Sizing Motor Disconnects (2014-02-24)

Motor disconnects are simple devices used to open a circuit providing power to a motor. Disconnects are rated by amperage, fused or non-fused, and enclosure rating. Typical amperage ratings for a disconnect switch are 30, 60, 100, and 200 amp frames and higher. To properly size a disconnect for the circuit, we need to find the motor’s full load amperage (FLA). Once the FLA is obtained, a designer should consult with the National Electric Code. 2012 NEC section 430.110.A states “The disconnecting means for motor circuits rated 1000 volts, nominal, or less shall have an amperage rating not less than 115 percent of the full-load current rating of the motor.” For the purpose of this blog, I am going to use the general code. For those attempting to size a disconnect for a specific circuit, please consult the NEC for any additional requirements or exceptions to your specific circuit (especially fire pump motors). Based on the general statement of the NEC, we will multiply the motor’s FLA rating by 115% for the minimum rating required of  a disconnect. Since manufacturers mass produce equipment, we round up 115% of the FLA to the next available disconnect size i.e. the 30A, 60A, 100A, …
The next step is to figure out the requirements of fused or non-fused disconnect. Some may wonder why do we need fuses if the circuit is on a circuit breaker. The answer is that the circuit breaker providing power to the circuit is designed to protect the circuit (conductors, transformers, panels, etc.). Fuses are designed to protect the equipment. The answer to whether a fused disconnect is required lies in the vendor’s power requirements. Does the vendor require a fused circuit? What ampacity fuses are required? Does the vendor provide equipment overcurrent protection that doesn’t require a fused disconnect?
The last step in sizing a disconnect is to find out where the disconnect will be located. Typical locations maybe indoors, outdoors, corrosive environment, hazardous environment, etc. Please refer to the NEMA enclosure ratings and identify the specific environment that the disconnect will be mounted.

Although I am making an attempt to simplify the process and give the general public an idea of the inner workings of electrical engineering, please consult with a professional engineer for sizing and selecting equipment and materials for electrical circuits.