Calculator / Working it out in the US and Canada
120 V, 15 A and 20 A circuits, GFCI
Working it out in the US and Canada
Almost every guide online is written for this region — and most of them still run the circuit sum and the chaining sum together as though they were one number.
Where the 80 percent comes from
Circuit capacity is volts times amps: a 15 amp breaker at 120 volts gives 1,800 watts, a 20 amp breaker 2,400. Neither of those is the figure to plan against.
The NEC defines a continuous load as one that draws current for three hours or more, and requires the conductors and the breaker to be rated at 125 percent of it. Holiday lighting plainly qualifies — it goes on at dusk and stays on. Sizing the protection at 125 percent of the load is the same arithmetic as holding the load to 80 percent of the breaker, which is where 1,440 and 1,920 watts come from.
This is a limit with a safe failure mode. Exceed it and the breaker trips, which is the breaker doing its job. The chain limit is the one with no such backstop.
The chain limit is printed on the tag
UL 588 is the listing standard for seasonal light strings, and it is why your box carries a caution reading connect end-to-end up to a maximum of 210 watts, or 1.75 amps at 120 volts. That figure is set by the flex in the first set, not by anything at the panel.
Heavier sets go further — around 420 watts on 20 AWG cord, and 630 watts on the 18 AWG used for commercial C9 runs. Where the box gives a number of sets rather than a wattage, use the number of sets. It is the manufacturer's own testing and it beats any general figure, including ours.
Typical US and Canadian limits
| Protection | Rating | Watts at 120 V | What it governs |
|---|---|---|---|
| Branch circuit | 15 A | 1,440 W | The whole circuit, after the continuous-load factor |
| Branch circuit | 20 A | 1,920 W | The whole circuit, same factor |
| String tag, 22 AWG | 1.75 A | 210 W | One run end to end — the usual UL marking |
| String tag, 20 AWG | 3.5 A | 420 W | Heavier sets |
| String tag, 18 AWG | 5.25 A | 630 W | Commercial C9 runs |
Outdoors: GFCI is required, not advisory
The NEC requires GFCI protection for outdoor receptacles at dwelling units. It is also the protection that actually matters out there: water and a damaged cord leak current to earth, and that is a fault a GFCI catches and a breaker never sees, because the total load is still perfectly normal.
If your outdoor receptacle predates the requirement, a plug-in GFCI adaptor costs very little. Whether the receptacle itself should be replaced is a question for a qualified electrician, not a web page.
Canada is close, but the code is not the NEC
The supply is the same 120 volts and every sum on this page holds. What changes is the authority: installations are held to the Canadian Electrical Code, published as CSA C22.1, and light strings carry a CSA or cCSAus mark rather than a UL one.
Provincial amendments exist and they differ. Where a number here and your inspector disagree, the inspector is right.
Common questions
How many Christmas lights can I put on a 20 amp circuit?
A 20 amp circuit at 120 volts carries 2,400 watts, but holiday lighting runs for hours, so the NEC continuous-load rule brings the working figure to 1,920 watts. That is the whole circuit, not one run of lights — the chain limit is a separate and usually much smaller number.
Why is the limit 80 percent of the breaker rating?
Because the NEC treats anything drawing current for three hours or more as a continuous load, and requires the conductors and the breaker to be rated at 125 percent of it. Sizing the protection at 125 percent of the load is the same arithmetic as holding the load to 80 percent of the breaker.
Do outdoor Christmas lights need a GFCI?
Yes. The NEC requires GFCI protection for outdoor receptacles at dwelling units, and it is the protection that matters most outdoors: a damaged cord in water leaks current to earth, which a GFCI catches and a breaker never sees. If the outlet is not protected, a plug-in GFCI adaptor is cheap.
Does any of this change in Canada?
The supply is the same 120 volts and the arithmetic is identical. The rules come from the Canadian Electrical Code rather than the NEC, and light strings carry a CSA or cCSAus mark rather than a UL one. Use the maximum printed on the set either way.
Sources
- NFPA 70, National Electrical Code — Article 100 and 210.19(A)(1), 210.20(A) — The definition of a continuous load as one running three hours or more, and the requirement that conductors and overcurrent protection be rated at 125 percent of it. This is where the 80 percent working figure comes from. www.nfpa.org
- NFPA 70, National Electrical Code — 210.8(A)(3) — GFCI protection for receptacles installed outdoors at dwelling units. www.nfpa.org
- UL 588, Seasonal and Holiday Decorative Products — The listing standard for light strings. It is the source of the caution marking that gives a run its maximum in watts — commonly 210 watts, which is 1.75 amps at 120 volts. www.shopulstandards.com
- CSA C22.1:24, Canadian Electrical Code, Part I (26th edition, 2024) — The Canadian equivalent of the NEC, and the code an installation in Canada is actually held to.
Work out your own load
The load calculator checks both limits against your display.