Is it legal to mix 12 AWG and 14 AWG on the same circuit?

Only on a 15 ampere circuit. NEC 240.4(D)(3) caps 14 AWG copper at 15 amperes, so introducing 14 AWG anywhere on a 20 ampere circuit is a violation.

What connector do I use to splice 12 AWG to 14 AWG?

Any splicing device identified for that combination under NEC 110.14(B). Check the listed combination chart on the wire nut package or use a push in connector rated per port.

Can a 15 amp receptacle go on a 20 amp circuit?

Yes, where two or more receptacles are supplied, per NEC Table 210.21(B)(3). That rule applies to device ratings and does not permit 14 AWG conductors on a 20 ampere circuit.

Can I splice 12 AWG to 14 AWG for a receptacle?

The splice is not the problem. The breaker is. Whether this is legal comes down entirely to the rating of the overcurrent device protecting the circuit.

The small conductor rule

Section 240.4(D)(3) of the 2023 NEC states that unless specifically permitted in 240.4(E) through 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG copper. Section 240.4(D)(5) sets 20 amperes for 12 AWG copper. Those are absolute caps applied after any ampacity adjustment or correction. The 14 AWG conductor does not care that it starts life spliced to a larger conductor. If it is on a 20 ampere breaker, it is unprotected.

So if the circuit is a 15 ampere circuit, splicing 12 AWG to 14 AWG is fine. You are simply using a heavier conductor than required for part of the run, often for voltage drop or because that is what was on the truck. If the circuit is a 20 ampere circuit, the 14 AWG section is a violation and there is no exception that saves it in a normal branch circuit. Fixture whips and tap rules do not apply to a receptacle branch circuit.

Making the splice correctly

Section 110.14(B) requires conductors to be spliced or joined with splicing devices identified for the use. A wire nut is listed with a combination range printed on the package, and 12 AWG plus 14 AWG has to fall within a listed combination for that connector. Section 110.3(B) makes that listing enforceable, so read the chart on the box rather than guessing by feel. Push in connectors are usually listed per port for a range of sizes and handle mixed gauges cleanly.

Section 300.15 requires a box or conduit body at each splice point, and Section 314.29 requires that box to remain accessible without removing any part of the building. Section 314.16 governs box fill, and mixing conductor sizes means counting each conductor at the volume in Table 314.16(B) for its own size.

Where the device sits

Section 210.21(B)(3) and Table 210.21(B)(3) permit a 15 ampere receptacle on a 20 ampere circuit where two or more receptacles are on the circuit. That rule is about receptacle ratings, not conductor sizes, and it does not authorize 14 AWG on a 20 ampere circuit. The two questions get conflated constantly.

Field takeaway

Check the breaker first. On a 15 ampere circuit, splice 12 to 14 with a listed connector and move on. On a 20 ampere circuit, replace the 14 AWG section with 12 AWG or move the circuit to a 15 ampere breaker if the load allows.