How many sets can you chain together?

When you plug set two into the end of set one, the cord of set one carries the current for both. Add a third and it carries all three. The wire in the first set was sized for one set's load, and every set you add behind it makes that first cord work harder.

Exceed what that wire can take and the cord heats up. The breaker on the wall, meanwhile, sees a load well within its rating and does absolutely nothing. That is why manufacturers print a maximum on the box and why many sets have a small fuse in the plug — that fuse, not your breaker, is what is protecting the cord.

In North America, UL requires the maximum to be printed on the tag. The commonly cited ceiling is 210 watts for the 22 AWG cord used in most mini light sets, and heavier 20 AWG sets go to around 420 watts. In practice this works out at roughly five incandescent mini sets end to end — or dozens of LED sets, because they draw so little.

In the UK and Ireland the plug fuse usually decides it first. Light sets typically ship with a 3 amp fuse, which caps a run at about 690 watts at 230 volts. That sounds generous compared with the American figure, and it is: at twice the voltage the same wattage draws half the current.

In Australia and New Zealand plugs are not fused, so the printed maximum on the set is the only figure you have. Use it.

The common mistake is dividing circuit capacity by set wattage and treating the answer as a chain length. You will find sites confidently telling you that 1,440 watts of circuit capacity divided by 10 watts per set means 144 sets — and then, a paragraph later, that you must never exceed 210 watts in a chain. Both statements are true and they describe different things.

The correct reading: 144 sets is how many the circuit can feed in total. 21 sets is how many can be joined end to end. So you would need seven separate outlets, all of which can share the one circuit.

The calculator on the front page reports both numbers separately for exactly this reason.