The chain limit and the circuit limit are different animals
A string of lights is a piece of wire with a fuse in the plug. When you join strings end to end, the first string in the chain carries the current for every string behind it, and the manufacturer prints the number of strings that wire and that fuse were designed to carry. That number belongs to the product. It does not change if you plug the chain into a bigger breaker, and it does not change if the chain is the only thing on the circuit.
A branch circuit is a completely different constraint. It does not know or care how the strings are grouped. Six chains of five strings fed from six different outlets that all trace back to one breaker are, to that breaker, thirty strings. This is the part that catches people, because the six chains feel like six separate things — six plugs, six locations, six decisions — and electrically they are one.
The default figures make the gap visible. Thirty strings at 4.8 W each is 144 W and 1.2 A at 120 V, which is not much of anything. But a chain maximum of five splits those thirty strings into six chains needing six feed points. The binding constraint here is the chain limit, and it will keep being the chain limit right up until the wattage per string changes.
Change one number and the answer inverts
Swap LED strings for incandescent ones and the wattage per string can go from under 5 W to 40 W or more. Thirty strings at 40 W is 1,200 W and 10 A. Against an 80 percent working figure on a 20 amp circuit — 16 A, or 1,920 W — those thirty strings now take most of it, and the chain maximum for incandescent strings is usually a much smaller number as well. The same display, the same house, and both limits have tightened at once.
That is why this page has no built-in wattage and no built-in chain limit. The two products behave so differently that a default would be actively misleading, and the numbers are printed on the box precisely so nobody has to guess.
What the working fraction is doing here
The field asking what fraction of the breaker rating you work to exists because people do plan to a figure below the rating for something that runs for hours at a time, and different people are given different figures by different electricians. This page takes yours and multiplies. It supplies no value of its own, and it does not check the one you enter against anything.
Everything on this page is arithmetic on numbers you provide. It does not say your circuit is adequate, it does not say a chain is acceptable, and it does not know what your breaker actually is — only what you typed. The question of whether a particular display may be fed from a particular circuit is a question for a licensed electrician who can see the panel, the wire, the outlets and the display, and none of those are visible from here.
Finding out how many circuits you really have
Two outdoor outlets at opposite ends of a porch look like two circuits and are very often one. Outdoor receptacles frequently share a circuit with a garage, a basement, a bathroom, or each other, and the way people discover this is by adding the last string and putting out the whole front of the house. Whoever wired the place knows, the panel directory might know, and an electrician can tell you in ten minutes. Guessing has a well-known failure mode.
The string count for the display comes from the christmas light string calculator. Once the display is fed, the cord runs out to it are on the extension cord size calculator, and what the whole thing costs over the season is on the holiday light timer and run hours calculator. If you are weighing incandescent strings against LED ones on running cost alone, the LED conversion savings calculator handles that comparison properly.
Questions people ask
How many strings of lights can I connect end to end?
Whatever the manufacturer prints for that product, and nothing else. It is set by the gauge of wire in the string and the fuse in the plug, both of which vary between products and between LED and incandescent. This page has no figure for it and will not invent one — it takes the number you enter and works out how many separate chains your display splits into. If the box is gone and the listing does not say, that is worth finding out rather than estimating.
How many amps do Christmas lights draw?
Amps are watts divided by volts, so it comes straight off the box. Thirty strings at 4.8 W each is 144 W, which at 120 V is 1.2 A. The same thirty strings at 40 W each is 1,200 W and 10 A. The factor between those two is the difference between LED and incandescent and it is the single largest number in the whole calculation, which is why this page asks for it rather than assuming.
If I plug chains into different outlets, are they on different circuits?
Not necessarily, and outdoors it is often the opposite. Several exterior receptacles commonly trace back to one breaker, sometimes along with a garage or a basement. The chains feel separate because they are in separate places, but the breaker sees the total. If it matters — and once the wattage per string is high, it does — find out which breaker each outlet is on rather than assuming that separate plugs mean separate circuits.
Does the calculator tell me whether my circuit can take the display?
No, and it is built not to. It multiplies your string count by your watts per string, divides by your voltage, and puts the result beside the breaker rating and the working fraction you typed in. Every one of those is your number. Whether the circuit is suitable depends on the wire, the length of the run, what else is on it, the condition of everything involved and rules that vary by jurisdiction, none of which a web page can see. That is a licensed electrician question.
Why does the display need six feed points when it only draws 1.2 amps?
Because the chain limit is not about current at the panel, it is about current in the string. With a maximum of five strings end to end, thirty strings is six chains, and each chain needs somewhere to plug in. That is true even though all six together draw about as much as a single incandescent lamp used to. The two limits are independent, and on a low-wattage LED display the chain limit is usually the one that decides the layout.