Originally Posted by Bob Landry
...Unless some jack leg electrician installed the outlet incorrectly, it it going to be ,limited by the C/B to 15A...
Sorry to butt in, but I just stopped by...
C/B = Circuit Breaker in the above context.
It is completely acceptable per the NFPA National Electrical Code to install 15 amp duplex convenience receptacles on a #12 awg branch circuit protected by a 20 amp circuit breaker. In fact it helps to prevent low voltage conditions on 15 amp receptacles. It is therefore common practice for many residential and commercial 15 amp configuration duplex receptacles to be fed by a 20 amp protected branch circuit.
You are always better off using a higher amperage rather than a smaller amperage extension cord because voltage drop is greater for a lesser rated extension cord of a given length than a higher rated one. Low system voltage is detrimental to HVAC units, refrigeration, some electronics, and other higher load appliances.
If you are not familar with electrical systems a standard practice of using a minimum 12 awg temporary adapted extension cord to feed your trailer regardless of 15 amp or 20 amp branch circuit rating is the better practice.