No, you can't lengthen or shorten standard series-wired incandescent and LED Christmas tree light strings that are made with 2 or 3-wire harnesses. Even if they work at first, they won't last long. If, however, you are installing C7/C9 18 AWG Christmas light cords with sockets and screw in bulbs that are wired in parallel, yes, you can shorten them.
For a full explanation of series and parallel wiring, look at the post: Are Christmas lights wired in series or parallel?
Mini Christmas Tree lights and Pre-wired LED light strings
These light strings have smaller bases, are made with smaller gauge wire and are wired in series.
Incandescent mini light strings are like these.
LED strings are like these.
We know you've seen those videos on the web showing how you can lengthen or shorten these light strings but the fact is that either more or less current will flow through a different number of bulbs than the lights were specified for and this means you'll run into one of the two following scenarios:
- light strings will either burn too bright and burn out in a short perior of time
- light strings will be dim, won't match other lights in the installation and will be generally disappointing
For ultra-low current LED sets, the investment, especially, isn't worth the risk.
What am I supposed to do?
Unfortunately, there isn't too much that can be done at the end of a run if there are a few leftover inches/feet of Christmas light strings. Wind them up, secure the coil with electrical tape and tuck the rest of the light string out of the way. Wire ties can be handy at times like this.
Jumping between windows?
Visually disguise the bulbs that are messing with your perfect rectangular window vibe but hiding them with a couple of quick turns of electrical tape.
18-AWG Cords and Bulbs
Cords and bulbs wired in parallel have the advantage that their constant voltage, varying current configuration allows them to be trimmed to fit.
These are light cords and bulbs like those shown in these categories.
Work with a Master Electrician and clip your string to fit using proper termination.
Proper termination can include:
- installing a female Gilbert plug on the end of the wire
- dipping the end in a product type referred to in the field as "liquid electrical tape"
- wrapping the end of the cord with electrical tape
The best help you can find with this is counting on the professional assistance of an electrician.