"If we look at the owner’s manuals of some of these appliances we will find that plugging them into a 20 amp circuits is beyond the recommendations of the manufacturer."
the appliance does not care what is it plugged into, as long as the prongs fit.. as you stated, the OCP for the wire in the wall is the 20a breaker at the panel. as for the 18 ga cord, that is up to the appliance to be fused. 20a vs 15a, it's still going to blow that fuse in the radio/tv/whatever.
Section 110.3(B) of the NEC states that any appliance of piece of equipment must be installed by the instructions so to install an appliance on a 20 ampere circuit when the manual states for it to be installed on a 15 amp circuit is a code violation.
As far as the appliance itself one could use jumpers from the meter base and the appliance wouldn’t care as long as it had current flow. The overcurrent device is installed to protect the conductors not the appliance so it wouldn’t care if it was wired straight to the transformer. Believe it or not but that surge protector or plug strip with the radio/tv/whatever plugged into it don’t care either but the owner might when the insulation starts smoking. These devices are one of the leading causes of fire in our homes today so why put them to the test?
i guess i don't see your point. if 12 ga was 'incorrect' to use, it would not be able to be used for such a purpose, don't you think? our current house is wired the same way ( i didn't wire it, bought it like that in 2005, new build) and have had zero issues, except from where the 'rockers' got carried away with a rotozip and hit a wire i had to fix.
It would only by a violation if there was something in one of the manuals for one of the appliances that called for a 15 amp circuit but common sense plays a role here also. When using things like extension cords and plug strips the fact that 18 gauge conductors are only good for 7 amps and 16 gauge conductors are only good for 10 amps why would someone want to plug one into a circuit that has 25% more heat energy available? Could it be that they believe that that 20 amp circuit is somehow better?
" i just rewired my rental house. i used 12 ga in most receps, and 14ga for all lighting, outlets separate from lighting. if you are wiring a living room with 10 receps, with your tv/stereo, table lights, etc and then run a something like a vacuum, it gives you what, 600 more watts to play with? yeah, i'll take that, rather than string another dedicated line for a living room circuit.
What role does 10 receptacles play in this discussion? This is a dwelling unit not a commercial or industrial installation where the load is calculated on the device but is instead calculated on the square feet.
In commercial and industrial installations all duplex receptacles are to be calculated at 180 volt amperes each. The reason for this is that one will not be installed unless it is planned on being used. In a dwelling unit we must install receptacles per 210.52 at regular intervals. These devices are sometimes never used so there is no need to figure them at 180 volt amperes each but instead at 3 volt amperes per square foot.
All of this thinking of 20 ampere circuits for dwelling receptacles and separating the lights from the receptacles comes from commercial and industrial installations. The adage was passed from the commercial trade to the residential trade through commercial electricians that only did commercial and industrial installations then one day doing a dwelling and using what they had learned.
A quick look at 210.21 through 210.23 one will learn that if a 15 amp receptacle is being installed then the maximum load that can be plugged into it is 12 amps and it doesn’t matter if it is a 15 or 20 overcurrent device.
So what does one gain by installing 20 amp circuits and not installing 20 receptacles? Nothing. What does one gain by installing 20 amp circuits and 20 amp receptacles? The cost of the 20 amp receptacles are twice that of 15 amp receptacles. Their only gain is more cost and a waste of our natural resources.
So is the 20 amp circuit for a dwelling unit better than a 15 amp circuit? It depends on who you talk with. Those that I talk with say no the 20 amp circuit is more dangerous than a 15 amp circuit. These are some of the greatest minds in the electrical trade not some master electrician that has retired nor is it some unfounded wives tale that was started many years ago. It is those who are charged with writing, testing, and researching the codes and installations for electrical energy.