How Many Strands of Christmas Lights Can You Connect?

The answer depends on the lights, and the label on the set is the final word. For incandescent lights, the Electrical Safety Foundation International (ESFI) says to never connect more than three strings. For LED lights there is no one number. The limit is printed on the package or the tag near the plug, and it changes from set to set.

That is less tidy than most people want, but it is the honest answer. This guide shows you where to find your limit, how to do the quick math, and what to do when you want more lights than one outlet should carry.

Why there is a limit at all

Two separate limits apply, and people often only think about one.

1. The limit of the string itself. When you plug sets end to end, the power for the later sets passes through the wire and plug of the first one. That wire and plug can only handle so much. A set's fuse, wire size and heat tolerance are why the maker prints a maximum on the label.
2. The limit of the circuit. The outlet is on a circuit with a breaker. Everything on that circuit, including other rooms and appliances, adds to the load.

You can stay under the circuit limit and still go over the string's limit. That is the main reason a long chain of lights gets hot or blows a fuse in an outlet that seems to have plenty of room.

ESFI also warns that more than three incandescent strands may not only blow a fuse but can also cause a fire. It also says to avoid overloading outlets and to plug only one high-wattage item into each outlet at a time.

Incandescent vs LED: what to expect

I did not find an official source that gives a single LED number. Websites that do give one, such as "up to 40 or more sets," are describing certain products. Do not use that figure unless it is printed on yours.

Where to find the limit on your lights

Look in this order:

1. The tag near the plug. Many sets have a small tag on the cord that lists the watts, the fuse rating and the maximum number of sets to connect.
2. The box or the instruction sheet. It may say "connect up to X sets end to end."
3. The plug itself. Some plugs have the fuse rating printed or molded on them.
4. The maker's website. Search the model name.

Also look for a safety mark from an independent lab, such as UL, CSA or ETL. ESFI recommends buying decorations that carry one.

If you cannot find any limit, do not chain the set to another one.

The quick math for a circuit

This tells you whether the outlet's circuit can carry your lights. It does not replace the string's own limit.

1. Find the watts on each label. If a set only lists amps, multiply by 120 volts.
2. Add up all the sets. Include everything you plan to plug into that circuit: inflatables, timers, a light-up display.
3. Know your circuit. Most US household circuits are 15 or 20 amps at 120 volts. That is about 1,800 or 2,400 watts at the limit.
4. Stay under 80 percent for steady loads. A common guideline is about 1,440 watts on a 15 amp circuit, or 1,920 watts on a 20 amp circuit. Lights that run for hours count as a steady load.
5. Remember what else is on that circuit. Other outlets and rooms may share the same breaker.

Example only, not a rule: say each of your three incandescent sets is labeled 40 watts. That is 120 watts total, or about 1 amp. That is a small part of a 15 amp circuit. But the three-string limit still applies, because it is about the string, not the circuit.

Another example only: suppose each LED set is labeled 5 watts and the package says you can connect up to 20 sets. Twenty sets would be 100 watts. That is also small for the circuit, but you still must not go past 20 sets on one run.

Your own labels will give different numbers. If you would like a calculator, the calculator that checks a circuit against its breaker adds up watts and compares them to a breaker size.

Can you mix LED and incandescent sets?

Do not do it unless both labels allow it. Websites disagree on this, and the official sources I read, ESFI and NFPA, do not address mixing. The two types draw very different amounts of power and may have different limits, so the clean approach is to keep each type on its own run, plugged into its own outlet or power strip.

Extension cords and power strips

An extension cord is another place where a safe setup can turn unsafe.

• Use a cord rated for the load. A thin cord carrying a lot of watts runs warm. The extension cord size calculator helps you check.
• Use lights and cords made for outdoor use if they will be outside.
• Do not plug one extension cord or power strip into another.
• Remember ESFI's advice to plug only one high-wattage item into each outlet.
• Stop using any cord with a damaged or cracked cover.

Signs you have connected too many

• A fuse blows in the plug. That means the set protected itself. Replace it only with a fuse of the same rating printed on the plug, and find out why it blew first.
• The plug, cord or outlet feels warm or hot.
• The lights are dim or flicker.
• The breaker trips or a GFCI outlet shuts off. Our guide on why a GFCI keeps tripping on Christmas lights covers the outdoor and wet-weather causes.
• You smell burning or melted plastic.

If you notice any of these, unplug the lights. If the breaker keeps tripping on a normal load, read what makes a breaker keep tripping.

What to do when you want more lights

You do not need one long chain. Try these:

• Split the display. Run several short chains from different outlets, and when you can, from different circuits.
• Switch to LED for new sets. They use much less power per string. Check each label for its maximum.
• Use a timer so lights are not on all night.
• Plan the count before you buy. The tree light count tool estimates how many lights and strands a tree of your size needs, plus power use.
• Fix dead sections first. If a section of a pre-lit tree is out, see what to do about a dead section before you add another chain.

Quick reference

When to call an electrician

Call a licensed electrician if:

• The breaker trips even with a small load
• An outlet or switch is warm, discolored or smells burnt
• Lights dim whenever something else turns on
• You are not sure what is on the circuit or what size the breaker is

Do not replace a breaker or fuse with a larger one to stop a trip. That removes the protection.

What this guide cannot tell you

I read ESFI's Top 10 Holiday Safety Tips and NFPA's winter holiday safety tips. Neither gives a number for LED lights, so this guide does not either. Wattages in the examples are made up to show the math and are not from a real product. Your labels, your circuit and your local code decide what is safe in your home.

The short version

For incandescent lights, never connect more than three strings, and never go past the number on the label. For LED lights, use the maximum printed on the package. Then check the circuit with simple wattage math, keep the total well under the breaker size, and spread big displays over more outlets.