The UL Directory under the subject of receptacles includes the following statement:
"Screwless terminal connectors of the conductor push-in type (also known as "push-in-terminals") are restricted to 15 amp branch circuits and are for connection
with No. 14 AWG solid copper wire only.
They are not intended for use with aluminum or copper-clad aluminum wire, No. 14 AWG stranded copper wire, or No
12 AWG solid or stranded copper wire."
