What extension cord should I use for a block heater in extreme cold?

Get a 12-gauge cold-weather extension cord rated SJTW or SJOW. Normal cords turn brittle at -30°C, and a block heater’s 1000–1500 watts can melt undersized insulation. For runs up to 50 feet, 12-gauge copper is safe; 14-gauge only works for short garage runs. Also choose cords with tight-sealing ends to keep out snow and slush.