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This is in a two story, ~1000square foot per floor building. With the advent of affordable, decent (IMHO) LED fixtures (finally!) the entire lighting load for the structure looks to be 500W - 1000W at the outside - well below even the 1440VA that a paranoid or cautious soul might derate 15 amp lighting circuits to on the basis that they are (at least potentially, in the long dark winter) pretty much like a continuous load. Great news for my power bill.

There will be NOTHING but lighting on the lighting circuit (or circuits.) I've been on the other side of that, not going back. I have never personally seen a lighting-only circuit blow, though I'm sure it must happen - very occasionally. These are lights in the ceiling, not outlets/receptacles that could have "whatever" plugged into them.

Do I need to split lighting into several circuits to satisfy a code requirement, or not?

I may anyway, to ease the wild hare of "I suppose the lights could trip a breaker" but is there a code-driven need to? As far as I can see, from a practical point of view, all the lighting needs of my building in the modern age can be met with a single 15A 120V breaker.

Say, for instance, the one lighting one light at the panel right now. (which I've given a separate question.)

If you want to get picky about edition, I believe that 2011 NEC is the currently adopted code for the USA location.

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Are all the lights fixed LED lamps, or are you using cans with PAR screw-ins? If the latter, you have to consider the amount of watts the fixtures are rated for. If they are rated 100 Watts, that will necessitate separate circuits. –  Edwin Mar 18 at 3:13
    
Fixed LED. Some are can-type LED fixtures, but not screw-ins. I've ben far less happy with "LED Bulb Replacements" than "LED fixtures." –  Ecnerwal Mar 18 at 3:24
    
Sounds like you are doing quite a bit of electrical work, maybe it's time to invest in a copy of the National Electrical Code. –  Tester101 Mar 18 at 11:53
    
Having a copy and knowing where they hid everything are two different things. Especially when looking for things that might not actually be in there. As such, the brain trust here serves its purpose. –  Ecnerwal Mar 18 at 12:57

3 Answers 3

up vote 3 down vote accepted

As far as the NEC is concerned, there is no limit to the number of outlets (lighting outlets in this case) that are allowed on a single circuit assuming that the attached fixtures don't exceed 80% of the rated capacity of the circuit.

However, @Edwin raises exactly the correct point in the comment - what matters is what the fixtures are rated for, not what they are currently being used for. The general rule of thumb that electrical inspectors tend to use is around 8 boxes for a 15A circuit and 10-12 for a 20A circuit. Keep in mind that they are counting boxes, not what gets put into them. The idea is that you can put in a receptical rated at 5W, decide you don't like it, and throw something with a 150W load in the same receptical. The only thing that makes something a "light" in the eye of the inspector is that it is on the ceiling and has a switch running to it.

Outside of the load consideration you actually bring up a good point in the question, which is safety. While the NEC doesn't specify a minimum number of lighted branches (other than the panel lighting requirement), your local authority very well could. I know my jurisdiction only allows 2 rooms to share the same branch for lighting purposes, precisely for the reason you mention - you pop a breaker, you're in the dark.

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Article 210.70 of the 2011 NEC, tells you what lighting outlets are required.

National Electrical Code 2011

Chapter 2 Wiring and Protection

Article 210 Branch Circuits

III. Required Outlets

210.70 Lighting Outlets Required. Lighting outlets shall be installed where specified in 210.70(A), (B), and (C).

(A) Dwelling Units. In dwelling units, lighting outlets shall be installed in accordance with 210.70(A)(1), (A)(2), and (A)(3).

(1) Habitable Rooms. At least one wall switch–controlled lighting outlet shall be installed in every habitable room and bathroom.

Exception No. 1: In other than kitchens and bathrooms, one or more receptacles controlled by a wall switch shall be permitted in lieu of lighting outlets.

Exception No. 2: Lighting outlets shall be permitted to be controlled by occupancy sensors that are (1) in addition to wall switches or (2) located at a customary wall switch location and equipped with a manual override that will allow the sensor to function as a wall switch.

(2) Additional Locations. Additional lighting outlets shall be installed in accordance with (A)(2)(a), (A)(2)(b), and (A)(2)(c).

(a) At least one wall switch–controlled lighting outlet shall be installed in hallways, stairways, attached garages, and detached garages with electric power.
(b) For dwelling units, attached garages, and detached garages with electric power, at least one wall switch controlled lighting outlet shall be installed to provide illumination on the exterior side of outdoor entrances or exits with grade level access. A vehicle door in a garage shall not be considered as an outdoor entrance or exit.
(c) Where one or more lighting outlet(s) are installed for interior stairways, there shall be a wall switch at each floor level, and landing level that includes an entryway, to control the lighting outlet(s) where the stairway between floor levels has six risers or more.

(3) Storage or Equipment Spaces. For attics, underfloor spaces, utility rooms, and basements, at least one lighting outlet containing a switch or controlled by a wall switch shall be installed where these spaces are used for storage or contain equipment requiring servicing. At least one point of control shall be at the usual point of entry to these spaces. The lighting outlet shall be provided at or near the equipment requiring servicing.

While 220.12 gives you the lighting load per square foot for load calculations.

Article 220 Branch-Circuit, Feeder, and Service Calculations

II. Branch-Circuit Load Calculations

220.12 Lighting Load for Specified Occupancies. A unit load of not less than that specified in Table 220.12 for occupancies specified therein shall constitute the minimum lighting load. The floor area for each floor shall be calculated from the outside dimensions of the building, dwelling unit, or other area involved. For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Table 220.12

Since you're in a dwelling unit, you'll use 3 volt-amperes per square foot. However, this 3 VA also includes the required general use receptacle load as well. If each floor is 1000 sq.ft., that's 3000 volt-amperes per floor, or 6000 volt-amperes total for general lighting and receptacle loads.

3 volt-amperes per sq. ft. * 1000 sq. ft. = 3000 volt-amperes
3000 volt-amperes * 2 = 6000 volt-amperes

A 15 ampere circuit would support 1800 volt-amperes, while a 20 ampere circuit can support 2400 volt-amperes.

120 volts * 15 amperes = 1800 volt-amperes
120 volts * 20 amperes = 2400 volt-amperes

So for a 2000 sq. ft. dwelling unit, you'll need 4 15-ampere or 3 20-ampere branch circuits for general lighting and general use receptacles.

6000 sq. ft. / 1800 volt-amperes = 3.33333
6000 sq. ft. / 2400 volt-amperes = 2.5

Depending on the lighting fixture ratings, you could technically use 3 of the 15-, or 2 of the 20-ampere circuits for receptacles. Then you'd use the remaining circuit for all the lighting. But again, that depends on the fixtures being installed.

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Code doesn't just protect you. Is the next owner of your house going to want only LED lights everywhere?

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If an Incandescent-Loving Individual purchases the place and wants to change the fixtures, they can change the wiring to suit the fixtures they have changed. I see "inspector rule of thumb" referred to; I cannot find anything in code, and unless someone does, I don't actually care about the I.L.I. They like to throw money away, so they can hire an electrician at that point. From my point of view as a vernacular builder, this is one of the several ways that LED lighting saves money - If code really caught up to them, you could wire the things with 24 gauge; I don't expect code ever will. –  Ecnerwal Mar 18 at 18:23

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