If you've been shopping for Christmas lights for a couple of Seasons, you may have noticed that most measurements on Christmas lights are followed by approximate, or approx. And you may be asking yourself, "Why is that?"
Interestingly enough, the answer is simple. There is a large human component to manufacturing Christmas lights and sometimes distances and "twist" will change the spacing between lights and sockets just a little bit.
Next time you put your hands on a set of Christmas lights, note how the wiring harness is twisted and if you twist it more or take time to untwist it, you'll see that the spacing between the bulbs will change slightly which affects the total length.
Why is this important?
For most folks a difference of a half an inch isn't noticeable for their projects—wrapping trees, branches, swagging from one side of the patio to another—but for some customers who are building static custom projects, this variation could be super important.
For instance, if a project requires that a mini light bulb be EXACTLY 2.5 inches apart, if there is a 1/4 inch drift between a couple of bulbs in a 50-light string, it would throw off the perfection of the measurements. For projects where lights are inserted from the back into pre-drilled holes, this variation would be a problem.
The solution: Choose a light string with a wider spacing to allow for drift in widths and know there will be a little extra wire on the back of the project.
Example #1 : Holes on a prop marquis for a theater production require a 2.5-inch spacing. Choose a light set with 4-inch spacing to allow for any small drift in bulb spacing.
What could also cause a variation in socket spacing?
C7 and C9 sockets are manufactured by machine onto "lamp cord," but there can be a small drift in socket installation due to a difference in tension or just the natural imperfection of the world—like a truly plumb wall is a rare and fantastic thing in construction.
For most Christmas light installations, this is imperceptible, and most of the time won't be noticed.
When will it be noticed?
When you pre-install flat mount Christmas light clips and measure them EXACTLY every 12 or 15 inches (or, insert the spacing of your project here). A different approach here would be to install the clips and cord at the same time varying the distance between the sockets ever so slightly as you go.
If the clips MUST be exactly 12, 15, 18, etc. inches apart with scientific measurement precision, then choosing a spool of wiring with slighly wider spacing would be a better approach. For example, install a 15-inch spool of a project requiring perfect 12-inch spacing between socket centers.
The Wrap Up
Will most folks notice the small natural variation between lights or sockets on Christmas light strings?
But for those folks who do require super-precision in their spacing, stepping up to a wider spacing with extra wire to allow for play in the lights is a great solution.