Unit
- Load types: Lighting, receptacle, HVAC, and motor loads each have distinct demand characteristics. Lighting loads are continuous when occupied, while motor loads have starting surges lasting seconds.
- Demand factor: The ratio of maximum demand to total connected load, expressed as a decimal. For example, a panel with 100A connected but 70A demand has a factor of 0.7.
- Diversity factor: The reciprocal of demand factor, used to size feeders when loads are unlikely to operate simultaneously. A diversity factor of 1.2 means feeders can be sized for 83% of total load.
- Load categories: Continuous loads (operating 3+ hours) must be calculated at 125% for conductor and overcurrent sizing per NEC 210.19(A). Non-continuous loads use 100%.
- General lighting load: Based on floor area and occupancy type. For offices, NEC Table 220.12 specifies 1.5 VA per square foot for general lighting and 1.0 VA for receptacles.
- Motor loads: Use full-load current from NEC Table 430.248 or manufacturer data. Multiply by 125% for feeder sizing if a single motor, or use demand factors for multiples.
- HVAC loads: Include compressor, fan motor, and electric heat. Heat strips are often the largest load; use nameplate ratings and apply demand factor per NEC 220.82(C).
- Optional method: For dwellings and some commercial, NEC 220.82 allows simplified calculation using 100% of first 10 kVA plus 40% of remainder.
- Transformer sizing: The calculated demand load determines the minimum kVA rating. Add 25% future growth margin and account for voltage drop and harmonics. A 150 kVA transformer serving a 120 kVA demand allows safe overload capacity during short peaks.
- Receptacle demand: For general use receptacles in commercial buildings, NEC 220.14(H) permits 180 VA per strap for non-dwelling occupancies. Apply demand factors from Table 220.44 when more than 10 receptacles are on the same feeder.
- Three-phase balancing: Distribute single-phase loads evenly across A, B, and C phases to minimize neutral current and avoid overloading any phase. Use a load schedule showing each phase's amperage. The maximum phase imbalance should not exceed 10 percent of the average.
- Power factor correction: Low power factor increases current and conductor losses. For inductive loads like motors, add capacitor banks at the panel or motor starter. Target a power factor of 0.95 or higher to reduce demand charges and free up transformer capacity.
Code Example
Python
def calc_panel_demand(connected_loads, demand_factor):
total_connected = sum(connected_loads)
demand_load = total_connected * demand_factor
return demand_load, total_connected
# Example: panel with 3 circuits
loads = [1200, 800, 1500] # watts
df = 0.8
demand, total = calc_panel_demand(loads, df)
print(f'Total connected: {total} W, Demand: {demand} W')This function calculates demand load from a list of connected loads using a uniform demand factor. In practice, demand factors vary per circuit type; this simplifation is useful for initial estimation.
Quick Revision
- Load calculation begins with identifying continuous and non-continuous loads.
- Demand factors reduce connected loads based on usage patterns.
- NEC provides standard demand factors for lighting, motors, and HVAC.
- Diversity factor helps size feeders for non-coincident loads.