"Mechanical Contractor" means a contractor whose services are unlimited in the execution of contracts requiring the experience, knowledge, and skill to install, maintain, repair, fabricate, alter, extend, or design, when not prohibited by law, central air conditioning, refrigeration, heating, and ventilating systems, including duct work in connection with a complete system only to the extent such duct work is performed by the contractor as is necessary to make complete an air-distribution system, boiler and unfired pressure vessel systems, lift station equipment and piping, and all appurtenances, apparatus, or equipment used in connection therewith, and any duct cleaning and equipment sanitizing which requires at least a partial disassembling of the system; to install, maintain, repair, fabricate, alter, extend, or design, when not prohibited by law, piping, insulation of pipes, vessels and ducts, pressure and process piping, pneumatic control piping, gasoline tanks and pump installations and piping for same, standpipes, air piping, vacuum line piping, oxygen lines, nitrous oxide piping, ink and chemical lines, fuel transmission lines, and natural gas fuel lines within buildings; to replace, disconnect, or reconnect power wiring on the load side of the dedicated existing electrical disconnect switch; to install, disconnect, and reconnect low voltage heating, ventilating, and air conditioning control wiring; and to install a condensate drain from an air-conditioning unit to an existing safe waste or other approved disposal other than a direct connection to a sanitary system. The scope of work for such contractor shall also include any excavation work incidental thereto, but shall not include any work such as liquefied petroleum gas fuel lines within buildings, potable water lines or connections thereto, sanitary sewer lines, swimming pool piping and filters, or electrical power wiring.