Actually that is per NEC. I'm surprised it has not changed over the years and I've had the discussion with a couple of people on the NEC Committee. I don't know that it has ever even been recommended. I mean, what are you going to base load off of? I guess one way would be to base it of service size calculations at 3w/sq-ft....... So let's see; load on 15amp cicuit at 80%. means 12a x 120v = 1,440w. 1440w / 3w = 480.......... So, 1 15a circuit per 480sq/ft.............. Makes no sense to me.
Maybe I'll write a recommendation for the next cycle and see what happens.
I always thought the logic behind the NEC is that it may prevent people from using those cheap brown or white, extension cords.
Now talk about something that should be outlawed....