What is the difference between a commercial electrician and a residential electrician?

simple. residential electricians wire dwelling units (single houses, duplexes, small apartments, high rise apartments) most common wiring method is romex cable, but when getting into larger residential projects, MC will be used. most of the rules for placing devices comes from the NEC such as one receptacle no further then 12 ft and so forth. the voltage will almost never be above 240volts. in commercial the wiring methods used will be EMT, MC, PVC, and sometimes RMC. so a good knowledge of conduit bending is needed. also since commercial is the broadest field as far as structures (anything from a small fruit stand to strip stores, gas stations, grocery stores, banks, schools, shopping malls, sporting venues, to high rise skyscrapers) the wiring methods and applied voltages will vary. a typical house can be wired in a few days. commercial projects can last months to years.