When wiring a house, it is all but impossible to know which appliances, lighting, heating, and other loads will be turned on at the same time. Different families lead different lifestyles. There is tremendous diversity. There is a big difference between “connected load” and “actual load.” Who knows what will be plugged into a wall receptacle, now or in the future? It’s a guess at best. Over the years, the NEC has developed procedures for calculating loads in typical one- and two-family homes.
The rules for doing the calculations are found in Article 220. For lighting and receptacles, the computations are based on volt-amperes per square foot. For the small-appliance circuits in kitchens and dining rooms, the basis is 1500 volt-amperes per circuit. For large appliances such as dryers, electric ranges, ovens, cooktops, water heaters, air conditioners, heat pumps, and so on, which are not all used continuously or at the same time, there are demand factors to be used in the calculations. Following the requirements in the NEC, the various calculations roll together in steps that result in the proper sizing of branch circuits, feeders, and service equipment.
Inch-pounds versus Metrics When Calculating Loads
Converting inch-pound measurements to metric measurements and vice versa results in odd fractional results. Adding further to this problem are the values rounded off when the NEC Code-Making Panels did the metric conversions. When square feet are converted to square meters and the unit loads are calculated for each, the end results are different—close, but nevertheless different. To show both calculations would be confusing as well as space consuming. Many of the measurements in this text are shown in both inch-pound units and metric units. Load calculations throughout this text use inch-pound values only, which is in agreement with the Examples given in Annex D of the 2011 NEC.
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