Three different limits decide your display
A holiday display is constrained by three things at once, and most people only think about the first.
Length is the obvious one. Your gutter is a certain number of feet and a string is a certain number of feet, so you divide and round up. The subtlety is that you round up per run, not once at the end: a 40 ft porch rail and a 120 ft roofline both take partial strings, and you cannot lend the leftover from one to the other.
Circuit capacity is the one that trips breakers. The National Electrical Code (NFPA 70) defines a continuous load as one expected to run for three hours or more, and requires that the branch circuit supplying it be loaded to no more than 80% of the overcurrent device's rating. Holiday lights are on from dusk to bedtime, so they are continuous by definition. On a 15 A, 120 V circuit that means 1,440 W, not 1,800 W. Note that this limit is shared with everything else already on that circuit — the porch light, the garage door opener, whatever is plugged in inside the wall on the same run.
The manufacturer's end-to-end limit is separate and often stricter. It is set by the current-carrying capacity of the thin wire inside the string, not by your breaker. Incandescent mini sets are typically limited to three to five strings plugged end to end; LED sets, drawing an order of magnitude less, commonly allow forty or more. Exceeding it overheats the first string in the chain, which is the one carrying every downstream string's current, and that is a fire mechanism the breaker will never see.
This calculator reports all three, because a display can pass one and fail another.
How each number is worked out
Linear runs. Add the roofline and every other linear feature, divide by the lit length of one string, round up. Measure the roofline properly: for a gabled end, the two rakes are longer than the wall beneath them, by the ratio of the rafter length to the run. A 30 ft wide gable at a 6:12 pitch has rakes of about 16.8 ft each, not 15.
Trees. Lights on a tree are budgeted per vertical foot, not by the length of string wrapped, because the wrap length depends on the branch structure and nobody can measure it in advance. The trade rule of thumb is 100 lights per vertical foot for good coverage, and this calculator lets you halve or double it. A 7 ft tree at the standard density therefore takes 700 lights, which at 100 lights per string is 7 strings. Multiply by the number of trees.
Shrubs are counted directly: lights per shrub, converted to whole strings, times the number of shrubs.
Load and current. Multiply total strings by the watts printed on the plug tag. Divide by the supply voltage to get amps. Both figures matter: the code limit is expressed in amps, but the arithmetic is easier in watts because the string is labelled in watts.
Circuit count. Apply the 80% factor to the breaker rating and the supply voltage, divide by the watts of one string, and round down — you cannot legally connect a fraction of the string that would put you over. Divide the strings you need by that limit and round up to get circuits. Rounding down on the first division and up on the second is deliberate: both errors must fall on the safe side.
Cost. Convert watts to kilowatts, multiply by hours per night and nights in the season, and multiply by your all-in rate. Use the all-in rate — total bill divided by total kWh — because delivery and rider charges are typically as large as the supply charge itself.
Worked example: 160 ft of roofline and rails, two 7 ft trees, four shrubs
Take the defaults: 120 ft of roofline, 40 ft of railings and windows, two 7 ft trees at standard density, four shrubs at 100 lights each, and 25 ft LED strings of 100 lights drawing 4.8 W, on a 15 A, 120 V circuit, lit 6 hours a night for 45 nights at $0.17/kWh.
- Linear strings. 120 + 40 = 160 ft. 160 ÷ 25 = 6.4, rounded up to 7 strings.
- Tree lights. 7 ft × 100 lights/ft = 700 lights per tree. 700 ÷ 100 = 7 strings per tree, × 2 trees = 14 strings.
- Shrub strings. 100 ÷ 100 = 1 string each, × 4 shrubs = 4 strings.
- Total. 7 + 14 + 4 = 25 strings, which is 25 × 25 ft = 625 ft of light string.
- Connected load. 25 × 4.8 W = 120 W. Current: 120 ÷ 120 = 1.00 A.
- Circuit limit. 0.80 × 15 A × 120 V = 1,440 W. 1,440 ÷ 4.8 = 300 strings. You need 25, so one circuit carries the lot with vast headroom.
- Season energy. 0.120 kW × 6 h × 45 nights = 32.4 kWh. At $0.17: $5.51.
- Incandescent comparison. The same 25 strings at 40 W each is 1,000 W — 8.33 A against a 0.80 × 15 = 12 A continuous allowance, so 8.33 ÷ 12 = 69% of the circuit. 1.0 kW × 6 × 45 = 270 kWh, costing $45.90.
The LED-versus-incandescent gap here is 45.90 ÷ 5.508 = 8.33×, which is exactly the ratio of the two wattages (40 ÷ 4.8), because everything else in the cost formula is identical. The energy saving is $40.39 for one season; the more consequential difference is that the incandescent version needs 8.33 A of a 12 A continuous allowance, leaving almost nothing for anything else on that circuit.
Reading the result before you climb the ladder
If circuits required is 1, you still are not finished. The 80% figure applies to everything on that branch circuit, not just to the lights. Outdoor receptacles are frequently on a circuit shared with a bathroom, a garage, or the porch and hall lights. Before loading a circuit to its calculated maximum, find out what else is on it — switch the breaker off and walk the house noting what died.
