Ampacity is a combination of two words: "Amps" and "capacity." The amperage draw of every motor is different. A small 1/4 horsepower motor draws significantly fewer amps than a 200 horsepower unit. According to Siemens Electrical, contactors are matched to the rating of the load. . A complex series of equations is utilized by electrical engineers to determine the final ampacity rating of the starter.
Another criteria when a starter is recommended is regulatory compliance. In the United States, the National Electrical Code (NEC) is the overriding regulation. This regulation defines how a starter is manufactured, and what the load handling capabilities of a particular starter is. On the surface, it may seem simple to adhere to regulations. However, starters manufactured outside of the United States may be substandard. For example, in California, the Bay Area Rapid Transit Authority (BART) states that starters and contactors must adhere to the California Electrical Code.
The National Electrical Manufacturer's Association (NEMA) and the International Electrotechnical Commission (IEC) are two organizations that also define the standards for starters. NEMA is predominantly in the United States, whereas the IEC is worldwide. Both organizations do not work by themselves in a vacuum. They read the regulatory statutes of each nation, and issue recommendations accordingly.
Issuing the final recommendation is a multistep process. First, the electrical engineering team determines what the ampacity requirements are. Second, they go about looking for a starter that meets the criteria. Engineers then draw up blueprints, also called schematics, for the electrician to wire the motor in. The engineer must state on the blueprints that all starters must adhere to the NEC, NEMA and IEC standards. The electrician doing the installation must ensure the starter is in compliance with the ratings and the regulatory regulations.