What is the minimum unit load when sizing feeder conductors for lighting and convenience receptacle load per square foot?

The minimum unit load for sizing feeder conductors for lighting and convenience receptacle loads is defined in the National Electrical Code (NEC) guidelines. According to NEC, a common standard for calculating the load in commercial buildings is 15 volt-amperes per square foot. This value provides a reasonable estimate to ensure that electrical systems can sufficiently handle the typical demands of lighting and receptacles, ensuring both safety and reliability.

When calculating feeder sizes, using 15 volt-amperes allows for a practical approach to accommodate typical lighting schemes and the usual appliance loads that may be plugged into receptacles. Adopting a higher unit load could lead to unnecessary oversizing of conductors and equipment, thus incurring additional costs, while a lower unit load might risk underestimating the required capacity and lead to potential overloads or failure to meet the load demand.

The other options provided would either exceed what the NEC suggests for general applications or fall short of providing an adequate margin for expected electrical loads in typical scenarios. Choosing the correct value ensures compliance with industry standards and promotes safe electrical system designs.