We can calculate light loads for residential dwellings, to make a calculation of general - purpose branch circuits we will use a general unit load of 3 volt - amperes per square foot for one - and two - family, this is derived from NEC table 220.12. You can learn from Hyperlite.
What is the NEC? National Electrical Code (NEC) is a set of regularly updated standards for the safe installation of electric wiring in the United States. First published in 1897, the NEC is updated once every three years. The NEC provides guidelines for electrical installation in order to prevent fires and other electrical accidents. Electricians and contractors need to be aware of NEC where applicable to ensure that installations are safe, legal and up to code. The codes are a part of the National Fire Protection Association, a private trade association. Despite its authoritative positioning and national title, NEC standards are not federal law. Instead, NEC standards are either adopted by local governments or local governments create and enforce their own electric code. The differing codes in various states are therefore something for contractors and electricians to remain aware of.
Why are we talking about this? Because you will find some formulas on internet like this: “W=VA (this means watts = volts x amps) in most lighting situations, you will know two of the three elements. From there, you can use the formula to find out the third. Household circuits in the U.S. are 120 volts, although a few circuits may have 240-volt capacity, and most allow 15 to 20 amps.” But as we see “state” could have their own codes or standards that is why we need to be aware of it when we are planning to install lights.
There are also commercial electrical load calculations the NEC covers commercial calculations also in Art. 220 but as an electrician must know the definitions in Art. 100, be familiar with what Art. 210 says about continuous loads and understand Art. 240. Two items associated with this type of calculation repeatedly need clarification: The voltage to use for your calculations depends on the system design voltage. Thus when you calculate branch-circuit, feeder, and service loads, you must use a nominal system voltage and if it is different from the standard need to be specified and the second is rounding when the ampere calculation exceeds a whole number by 0.5 or more, round up to the next whole number.