Demand Factor and Load Calculations per NEC 220
Without demand factors, a 200-unit apartment building with 12 kW per unit would require a 2,400 kW service entrance — a massive transformer, bus duct, and switchgear installation costing millions of dollars. That infrastructure would sit 75% idle because not all 200 apartments cook, heat water, and run air conditioning simultaneously. Demand factors capture this statistical reality: the mathematical certainty that connected loads never all operate at maximum simultaneously. NEC Article 220 codifies decades of utility metering data into standard demand factor tables, enabling engineers to right-size infrastructure without compromising safety.
For dwelling units, NEC 220.42 provides demand factors for general lighting and receptacle loads: 100% of the first 3,000 VA, 35% of the next 117,000 VA, and 25% for the remainder above 120,000 VA. NEC 220.54 covers household electric dryers with a sliding scale based on the number of units: 100% for 1-4 dryers, down to 25% for 12+ dryers. NEC 220.55 addresses cooking equipment with Column A (up to 1¾ kW rated), Column B (1¾-8¾ kW), and Column C (most household ranges at 8¾-12 kW) — 8 kW demand for a single range, but only 28 kW for 12 ranges (out of 144 kW connected).
Commercial and industrial load calculations offer two paths: the standard method (NEC 220.40-220.60) and optional methods for specific occupancy types. The standard method applies individual demand factors to each load category — lighting (Table 220.42), receptacles (Table 220.44 — first 10 kVA at 100%, remainder at 50%), and motor loads (Article 430 sizing rules). The optional method for New Construction Commercial (NEC 220.87) uses metered demand data: the measured maximum demand plus 125% of any new loads added, with a minimum of 100% of the largest motor.
Multi-family housing calculations per NEC 220.84-220.85 provide optional demand factors that dramatically reduce service sizing. For buildings with individual apartment meters, Table 220.84 applies: 100% of the first 3 kVA + 35% of the next 117 kVA + 25% of the remainder × number of dwelling units, with an additional factor based on unit count. A 100-unit building with 8 kW per unit (800 kW connected) might calculate to only 280-350 kW of demand — enabling a 500 kVA transformer instead of a 1,000 kVA unit.
Commercial kitchen demand factors (NEC 220.56) apply to thermostatic equipment (ovens, fryers, griddles, warmers) in restaurants, schools, and hospitals. Table 220.56 provides demand factors by number of units: 1-2 units at 100%, 3 at 90%, 4 at 80%, 5 at 70%, falling to 65% for 6+ units. The key distinction is that only thermostatically controlled equipment qualifies — continuous-draw equipment like exhaust fans, refrigeration compressors, and warming lamps must be calculated at full load. Hospitals and healthcare facilities have additional considerations per NEC 517.
Proper demand factor application requires engineering judgment beyond the code tables. A data center, for example, should not apply traditional office building demand factors — its loads run at near-maximum continuously (load factor approaching 1.0). Industrial facilities with process heating or arc furnaces may have diversity in their motor loads but sustained demand on heating elements. The engineer must understand the facility's operational profile: shift schedules, seasonal patterns, and future expansion plans all influence the appropriate demand factor. NEC demand factors represent statistical minimums — applying additional reduction beyond code tables requires documentation and client approval.