Its been a long day so my mind is fried. I know this is a simple one but just to be sure.
Living room lights added in ceiling. Circuit is 20amp 12G and junctioned in attic, then ran to the lights.
Added wiring from junction boxes to lights is 15 amp 14 g… Either the wiring needs replaced or the breaker needs to be changed correct…
Yes, the general rule is that a #14 AWG conductor must be protected at not more than 15 amps {240.4(D)(3)} so in this case the OCPD would need to be changed to 15 amps. But there are exceptions, for example fixture wires tapped to a light fixture can be protected at greater ampacites. Even #18 AWG conductors can be tapped to a 20 amp circuit. Not sure that this would apply to this installation but this is good as a point of reference.
Good point to bring up…
I was not sure if the wires just fed the lights only or continued to feed the rest of circuit. If it was just one added fixture at the end or off a junction would it still need to be 12g or could you use 14 if it were just a single light??
The entire run needs to be sized for the OCPD. you cannot have the #14 on the 20 amp circuit, regardless of it’s position in the circuit.
The load on the fixture wires is finite. That is why the smaller conductors are allowed.
The load on the branch circuit conductors is limited only be the OCPD. Smaller conductors like the #14 in this example could overheat before the breaker tripped.
To determine if a conductor is rated as a fixture wire it would need to listed as per Table 402.3. As Jim stated the load on the fixture wire is limited by the fixture so smaller conductors are permitted to be tapped and protected by a larger OCPD. The load cannot exceed the values in Table 402.5.