I am replacing a front yard light post that has a GFCI outlet attatched. The old post has 12 gauge wire connected from the house thru the post and outlet to the the light. The new post is prewired with 18 gauge wire and also includes a GFCI outlet. Can I connect the old 12 gauge to the new prewired 18 gauge? there is a 20 amp circuit breaker. What is ETL?
The fixture is a listed device and manufacturers don’t have the same limitations for the wiring that electrical wiring in your home has. YES you can connect the 12 AWG wire to your light pole fixture. This is exactly the same as almost every light fixtures in your home mfg’s regularly use smaller wires because the load is limited. Note they have a maximum wattage listed for the light(s) when used according to the manufacturers instruction it is safe. So yes this is normal for a fixture.
Yes, that's perfectly fine. The reason is that the appliance is supplied this way, and it is listed by a competent NRTL, meaning that it follows (most likely UL's) rules for internal construction.
The rules inside equipment are different than the rules for in-wall wiring. For instance wiring inside equipment can exceed 90C if its insulation is rated for that, and if that won't damage the appliance. You can bet that part of the ETL testing included throwing 20A of load on that receptacle.
If you saw this in equipment that lacked an NRTL stamp and file number, then you'd want to take a hard look at it, and possibly a hard look at your purchasing practices. Keep in mind that NRTL listing is required for most parts of your mains electrical system (NEC 110.2 requires the local AHJ approve the equipment, and any sane AHJ will defer to the NRTLs.)
Luminaires are not one of them, however a thing can't have a GFCI receptacle and still claim to be a luminaire.