This is a question that I see on a fairly regular basis.
This is an interesting question and depending on the lights you are considering the answer could be yes .. or no.
The answer is dependent on how the lights you are looking at are wired and connected together – whether they are in series or parallel.
More often than not, people ask this question regarding string lights – mini lights. These lights are run in series and are at constant current. Since the traditional incandescent lights have a much higher resistance than LED sets, they pull alot more current – which runs through both types of sets simultaneously. As a result, you just can’t run then in series with the newer LED sets. More than likely, you’d either pop the fuse of the LED set or ruin the LED set.
So part of the answer to the question is… “No”, you can’t run traditional incandescent and LED string lights in series in the same electrical run.
But what about C7 or C9 bulbs? This is the second most common way folks ask about combining incandescent and LED lighting. Eighteen gauge C7 and C9 Christmas light cord is wired in parallel – so all the bulbs have constant voltage and variable current – each bulb only pulls the current that it “needs” – determined by its resistance. This is the reason you can cut them to fit and skip sockets where you want to. So, in this instance, the answer is “Yes”.
In parallel wired C7 and C9 stringers, you can mix incandescent and LED bulbs to your heart’s content… beyond the electrical specifications, taste and Christmas light decorum are your biggest considerations when mixing bulb colors and styles.