
Color: chip color vs shell color
Colored LED bulbs (red, green, blue, orange, yellow, purple) usually get their color from the LED chip itself, not from paint. The color doesn't fade the way painted incandescent glass does after a few seasons outdoors.
White LED bulbs are rated by color temperature in kelvin. Lower numbers (around 2600–3200K) look warm and close to incandescent; around 4000–4500K reads as neutral white; 5900K and up reads as cool or blue-white. Use one white across a job so every run matches. See the color temperature guide for installers for how each white looks on brick, siding and evergreens.
Wattage: LED vs incandescent
A standard incandescent C9 is rated at about 7 watts and a C7 at about 5 watts. LED C9 and C7 retrofit bulbs typically draw a fraction of a watt to around a watt each; the exact figure is printed on the package and is the number to use in load math.
The difference adds up on a full roofline. 500 incandescent C9s at 7 watts draw about 3,500 watts, more than a single 15-amp circuit can carry. The same 500 sockets with LED bulbs draw a small fraction of that, which is what lets one circuit and one timer run an entire house. The C9 lights per circuit guide walks through the amp math with the 80% rule.
Flicker and rectification
LEDs run on direct current, and household power is alternating current. Some LED bulbs light on only half of each AC cycle, which makes them flicker 60 times a second. Most people don't notice it head-on, but it can show in peripheral vision and in phone video. Bulbs with full-wave rectification light on both halves of the cycle and look steady.
If a job will be filmed or viewed up close, check the bulb's spec for rectification, and keep one bulb type on each run so any flicker is consistent.
Buying LED C9 bulbs
LED C9 bulbs are sold in 25-packs for small jobs and replacements and in 500-count cases for full rooflines. Christmas Light Supplier stocks LED C9 bulbs in warm, pure and cool whites plus red, green, blue, orange, yellow and purple, at $19.95 per 25-pack or $319 per 500-count case. Every order ships free the same day it's placed, with no minimum.
Frequently asked questions
What base does a C9 bulb use?
C9 bulbs use an E17 intermediate base, a 17 mm Edison screw. C7 bulbs use the smaller E12 candelabra base, so the two don't fit each other's sockets.
What does C9 mean on a Christmas light bulb?
C stands for cone, the candle-flame shape, and 9 is the diameter in eighths of an inch. A C9 is about 1⅛ in. (29 mm) across; a C7 is about 15/16 in. (24 mm).
Can I put LED C9 bulbs in old incandescent C9 socket wire?
Yes. LED C9 retrofit bulbs use the same E17 base as incandescent C9s and screw into standard C9 sockets.
What's the difference between faceted and smooth C9 bulbs?
Faceted lenses have cut facets that scatter the LED's light so the bulb sparkles and reads bright at an angle. Smooth lenses give an even glow closer to a painted incandescent bulb.
How many watts does an LED C9 bulb use?
Typically a fraction of a watt to about a watt, compared with about 7 watts for an incandescent C9. Use the wattage printed on the bulb package for circuit load math.