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2017 NEC 210.23.D says:

(D) Branch Circuits Larger Than 50 Amperes. Branch circuits larger than 50 amperes shall supply only nonlighting outlet loads.

What's bad about making light with more than 50 amps?

Surely the NEC isn't concerned with how high your electric bill would be. And surely they aren't concerned about the >6000 Watts of heat that'll be generated since apparently high amperage heaters are fine by this paragraph.

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  • Really 12,000 W because if you're looking at that much current, might as well jump to 240V and get twice as much on the same wires. But seriously, even before modern LED lighting, that's an awful lot on one circuit. And your wire size has to match, so why bump to 6 AWG (or whatever) when multiple circuits of 12 AWG will do the job for a lot less. And if you really need to control everything at once, wire up a switch that uses relays to turn on everything else. Or put all the lighting in a subpanel with a main breaker (switch rated...) to turn on/off everything at once.
    – manassehkatz-Moving 2 Codidact
    Commented Dec 5, 2021 at 4:42
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    And also consider the incandescent bulb - it offers a changing resistance on initial start so is that 50A at start or 50A when running stable?
    – Solar Mike
    Commented Dec 5, 2021 at 8:45
  • @SolarMike the amps of a branch circuit is the amps of its breaker. This accounts for startup load. A 50A circuit can support up to 40A continuous load, including whatever momentary startup inrush or LRA current it may have.
    – Harper - Reinstate Monica
    Commented Dec 5, 2021 at 9:37

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Nobody runs 50A/120V circuits, because you get munched by voltage drop, and it's easier to run a 25A/240V circuit once you're into that much power. So a 50A circuit is 12,000 watts not 6000.

"Outlet" does not mean receptacle. It means any point-of-use, plug-in or hardwired, which is not a subpanel.

Code here is not really thinking about one giant light that's over 12,000 watts. Code is talking about putting multiple lights in a building on one circuit. (think: a big-box store, with certainly more than 12KV of lighting). It is saying that you must break the lighting into multiple circuits so no one circuit is over 50A.

This is echoed in "tap conductor" rules, which say you can come off a commercial lighting circuit with smaller taps to feed individual fixtures. (that doesn't apply to residences anyway).

As well as the UL Listing of the commercial lighting fixtures, which will call out a maximum breaker of certainly under 50A. If it said over 50A, UL would not certify it.

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  • Thanks for this answer. However I still don't understand what's "bad" about high amperage lighting circuits. What's "bad" about even a single 12kW light constructed (safely) from, say, a graphite rod on some 6 AWG wire? I'm not actually going to do that (even though it would be fun). But is it only for pragmatic reasons that I shouldn't do this? It just seems odd that the NEC would prohibit something merely because it's impractical or expensive. And it's also odd that apparently a 12kW heater is fine--as long as it doesn't glow too brightly ;)
    – Matt Thomas
    Commented Dec 6, 2021 at 3:20
  • @MattThomas There's nothing wrong with a single >12KW light. It must be UL Listed (NEC 110.2). UL approves the instructions as part of the UL listing, and you're required to follow those instructions (NEC 110.3). They will say e.g. "use 70A breaker" which you must do. It will definitely be hardwired. What NEC is saying is you can't put multiple smaller lights on a >50A circuit.
    – Harper - Reinstate Monica
    Commented Dec 6, 2021 at 4:38
  • "There's nothing wrong with a single >12KW light." Ahh, it's "210.23 Permissible Loads, Multiple-Outlet Branch Circuits" not "210.22 Permissible Loads, Individual Branch Circuits". Sorry for being dense. So is the NEC saying it's a bad idea to put a hundred 15 amp light fixtures on a single 1500 amp circuit because there's effectively no fault protection for the individual fixtures?
    – Matt Thomas
    Commented Dec 6, 2021 at 13:09
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    @MattThomas Yeah. Because a 1500A breaker will take 9-35 seconds to trip at 3000A, and is not even guaranteed to trip until 2025A. I don't think you want to bring 1500A wiring to each lamp so you'd be relying on the tap rules... and so one of the 15A lights could be shorting out by 100x its current rating, and the breaker wouldn't care for awhile if at all. Yes, anything 50A or larger will have a dedicated circuit, with breaker reasonably matched to the load.
    – Harper - Reinstate Monica
    Commented Dec 6, 2021 at 20:04

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