No efficiency difference that would be appreciable. Obviously you can run more wattage off one receptacle.
Its not advisable to “overwire” since your wire only needs to be rated for your breaker and outlet. Running 10AWG on a 20A circuit for instance is just a waste of your money. Copper is not cheap. Its more advisable to have as many branch circuits as your panel will fit.
If you need a nema 6-15R then thats what you shoukd install. The wire gauge depends on length of branch circuit, type of insulation on wire, and the environment it is installed in.
Running a larger wire runs cooler due to less resistance. Is it more efficient to run a large wire? Yes, is that efficiency measurable? Not really. Only reason to install a larger conductor if you expect the load to grow or change at a later day.
The wire should be able to sustain at least 15% overload because that subtle of a fault will not trigger the circuit breaker immediately.
That is a waste of copper. You don´t need every light on a seperate breaker and not every wall plug needs its own breaker either. Having seperate ones for IT-equpipment is recommended for offices though.
Yes, and I misled you on what I said. The code doesnt say 20%. It says you can only use 80% of your Over Current Protection with a continuous load and your wire must be sized to 100% of the OC protection. so for a given 20A circuit you’re only supposed to use 16A of it.
I use the word continuous because their definition of continuous dictates how much you can derate all the breakers in the panel for the actual load. The breakers in the panel when added up will be greater than the main breaker. Obviously you’re not going to use everything all at once so there is some calculation on derating circuits. Thats how I came up with that 7 15A outlets.
Most of my time spent earning the degree I have was in the electrical code book. Theres some stuff in there I agree with, and some I dont. I think their formulas for derating panels are a little silly but in the end more home runs to the main panel just means less work down the road should someone decide to add on. IMO 3-4 devices per breaker is ideal.