You seem to have changed the discussion from what the code allows you to do, to what would be the best way to design a circuit. This is two separate issues.We look at multi-outlet branch circuits differently you build your branch circuit conductors based on the load in this discussion a 10 amp load.
I build a multi-outlet circuit based on the rating of the circuit.
We do disagree on what a 15A circuit is. 210.3 says the "rating" of a branch circuit is "in accordance with the maximum permitted ampere rating or setting of the overcurrent device." There is nothing there about the ampacity of the conductors. A 15A c/b with 10A conductors, a 15A c/b with 15A conductors and a 15A c/b with 25A conductors are all 15A branch circuits.General purpose multi-outlet lighting circuit.
The one point you would most likely not agree is in 210.23 (A) it says a 15 amp circuit. That to me includes a circuit that has conductors rated at 15 amps Min. Without any initial consideration to what size load you put on it. After I build the circuit do not violated the rules , I build the circuit and limit the loads,
Because this is exactly what 210.19(A)(1) tells me to do.You build the circuit to handle the load. If t is was an individual branch circuit I build it the way you do.
Let's try another example and see how it fits into your method. Let's says we had six (6) 15A lighting circuits in the same raceway (12 ccc's.) The load on each circuit is 12Amps continuous.After I build my 15 amp circuit with 12 CCC in conduit I will increase my conductors to 12AWG THHN And put your 10 amp load on it. Someone wants to add a lighting load to my circuit throw an amp probe on the circuit decide if you can add the lighting load to my 15 amp general purpose lighting branch circuit.
You built your circuit with 10 amp rated conductors my amp probe doesn?t tell me if i can add a load on your 15 amp general purpose branch circuit
210.19(A)(1) tells me that the minimum branch conductor size, before any adjustment factors, must have an ampacity that is not less than 125% of the continuous load: 12*1.25=15. Therefore, #14 awg is my minimum conductor size.
210.20(A)(1) say the OCPD shall be not less than 125% of the continuous load, so a 15A c/b is OK.
I plan on using #14awg THHN (25A at 90degc) as my conductor. T310.15(B)(2)(a) says that I must adjust the ampacity of my #14 THHN because I have 12 ccc's in the raceway. Therefore, the ampacity of my conductors is 12.5 (50% ajustment factor.)
Per 210.19(A)(1), the 12.5 ampacity of my #14 THHN (12.5) is not less than the maximum load to be served (12), so the #14 is acceptable. Per 240.4(B), the next higher standard overcurrent device is allowed, so the 15A c/b is acceptable for the #14 THHN. Per 240.4(D)(3), 15 Amp is the max OCPD for #14 copper, so we are compliant with this section as well.
By your definitions, we do not have a proper 15A circuit because the ampacity of the conductors is only 12.5A. But the circuit is already fully loaded, so no additional load can be added to the circuit. Why increase the conductors to #12 to get a 15A ampacity conductor when the #14 is properly sized, properly protected, and no additional load can be added?