The 80% "rule" (which is actually 125% of a breaker's rated capacity) applies to continuous maximum loads (like pumps and motors). Induction units are a non-continuous load, which is why an 1800W unit is supplied with a NEMA 5-15P molded plug, and can be used on a 15A circuit. Note this does not apply in Canada, where the rule is different. Many manufacturers (including Hatco) offer a 1440W version with a NEMA 5-15P plug in order to comply. Just to confuse things, California allows the use if two exempt pieces of equipment at up to 1500W each without an exhaust hood. Induction units are on the exempt list, hence the Canadian model is often used in California also in such applications.