How do you size a breaker for a continuous lighting load?

Multiply the continuous load by 1.25 and pick the next standard breaker at or above that number. The rule is Section 210.20(A) of the 2023 NEC, and the same 125 percent factor applies to the conductor through Section 210.19(A)(1).

What counts as continuous

Article 100 of the 2023 NEC defines a continuous load as a load where the maximum current is expected to continue for 3 hours or more. Store lighting, office lighting, parking lot and site lighting, warehouse aisle lighting, and sign circuits are the classic examples. A residential bedroom light is not continuous. A dimmer that never gets touched in a commercial lobby is.

The 3 hour test is about the specific circuit, not the building. If you are unsure, treat it as continuous. Being conservative here costs one wire size and buys you a compliant install.

The calculation

Section 210.20(A) requires the rating of the overcurrent protective device to be not less than 100 percent of the noncontinuous load plus 125 percent of the continuous load. Section 210.19(A)(1) requires branch circuit conductors, before the application of any adjustment or correction factors, to have an allowable ampacity not less than the same figure.

Work an example. A retail space has 32 LED fixtures at 54 volt amperes each on a 277 volt circuit. That is 1,728 volt amperes, or 6.24 amperes. Multiply by 1.25 and you get 7.8 amperes. A 15 ampere breaker with 14 AWG covers it easily, and in practice the limit on that circuit is voltage drop and the number of fixtures, not the ampacity.

Now flip it. If you have a 20 ampere circuit and the load is entirely continuous, the maximum continuous load you may connect is 20 divided by 1.25, which is 16 amperes. That is the 80 percent figure everyone quotes. It is the same rule stated from the other direction.

The exception people miss

Section 210.20(A) has an exception. Where the assembly, including the overcurrent devices protecting the branch circuits, is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device is permitted to be not less than the sum of the continuous load plus the noncontinuous load. One hundred percent rated equipment exists, it is expensive, and it is almost never in a standard panelboard. Unless the equipment label specifically says 100 percent rated, apply the 125 percent factor.

Feeders follow the same math

Section 215.2(A)(1) applies the identical 100 percent plus 125 percent rule to feeder conductors, and Section 215.3 applies it to the feeder overcurrent device. Section 220.42 gives the general lighting load values by occupancy in Table 220.42(A), and Section 220.44 covers show window and track lighting. For a lighting panel in a commercial building you will run that 125 percent factor at the branch circuit, at the feeder, and again in the service calculation.

Field takeaway

Continuous load times 1.25 sets both the breaker and the conductor. Do not exceed 80 percent of the breaker rating with continuous load. Only skip the factor when the assembly is listed and marked for 100 percent operation.