If circuits required is more than 1, splitting is not optional. Two extension cords from the same outlet are still one circuit; so are two outlets on the same breaker. You need feeds from genuinely different breakers, which for most homes means running a cord from the garage as well as the porch.
Check the end-to-end limit even when the circuit is fine. Twenty-five LED strings is comfortably inside a typical 43-string manufacturer limit, so a single chain is legitimate. Twenty-five incandescent strings is five times a typical five-string limit, and would need at least five separate feeds regardless of how much amperage the breaker has spare. This is the constraint that surprises people who upgrade a display without changing the bulb technology.
Judge the cost against what it replaces, not against zero. $5.51 for a season of LEDs is less than most households spend on the extension cords. $45.90 for incandescents is real money but still modest — the argument for LEDs outdoors is mainly the circuit capacity and the string-chaining freedom, and the energy saving is a bonus. State it that way round and you will make better decisions about which sets to replace first: replace the ones on the most congested circuit, not the ones that are on the longest.
Use the season length honestly. Many households run lights for six weeks but light them with a photocell from dusk to midnight, which is closer to eight hours a night in late December than six. At 8 hours the default display costs $7.34 rather than $5.51.
How many strings one circuit carries, by string type
| String type (100 lights) | Watts | Max on a 15 A circuit | Max on a 20 A circuit | Typical end-to-end limit |
|---|---|---|---|---|
| LED mini, 5 mm | 4.8 | 300 | 400 | 40–45 |
| LED mini, C6 / C9 style | 8 | 180 | 240 | 20–25 |
| Incandescent mini | 40 | 36 | 48 | 3–5 |
| Incandescent C7 | 250 | 5 | 7 | 2–3 |
| Incandescent C9 | 350 | 4 | 5 | 2 |
Circuit columns are 1,440 W and 1,920 W divided by the string wattage and rounded down. End-to-end limits vary by manufacturer and are printed on the tag — always use yours rather than this column.
What the NEC actually requires
Three provisions of NFPA 70 bear on a holiday display. Article 100 defines a continuous load as one whose maximum current is expected to continue for three hours or more, which holiday lighting always is. Article 210 then requires the branch-circuit rating to be not less than 125% of the continuous load — the same rule expressed the other way round, and the origin of the 80% figure this page uses (1 ÷ 1.25 = 0.80). Article 590 covers temporary installations, including the ninety-day limit on decorative lighting for holidays and the requirement for GFCI protection on 125 V, 15 and 20 A receptacles used for temporary power.
The edition assumed here is the 2023 NEC. Adoption is by state and local jurisdiction and some areas are still on an earlier cycle, but the 80% continuous-load factor and the GFCI requirement for outdoor receptacles have been stable across editions. None of this replaces reading the tag on your own light strings, which carries the manufacturer's limits and any UL listing conditions.
Mistakes that cost you a trip back to the store, or worse
- Measuring the roofline along the ground. Gable rakes are longer than the wall below them. At a 6:12 pitch, multiply the horizontal run by 1.118 to get the rake length.
- Ignoring the end-to-end limit because the breaker has headroom. The limit protects the wire inside the first string, which carries every downstream string's current. The breaker cannot see that overheating.
- Assuming an outdoor outlet is on its own circuit. It usually is not. Find out what shares it before you load it.
- Using indoor extension cords outside. Outdoor cords are jacketed for UV, water and cold, and are rated for the current. An indoor cord in freezing rain is a different object by February.
- Skipping GFCI. Every temporary outdoor receptacle needs it. If the existing outlet is not GFCI-protected, use a GFCI adapter or an in-line protected cord.
- Buying by bulb count rather than by lit length. Two 100-light strings can be 25 ft and 50 ft depending on spacing. For a roofline, the feet are what matter; for a tree, the lights are.
- Rounding down on the circuit limit. The 80% figure is a ceiling, not a target. Leave real headroom for whatever else is on the circuit.
Related tools and where this stops
This calculator sizes and prices the lights; it does not design the electrical installation. If your display needs more than one or two circuits, or a dedicated outdoor receptacle, that is work for an electrician and it falls under the permanent-wiring articles of the code rather than the temporary ones.
The running-cost half of this page is the same arithmetic as any other plug load, so if you want to compare the display against the other things on your bill, the appliance energy cost calculator and the electricity bill calculator use the identical kW × hours × rate structure with your tariff's tiers applied.
Most of the injuries in holiday lighting come from the ladder, not the electricity. Before you go up, the ladder length and angle calculator gives the working length and the 4:1 setback that OSHA and ANSI both specify for an extension ladder against a gutter.
The rest of the seasonal-home arithmetic lives nearby: the snow melt salt calculator for the path under the lights, the gift wrap paper calculator for the indoor half of the season, and the moving boxes needed calculator if this year's display is going into storage rather than up.