Determining the electric load of a building is an important part of designing and maintaining an electrical system. The electric load is the total demand for electricity placed on the system by all equipment, lighting, and other electrical devices operating in the building. A properly sized electrical system is critical for providing adequate power safely and efficiently. Calculating the electric load accurately helps avoid issues like tripped circuit breakers, dimming lights, and damage to equipment from underpowered supply. Here is an overview of how to calculate the electric load for a building.
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Types of Electric Loads
There are two main types of electric loads to consider when determining the total load on a building:
Continuous Loads
Continuous loads are those that are always in operation or expected to run for long periods of time. Examples include:
- Heating, ventilation, and air conditioning (HVAC) equipment
- Refrigeration
- Machinery
- Computers and electronics
- Lighting
Non-Continuous Loads
Non-continuous loads operate intermittently, for shorter periods of time. These include:
- Appliances like microwaves, toasters, etc.
- Office equipment
- Water heaters
- Electrical outlets
Non-continuous loads often require higher power for shorter durations than continuous loads. Both types of electric loads must be accounted for in load calculations.
Factors Affecting Electric Load
Several key factors determine the electric loads in a building:
Building Size and Function
Larger buildings require more power, as do facilities with equipment like industrial machinery and data centers. High-rise buildings also tend to have greater electric loads due to more extensive lighting, ventilation needs, elevator systems, and water pumping requirements.
Occupancy
The number of people using a building affects the electric load. More occupants equate to additional lighting, equipment operation, appliance use, and HVAC demand.
Geographic Location
Facilities in extremely hot or cold climates require more power for heating and air conditioning. Humidity and solar radiation levels also impact HVAC runtime and sizing.
Efficiency of Electrical Devices
Equipment age and energy efficiency rating determine the power draw. Newer, Energy Star rated devices typically consume less electricity than outdated, inefficient models.
Future Expansion
Projected facility additions and increased electric demand should be incorporated into load calculations to avoid under-sizing the electrical system.
Steps for Calculating Electric Load
Determining electrical load typically involves these major steps:
- Identify all electrical equipment, devices, and lighting.
- Determine power ratings for each load.
- Estimate demand factors based on usage patterns.
- Multiply the power rating by the demand factor for each load.
- Sum the individual electric loads.
- Incorporate allowable demand increase.
Going through this process methodically for all areas of the building ensures no part of the electric load is overlooked. The specifics of each step are explained in more detail in the following sections.
1. Identify All Electrical Loads
Creating a comprehensive list of all equipment, devices, and lights drawing power in the facility is crucial. This inventory can be developed by:
- Conducting thorough site surveys of all rooms and spaces.
- Reviewing building plans and electrical schematics.
- Interviewing facility managers and maintenance staff.
- Referencing equipment schedules and lighting plans.
- Inspecting nameplate data for power ratings.
Every electrical system and component should be included, from major HVAC motors down to small appliances and outlet loads. Missed loads can skew load calculations, so the inventory must be complete and accurate.
2. Determine Power Ratings
The electric power demand for each load must be known to estimate the overall load. Power ratings can be obtained from:
- Nameplate or equipment data – lists voltage, amps, watts, etc.
- Manufacturers’ specs and submittals.
- Minimum circuit ampacities.
- Historical data and meter readings.
- Assumed average power draws for lights and outlets.
Power is commonly rated in watts or kilowatts. Carefully identify and record the ratings for every item on the load inventory list for the next calculation step.
3. Estimate Demand Factors
Demand factors account for the fact that different types of loads operate at different times and durations. For example:
- Lights are typically on around 12 hours per day.
- HVAC units cycle on and off throughout the day.
- Motors and equipment run variably depending on production.
Typical demand factors fall in the range of 20% to 100% based on hours of expected operation. Historical meter data can provide guidance. Apply appropriate demand factors to each load item to represent realistic usage patterns.
4. Multiply Ratings by Demand Factors
For each piece of equipment, device, and light, multiply the power rating determined earlier by the corresponding demand factor percentage (converted to decimal form). This calculates the estimated actual power demand for each electric load.
Example:
- Rooftop AC unit rated at 10 kW capacity
- Estimated 50% demand factor since unit cycles on and off
- Calculation: 10 kW x 0.50 = 5 kW estimated actual demand
Performing this calculation for every item develops a truer picture of the electric load profile compared to just summing the nameplate ratings.
5. Sum the Loads
Add up the estimated actual electric demand for all loads determined in the previous step. This provides the estimated total power the electrical system must supply to the building under typical operating conditions.
A sample calculation:
| Load Description | Power Rating | Demand Factor | Estimated Electric Load |
| Rooftop AC Unit | 10 kW | 0.5 | 5 kW |
| Heat Pump | 15 kW | 0.2 | 3 kW |
| Exhaust Fan | 2 kW | 0.75 | 1.5 kW |
| Lighting | 40 kW | 0.8 | 32 kW |
| Outlets | 5 kW | 0.5 | 2.5 kW |
| Total Load | 44 kW |
Complete this for the entire building by sector if performing by hand. Load estimating software can automate and expedite the process for larger facilities.
6. Incorporate Demand Increase
Add margin to the total estimated load to account for future electrical needs. Recommendations vary by building type and often fall in the 10-25% range. Anticipated load growth, potential renovations, equipment replacements, and expanded occupancy should be considered. This helps ensure the electrical service is sized appropriately for the long term.
For the example above, adding 15% to the total load gives a final electric load estimate of 44 kW x 1.15 = 50.6 kW or 51 kW (rounded up).
Special Considerations
Several other factors come into play for more complex buildings and electrical distribution systems:
Voltage and Phase
Most commercial buildings are supplied with three-phase power. The distribution system configuration and line voltages impact how electric loads are analyzed and calculated. Power ratings and measurements must align with the supply voltage, and loads balanced across phases.
Load Diversity
For larger facilities with broader variety in load types, the peak demands will likely not occur all at once. Statistical diversity factors can be applied to the total load to account for this effect.
Power Factor
Certain equipment like motors and fluorescent lighting have reactive power draws that cause a lag between voltage and current. This must be incorporated into calculations through the power factor.
Harmonic Loads
Electronic loads can distort the voltage waveform and introduce harmonics. The reactive impact of harmonic loads may need to be considered in determining the building’s total power draw.
Generator Sizing
If the facility has backup generators, their capacity must match the estimated loads they will support. Critical and non-critical loads fed by generator power require separate load analysis.
Battery Storage
For buildings with battery energy storage systems, factoring in charging and discharging loads may require specialized calculations.
Conclusion
Estimating the electric load is vital for properly sizing electrical systems and equipment to meet a building’s power needs. By following the key steps outlined here and accounting for all usage factors, an accurate load calculation can be developed. While straightforward for simpler facilities, large or complex buildings may require more extensive analysis and modeling. Professional electrical engineers have the necessary expertise to evaluate variables like duty cycles, demand diversity, and reactive loads to determine precises loads and required capacity. Proper diligence during the load estimating process helps ensure electrical systems are designed correctly the first time, avoiding unnecessary costs and safety issues.