Electrical – How to mitigate 12 AWG to 14 AWG transition at GFCI

Yes it is sufficient.

A 15A receptacle is designed such that an appliance with a 20A plug should not be able to plug into it. This means that at most under normal operating conditions of a 15A appliance, that the receptacle itself will only draw 15A. If the circuit shorts then the breaker will kick at 20A however.

The tab of a 15A receptacle is designed to handle a 20A load.

EDIT for clarification above: A duplex 15A receptacle contains a tab that connects the hot lead to the hot load. This allows the receptacle to carry the load of another outlet by wiring the lead of Outlet(2) to the load of Receptacle(1). When you break off this hot tab then you are specifically wiring for a switch to control only of the receptacles of the outlet, or you are specifically wiring for each receptacle to have its own circuit on a shared neutral. For two circuits to be wired to a single receptacle on a shared neutral, then you need to ensure that the breakers are on different service legs or you risk overloading the neutral. If you also break the neutral tab then you are wiring the receptacle to have two separate circuits each with its own dedicated neutral. These should be considered fringe cases for residential so it is probably best not to break the tabs if not needed so as to avoid confusing future DIY'ers and licensed electricians. Every time a DIY'er or electrician gets confused then they are potentially risking themselves.

The NEC states that this is acceptable as long as:

1. The wire is 12 AWG sized for a 20A circuit

2. There is more than one receptacle on the circuit.

It is important to note that under no circumstances should you install a single 15A receptacle onto a dedicated 20A circuit. This is against code.

210.21(B) Receptacles (1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.

(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or where larger than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.

Table:

Circuit Rating (Amperes)
Receptacle Rating
(Amperes)
15 Not over 15
20 15 or 20
30 30
40 40 or 50
50 50

In conclusion, a standard 15A receptacle that one would buy in your hardware store is quality tested and approved to comply with all NEC codes to be legally sold in the United States. If the tab couldn't handle a 20A load without overheating then they would call this out specifically.

No, Code says nowhere that a receptacle-type AFCI or GFCI is required; breaker-type GFCIs and AFCIs are also usable whenever AFCI or GFCI protection is called for by the NEC or otherwise desirable. In fact, circuit breaker manufacturers now offer a device that's both a GFCI and a CAFCI in the same package -- look for a DFCI (Dual-Function Circuit Interruptor) breaker.

As to your clarified question (cites from the 2014 NEC):

The refrigerator receptacle and the dedicated microwave receptacle do not require GFCI protection, as those receptacles are not installed to serve the kitchen countertop surfaces, and likely are more than 6' from the edge of your kitchen sink as well.

The DW circuit, however, does require a GFCI, even though it is hardwired, as per 210.8(D):

(D) Kitchen Dishwasher Branch Circuit. GFCI protection shall be provided for outlets that supply dishwashers installed in dwelling unit locations.

Note the word "outlets" in this passage, vs. the use of the term "receptacles" in 210.8(A) -- in Code parlance, an "outlet" is a place where power is tapped from a circuit to serve a utilization device, whether it be a hardwired device, a luminaire, or a cord-and-plug connected device, while a "receptacle" is what you plug cords into.

The disposal does not require GFCI protection either, unless its receptacle is within six feet of the top inside edge of the sink bowl "as the cord flies". (In other words, the receptacle, while not covered by 210.8(A) point 6, may fall under 210.8(A) point 7.)