Every December the same call comes in — the outdoor receptacle that runs the Christmas lights keeps tripping the breaker or the GFCI. The Saturday afternoon fix is to plug fewer strings into it, but the better answer is to know what the circuit can carry before the first strand goes up. Here is the planning math we walk homeowners through, and the conditions where adding a dedicated exterior circuit is actually the right call.
The two numbers that matter
A typical exterior receptacle in an Ontario house is on a 15A or 20A circuit at 120V. The continuous-load limit on that circuit (what you can safely run for hours without nuisance trips) is 80% of the breaker rating:
- 15A circuit → 12A continuous → 1,440 W
- 20A circuit → 16A continuous → 1,920 W
That is the headroom. Everything plugged into the circuit (front-yard string lights, the inflatable reindeer, spotlights on the spruce, path lights on the timer) has to fit under that ceiling. And critically, the circuit is almost never dedicated to the exterior. It usually feeds something inside too, which means the headroom is smaller than the panel number suggests.
LED vs incandescent: the order-of-magnitude difference
This is the single most important shift in Christmas-light load planning over the last decade. LED strings draw a fraction of what incandescent strings draw. The numbers:
- Mini-incandescent string, 100 bulbs, C7/C9. ~40W per string (legacy). 36 strings would exceed a 15A circuit.
- LED mini string, 100 bulbs. ~5–8W per string. 200+ strings on a 15A circuit before you run out of capacity.
- LED C9 retrofit bulbs. ~0.5–1W per bulb, vs. 7W per incandescent C9.
- Inflatable yard decorations. 60–100W each (the blower motor is the load, not the LEDs inside).
- Outdoor spotlights, LED PAR38 type. 10–18W each.
- Outdoor spotlights, halogen PAR38 type. 75–150W each.
If your display is fully LED, you will almost never hit the circuit ampacity ceiling — the limit becomes how many fixtures you have receptacles for and what the GFCI sees. If your display is still legacy incandescent C7/C9, the ampacity ceiling is real and the math has to be done up front.
The GFCI nuisance-trip problem
The OESC requires GFCI protection on all outdoor receptacles. The GFCI device compares hot current to neutral current; any leakage to ground above ~5 mA trips the device. With one string of LED lights, leakage to ground is essentially zero. With twenty strings, twenty extension cords, six in-line transformers (for low-voltage LED bullets), and a wet first-week-of-December cold rain, accumulated leakage can hit the trip threshold.
That is the source of the most common Christmas-light call: "the lights worked yesterday, the GFCI keeps tripping today." The display didn't change; the weather did. Diagnosis is one string at a time — plug one string into the GFCI, see if it trips, add another, repeat until you find the offender. Often the culprit is a single damaged cord or a transformer that has water inside.
What does not fix the problem: replacing the GFCI receptacle with a non-GFCI receptacle. That is a code violation, an insurance problem, and a real shock hazard.
Timer wiring: the question we get asked most
Three options for switching the lights on and off:
- Plug-in mechanical or digital timer. Cheapest and most common. Reliable, but the timer itself counts against the receptacle load and adds a connection point that can fail.
- Smart plug or smart outlet. Wi-Fi or Zigbee/Z-Wave controlled. Works great when the network is up; less reliable when the router reboots at 2 AM.
- Hardwired exterior timer or smart switch on a dedicated circuit. Cleanest install, controlled from inside the house or from your phone, no plug-in device at the receptacle. This is the option we wire when homeowners want a permanent solution.
For homeowners running serious displays (multi-zone, music-synced controllers, projector lighting, the works) we wire dedicated exterior circuits on smart switches: Lutron Caseta or RA3, or Leviton Decora Smart. The display gets reliable power, the controls are integrated with the rest of the home, and the GFCI protection is on a circuit that does not also feed the garage compressor.
When to add a dedicated exterior circuit
The right scope-of-work question is not "can the existing receptacle handle it" but "is it worth pulling a new circuit." Add a dedicated exterior circuit if:
- Your display has been growing each year and you are now consistently maxing out the existing receptacle
- You are doing a serious permanent installation (Trimlight, Jellyfish, Permalit-type permanent eave lighting) — that wants its own circuit
- The existing exterior receptacle is shared with a garage circuit and the garage compressor or freezer is on the same line
- You are planning controlled scenes integrated with smart-home lighting
A new 20A exterior circuit is a half-day job in most cases — pull from the panel, through the wall to a new in-use covered receptacle near the display, GFCI breaker at the panel. ESA permit, inspection, and you are set for every future year.
Extension cord rules: the part homeowners get wrong
- Outdoor-rated cords only outside. Indoor (SJT-rated) cords used outside get hard within one cold cycle, develop cracks, and fail GFCI test.
- 16 AWG is for low draw, short runs. Over 50 ft or over 10A continuous, step up to 14 AWG or 12 AWG.
- Daisy-chaining cords is risky. Each connection is a failure point. Run one cord to a powered junction with a GFCI face, not three cords in a chain.
- Cords resting in standing water or under snow. Acceptable for properly rated outdoor cord, but bring the connections up off the ground in an outdoor-rated splice cover.
When to call us
If you want a permanent exterior circuit for Christmas lighting, a smart-switched eave light system, or just a quick walk-through of what your current circuit can handle, we do residential electrical work across both clusters. The window to get this done before the first weekend in December is short — November fills up. Request an exterior lighting circuit quote and we will scope it in writing.