The National Electric Code (NEC) provides guidelines for calculating electrical loads and determining wire size. NEC 210.19(A) states that the minimum wire size for branch circuits, which supply power to individual devices or equipment, must have an ampacity of not less than the non-continuous load plus 125% of the continuous load. The ampacity of the wire must not be less than the ampacity of the overcurrent protective device protecting the conductor.
Branch circuits shall be calculated on the basis of a noncontinuous load of 125 percent of the continuous load plus the noncontinuous load. NEC 210.19(A)
To calculate the load, you must first determine the total wattage of the devices or equipment that will be powered by the circuit. This information can typically be found on the device specifications or nameplate. To determine the ampacity, divide the total wattage by the voltage (usually 120 volts). This will give you the amperage load.
Once you have calculated the load, you can use NEC 310-16 (Size of Conductors for Individual Conductors and for Multi-Conductor Cables) to determine the appropriate wire size. The table provides wire size options based on ampacity and conductor temperature rating. It is important to choose a wire size that meets both the ampacity and temperature requirements.
It is also important to note that local building codes may have additional requirements or variations to NEC codes and tables. Before beginning any electrical work, it is important to consult both the NEC and local building codes to ensure compliance and safety.