They do make light testers a special tool have wore them out before This is why i still do the tree with incandescent lights still. Been fixing them so long its like an old bike jump on and go. Had some led sets flickered too much and once they broke to the scrap bucket they went at 15$ each was kinda pissed have paid under 2$ for the others and getting on average 5 to 10 years out of them found some this year that 19 yrs old this year. I'll go snap picture then get it on the computer for all to see.

Ok got the picture yes it gets warm. lights or spread over 3 different cords to keep for starting a fire by overloading cord. care to guess the number of lights. lets just say i keep a pre made spread sheet to pull it off. Added 42 bubble lights after the photo the light you see is the light off the tree no flash.

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The tester is a light keeper pro the red version is for incandescent lights and the yellow is for led sets.

OK no guesses on numbers i have had to use a hand made form to track the lights and one which of the 3 cords to balance the load the total number before bubble lights is 4200 then the added 42 bubble lights. Keeps room warm lol.