what gauge extension cord for generator

{"cid":"91181013","title":"Universal Generator Auto Retractable Extension Cord Reel , Flexible Power Cable","imgUrl":"https:\/\/img.chinax.com\/nimg\/83\/5d\/93602c5aae8a60a81ac321ba411d-200x200-1.jpg","attrs":{"Price":"Negoation","Brand Name":"SFC","Model Number":"SFC-GFCI-02","Minimum Order Quantity":"100PCS","Delivery Time":"25Days","Certification":"ETL","Place of Origin":"Jiaxing,China","Payment Terms":"T\/T, LC","Supply Ability":"10000PCS Per Month","Packaging Details":"every one into plastrics poke and box. 5pcs into paper CTN.","Material":"ABS , PA66 UL94V0","Rating":"20a,125vac, 30A 125VAC","outlet quantity":"20A 125VAC GFCI duplex outlets","Color":"Black|Yellow","Power Cord Length":"15 ft","Cord":"UL approved 10AWG"}}