Christmas lights keep tripping the breaker

Short answer: if the breaker at the panel trips, you are drawing too much current — reduce the load or split it across circuits. If the GFCI at the outlet trips, there is almost certainly water somewhere it should not be, and it is protecting you. They are different faults and the fixes are different.

Work out which one tripped

On an outdoor circuit in December, the GFCI is by far the most common of the three, and it is doing its job. A GFCI detects a tiny imbalance between the current going out and coming back — which is what happens when some of it is finding a path through water. See GFCI and outdoor lights.

If the GFCI is tripping: find the water

1. 1

   Unplug everything from the outdoor circuit

   All of it. Then reset the GFCI. If it holds with nothing connected, the fault is in the display rather than in the wiring.

2. 2

   Plug in one run at a time

   Add one run, wait a couple of minutes, and see whether it holds. Work through the display methodically rather than plugging everything back and hoping.

3. 3

   When it trips, inspect that run's connections

   Every joint in that run: set to set, set to cord, cord to outlet. You are looking for a connection sitting in a puddle, lying in a gutter, or resting on wet ground.

4. 4

   Check the sockets on that run

   On a socketed set, look for green or white corrosion in the sockets, and for any socket with no bulb in it. An empty socket is an open path to the contacts.

5. 5

   Dry it, elevate it, re-test

   Dry the connection thoroughly, raise it off the ground on a brick or hook, add a drip loop, and try again.

6. 6

   If it still trips, that run is faulty

   Water has most likely got into the wire itself or a molded connector. Replace that set — do not tape it.

If the breaker is tripping: reduce the load

A breaker trips on current. Work through in this order:

1. Total the load. Add the wattage of everything on that circuit — including anything indoors on the same breaker — and divide by 120. Compare with 12 amps. See Christmas light amps and watts.
2. Find what else is on the circuit. Outdoor outlets are often shared with a garage or bathroom. A space heater or a power tool on that breaker is a much bigger load than your display.
3. Split the display across two feeds, ideally on different breakers. Roofline on one, ground level and inflatables on the other.
4. Replace incandescent runs with LED. This is usually the entire answer, because the saving is an order of magnitude.
5. If it trips with almost nothing connected, that is a short rather than an overload, and it is an electrician's job.

Why it happens on cold or wet nights specifically

Two reasons people find confusing.

Wet nights are obvious once stated: water finds its way into a connection that was fine when dry. A display that runs perfectly for a week and trips during the first rain has a connection sitting somewhere it should not be.

Cold nights are less obvious. Cold contracts materials, which can open a small gap in a connector that then admits moisture; and a breaker that has been operating near its limit all season will trip sooner as components age. A display sitting right on the edge of its budget will fail on the coldest, wettest night rather than on a mild one.

When to stop and call someone

• The breaker trips with nothing, or almost nothing, connected
• You smell burning, or see scorching or melting at an outlet
• The GFCI will not reset even with everything unplugged
• The outlet or the cover plate is warm to the touch
• You do not have a GFCI on the outdoor circuit at all

Each of those is a wiring issue rather than a decorating issue. Nothing about a display is worth working around them.