As far as I know, the NEC hasn't been updated to reflect LED replacement bulbs (and it probably shouldn't because they share base types with incandescent). In that case 10 lights on a 20A circuit is the max allowed (to get it inspected). 9 on a 15A circuit. Even though you could put a ton of LEDs on the same circuit.
I've never heard of that one and I used to design electrical systems on both commercial and residential (but that was long ago in a previous life...back in the 80's). Are you sure you're not thinking of outlets? We always assumed outlets to be 180W each and circuits are only allowed to be loaded to 80% capacity, so 120v x 20a x 80%= 1920w...or 10.6 outlets. When it came to lights though, we always loaded a circuit to 80% regardless of whether that was one light or fifty.
It's been a quite while since I read the information on this but I'll give it a try...but don't expect a lot of "technical words!"
It's because you're providing multiple places for lightning to enter the system. Say a bolt of lightning hits a tree immediately adjacent to the last light. That current (possibly in the millions of volts) rushes from the tree into the ground and can enter your grounding system since there is a ground wire at that light. The ground wires at the other lights aren't energized (because they're well away from the strike) so that sets up "current potential" (a difference in voltage) in your grounding system and that introduced voltage rushes down the system's ground wire towards the other fixtures whose ground wires are sitting there at 0 volts. The small wire at the first fixture is likely not enough to completely dissipate that (massive amount of) energy...it may take two fixtures...or twelve...before all the energy is dissipated. And that energy will likely damage each device it passes thru until it is completely dissipated.
Now, maybe you weren't going to run a ground wire with the circuits? That would be better, the lightning would likely only blow out the light it entered and possibly one or two others (because the transient voltage could very well jump from the grounding conductor to either the hot or neutral at the first fixture's box and still run up line.) I'm not sure individual ground that aren't tied together meets code though. I'm not saying it doesn't, I'd have to check, but it doesn't sound kosher.
It used to be that every parking lot light pole base was grounded on big commercial parking lots. They've gotten away from that for the reason described above. These are worse case scenario because they're big freakin' lightning rods sticking in the air inviting a strike. Low profile runway lights would be far less vulnerable.
Not grounding individual fixtures doesn't completely eliminate the problem, a lightning strike very near a fixture could still enter the wiring system that's direct buried. It's not like the cable's insulation is designed for lightning strikes. But it's less likely.
Summary: If you don't have a direct path for lightning to take into your wiring system at each fixture then you're far less likely to have transient voltage issues from strikes.
One other thing about lightning energy, it doesn't do 90 degree bends well, it'll arc off of the wire at a tight bend. Thus the ground wire should be kept as straight as possible and/or use very long sweeps when turning, especially the main ground from the panel to the ufer.
That only covers transient voltage that enters the system thru your own ground conductor(s). Lighting (and idiot neighbors) can create transient voltage that enters thru your service wires...both the hot and the neutral/ground. That's where a whole house surge suppressor comes into play...I've had one of those installed everywhere I've lived for at least 15 years now. They're a lot cheaper than replacing electronics (which are included most appliances these days).
I think all of that is pretty accurate but it's been awhile...
If I hard wire a runway lighting system, I plan to install wiring that has a grounding conductor (12/2 w/ a ground or 14/2 w/ a ground, or somesuch) and it will be grounded at the panel only. (Until someone shows me that I'm full of **** that is, which is quite likely!)
