Importance of an Accurate Electrical Load Calculation
An electrical load calculation is necessary when sizing branch circuits, protection devices, distribution boards, feeder circuits and service equipment. As previously mentioned, undersized components cannot accomplish their function safely, but oversized components waste money.
When the electrical load is known accurately, Con Edison and the building owner both save money by installing the optimal capacity. Consider that connection costs are split - the owner assumes the cost of all components behind the meter, while Con Edison is responsible for connecting the meter to the power grid.
- Appliances and equipment with electric motors are normally the largest loads in buildings, and this applies for both homes and businesses. The compressors of air conditioning systems typically have the highest power consumption in residential and commercial buildings. Other large loads that are driven by motors include ventilation and pumping systems.
- Lighting is often the second-largest electrical load after motor equipment, especially in commercial buildings with long schedules. The NFPA 70 National Electrical Code specifies a design load of 3 watts per square foot, even when using lighting fixtures that consume less power, such as LEDs.
For design purposes, the electrical load of a building is not simply the sum of individual loads. Electrical engineers use a metric called the coincidence factor to describe the total load of a building, which is the maximum total load divided by the sum of individual loads. For example, if the individual loads add up 500 kilowatts, but the peak electrical consumption is 350 kW, the building has a coincidence factor of 0.70.
Adding all the loads directly would result in an oversized system, since some loads are never used simultaneously.
- For example, it would make no sense to add all air conditioning and space heating loads, since they are used at different times of the year. Some buildings may use air conditioning in some areas even when others are heated, but the two systems will not operate together at full capacity.
- Power outlets are another example: Buildings have a large number of receptacles for convenience, but using them all at full capacity at once is very unlikely.
To get an accurate electrical load calculation, the best recommendation is working with a professional engineering firm. This way, you can avoid the risks of using undersized components, while making sure you don’t spend on oversized systems either.
Electrical load calculations must follow the NFPA 70 National Electrical Code and the NYC Electrical Code. Meeting these requirements is important, to ensure a quick project approval with the NYC Department of Buildings. You will need an electrical load letter to file a service connection with Con Edison, and load calculations are a necessary step.
An electrical load calculation is not only required in new constructions, but also renovations and additions in existing buildings. These projects change the electricity consumption of a building, and modifications to the electrical service may be necessary.