Electrical Load Calculator

A service load calculation asks how much power a dwelling will actually demand at once, not how much equipment it contains. NEC Article 220 answers this with demand factors that encode diversity. General lighting and general-use receptacles are computed at 3 volt-amperes per square foot of floor area measured from outside dimensions, and the two required 1,500 volt-ampere small-appliance branch circuits plus one 1,500 volt-ampere laundry circuit are added at full value. That general subtotal is then taken at 100 percent for the first 3,000 volt-amperes and 35 percent for the remainder, which is why a 6,900 volt-ampere general total becomes a 4,365 volt-ampere demand. A household electric range up to 12 kW counts as a flat 8,000 volt-amperes under Table 220.55, and heating and air conditioning are never added together because they cannot run simultaneously, so the larger of cooling at 100 percent or electric space heating at 65 percent is used. Dividing the demand total by service voltage, normally 240 volts for a single-phase dwelling, gives service amperes; three-phase services divide by voltage times the square root of 3. Typical results run 15,000 to 30,000 volt-amperes for an average single-family home against a connected load two to three times larger, so a diversity ratio near 40 to 60 percent is normal. The main limitations are that nameplate data must be real rather than estimated, and that newer concentrated loads such as electric vehicle supply equipment and heat pump backup strips can invalidate assumptions built into older demand factors.

Demand factors predate the modern code and come from utility practice. Electricity suppliers observed early that the sum of their customers' connected equipment vastly exceeded any peak they had to generate, and the ratio between the two became known as diversity. Translating that observation into building wiring rules gave designers permission to install a service smaller than the arithmetic sum of every nameplate, which is much of what made residential electrification affordable as appliance ownership grew through the twentieth century. The National Electrical Code collected these methods into Article 220, and its structure records the history of American household appliances. Dedicated small-appliance and laundry circuit allowances exist because those loads were repeatedly found to overload general lighting circuits. The flat 8,000 volt-ampere allowance for household ranges recognizes that surface units and the oven are thermostatically cycled and are never all at full output for long. The 65 percent factor on electric space heating against 100 percent on air conditioning simply states that a building does not heat and cool itself in the same moment. An optional dwelling calculation was added later, now in 220.82, using a simpler 100 percent of the first 10 kVA and 40 percent of the remainder, because the standard method proved conservative for typical homes. Recent cycles have had to address electric vehicle charging in Article 625 and energy management systems that cap simultaneous demand, since a single 48 ampere charger can add more load than an entire mid-century house.