How Many Lights Can I Plug In Together?

There are two parts to the “How many lights” question – the number of lights strings that can be connected together, and the number of light strings that can be on a single circuit. Before I go too far please remember that I’m not an electrician so if your house burns down because you followed my advice, don’t sue me.

How many lights can be in a single string?

Incandescent (old-school lights): The “rule of 3” is generally applied to incandescent bulbs. You can plug 3 strings in series (end-to-end) safely – generally this is true regardless of the number of bulbs per string (strings range from 25 to 200 bulbs) if the strings were manufactured after 2008. You can also plug the strings in stacks of up to 3 (the plug at the beginning of a string allows you to plug another string into the back of that plug). So, you can have 3 series’ starting from the same location, with 3 strings in each, for a total of 9 strings.

With LED lights the number of lights you can string together is much larger – generally the rule with strings manufactured in the last 3 years is up to 73 sets per series! Personally, I don’t generally go that far with a single series – I generally limit it to about 25-30 per series.

How many lights can be on a circuit?

As a caveat, the math below assumes that there’s no other load on the circuit (i.e. there’s nothing else using it). Because it’s outdoor receptacle, it’s definitely possible it’s on a circuit with other things (outdoor lighting or just another random circuit in the house). You may want to find the breaker, turn it off and see what loses power so you have at least a rough idea of what else is using that circuit.

Receptacle circuits run on 120 volts and are either 15 or 20 amps. You can usually tell a 20a circuit by the plugs – a 20a will have an extra horizontal slot along one of the two non-ground prongs. For safety’s sake, we’ll assume your circuit is 15 amp.

To calculate the number of strings your plug can power you need to determine how many watts the circuit can support. To calculate the number of watts supported, multiply the amperage by the voltage, i.e. 120 x 15, or 1800 watts. However, it’s a generally held convention that circuits should never be more than 80% loaded so we need to reduce that to 80% load  – 1800 watts x .8, or 1440 watts.

Now you just need to figure out how much wattage each of your light sets requires. While every product  is a little different, here are some good base values you can start with:

50 Incandescent mini – 16w
100 Incandescent mini – 20w
150 Incandescent mini – 40w
200 Incandescent mini – 60w
50-70 LED Strings – 2.6w

So, if you were using 100 incandescent mini light strings you could have about 36 strings (1440 watts / 20 watts) whereas with LED lights you can have about 550 strings (1440 / 2.6 watts)!

Hopefully that helps! The key is to watch your total load on the circuit and to keep your string series no longer than the manufacturer’s recommendations.