120 V and 230 V · US, UK, IE, AU, NZ

Two limits decide your display. Most guides only check one.

Enter what you are hanging up and this works out both: how many sets you can join end to end, and how much the circuit can carry. They are different numbers, and the one that catches people is the one no breaker protects you from.

The run limit: How much one chain can carry, set by the cord in the first set — and by the plug fuse in the UK. Exceed it and the cord overheats while the breaker sees nothing wrong.
The circuit limit: How much the breaker can feed in total, across every outlet on that circuit. Exceed it and it trips, which is annoying but safe.
Why it matters: You can be far under your circuit limit and still overload a run. That is the failure that starts fires, and it is the one most calculators skip.

Load gaugeTwo limits, checked separately

Guides

The limit no breaker seesHow many sets can you chain together?This is the limit people get wrong, and it is the dangerous one — because when you exceed it…230 V, ring finals, fused plugsWorking it out in the UK and IrelandNearly every guide online assumes a 120 V American branch circuit. Almost none of it transfers…230 V, unfused plugs, RCDsWorking it out in Australia and New ZealandSame voltage as the UK, but no fuse in the plug — which changes where the limit sits and what you…Cords, leads and reelsExtension cords and leadsThe cord is part of the circuit. A lead that is too thin or too long costs you brightness…The load differenceLED against incandescentThe difference is about eight to one, and it changes every number on this site — how many you can…Trees, rooflines and shrubsHow many lights do you need?Before the load question comes the quantity question. These are the rules of thumb that hold up…What actually causes firesWhat actually goes wrongHoliday lighting fires have a short list of causes, and none of them is the thing most guides spend…

The arithmetic, in full

Circuit capacity is volts times amps. A 15 amp breaker at 120 volts gives 1,800 watts; a 16 amp breaker at 230 volts gives 3,680. Holiday lighting runs for hours at a time, which makes it a continuous load, so the working figure is 80 percent of that — 1,440 watts and 2,944 watts respectively.

The chain limit is a different calculation entirely. It comes from the current-carrying capacity of the cord in the light set, because the first set in a chain carries the current for every set behind it. In North America that is commonly 210 watts for the 22 AWG cord used in mini sets. In the UK the fuse in the plug top usually decides it first, at about 690 watts for the 3 amp fuse light sets normally ship with.

Neither number overrides the other. You need to satisfy both, and which one binds depends on whether you are running a few heavy sets or a great many light ones.

Why voltage changes the answer

Power is voltage times current, so the same 500 watt display draws 4.2 amps at 120 volts and 2.2 amps at 230. That is why British and Australian households can hang far more lighting off one outlet than American guidance implies, and why copying American set counts is misleading in both directions.

This site asks where you are before it calculates anything, because almost every other guide silently assumes 120 volts.

What each circuit carries

Supply	Breaker	Theoretical	Working limit (80%)
120 V	15 A	1,800 W	1,440 W
120 V	20 A	2,400 W	1,920 W
230 V	10 A	2,300 W	1,840 W
230 V	16 A	3,680 W	2,944 W
230 V	20 A	4,600 W	3,680 W
230 V	32 A ring	7,360 W	5,888 W

Common questions

How many Christmas lights can I put on one circuit?

Multiply the breaker rating by the supply voltage, then take 80 percent. A 15 amp circuit at 120 volts gives 1,440 watts. A 20 amp circuit gives 1,920. At 230 volts a 16 amp circuit gives about 2,944 watts after the same margin.

Is the circuit limit the same as the chaining limit?

No, and confusing them is the most common error in holiday lighting advice. The circuit limit is how much the breaker can feed in total. The chain limit is how much one string of connected sets can carry through the first set's cord. Both apply at once.

How many watts does a 15 amp circuit handle?

1,800 watts at 120 volts in theory, but holiday lighting runs for hours, so the working figure is 80 percent of that — 1,440 watts.

Why do my lights trip the breaker?

Usually because something else large shares the circuit — a space heater, a microwave, a freezer. Enter that load in the calculator and you will normally see where the headroom went.

Do LED lights change the answer?

Substantially. LEDs draw roughly an eighth of what incandescent sets do, so chain limits stop being a practical constraint and a single circuit will carry an entire display.