An electrical building services engineer needs an organized methodology for calculating the electrical needs of a building. This goes with the challenge of determining both the current and future power requirements. An essential part of an electrical project planning and design is to align the power requirements of the project with the capacity of the supporting infrastructure such as the protective devices and the electrical cables. This post will provide you with the working knowledge needed to determine the current, future, and the total power requirements for your electrical project.
Let’s begin by identifying availability requirements
Successful design initiatives begin with assessing the building loads as underestimating the required capacity may result in future power disruptions. At the same time, overestimation can lead to high upfront installation costs and higher maintenance expenses. Over the years, the architecture of our building electrical load is based on five loads unit, which includes
1- Lighting load
2- Receptacle load
3- Low current load
4- Mechanical load
5- Conveyance system load
6- The future growth
Once the required loads have been identified, the next is to identify the load components and their power requirements.
Identification of Load Components
This is a systematic process of compiling a list of all devices in a renovation project or dedicated outlets and points in a new project and sorting them based on the five loads classification in the building industry listed above. This can be achieved by using a workable calculation worksheet called Electrical load estimate.
Determining the load requirements
Determining the load requirements is the most critical process to determine the power requirements for an electrical system. The process may begin by using the standard load for the square meter(watt/m2) for a proposed project or by compiling each Electrical load with the standard load per point value in an already designed or renovating project.
For the purposes of this example, we are using the standard load for the square meter(watt/m2) approach which does serve as a preliminary or primary load estimation approach to establish the Power Requirements for an Electrical Project. This approach gives a standard to each load classification and a summation is taken for the building load requirement.
Standard watt per square meter for Lighting Loads
Lighting loads account for all the lighting in the building. It changes from places to places according to the nature and types of the building project. It ranges from 15- 20watt/m2 for a residential and educational project, 20- 35watt/m2 for institutional, commercial, and business apartment project, to 35- 50watt/m2 for public assembly and recreational centers.
Standard watt per square meter for Receptacle Loads
The determination of the standard for receptacle load requires not only the knowledge of an area but to what extent the occupants use the outlets. For this reason, the standard receptacle load is empirical as it varies grossly for different project types and functions but a range of 40-60watt/m2 for most projects other than industrial projects is widely use by professionals.
Low current Load Requirements
As most often, low current systems rely on electrical power for their operation, which is also accomplished through battery backup. It is thus recommended to dedicate an electrical circuit for each system incorporated in the project. Also, because of the low current they drive, they are being neglected in calculating load estimates.
Standard watt per square meter for Mechanical Load
Mechanical load can be an HVAC system load or the plumbing and firefighting system load.
The HVAC systems vary widely in different clime but can be broken down into chilled water systems and direct expansion systems.
Chilled water systems are somewhat more efficient, and a general guide for power consumption is 70% of the total peak load while the Direct expansion systems require about 100% of the total peak load.
An average of 50watt/m2 is recommended for the HVAC load and the plumbing system can thus be neglected in this process unless the project uses services such as the water pumps, firefighting pumps, etc.
Standard watt per square meter for Conveyance system Load
The building conveyance comprises of the escalator and the elevator. The load requirement of all this equipment is a direct equivalent of their rated horsepower. This will then be converted to kilowatts and a little power will be added to cater for their essential services.
Planning for Future Growth
This involves looking ahead for future demands. It involves dedicating circuits or loads for anticipated usage. This will also enable us to ensure that our electrical system is capable of supporting peak power draws (due to some variations that might exist in the critical load).
An average of 20% – 30% is recommended for every electrical project.
Total Power Requirements Summary Calculation
For the purpose of demonstration, assuming the total square meter of our project which is a simple residential project is 100m2.
Project Area= 100m2
Standard lighting load= 20- 35watt/m2 = Assuming a value of 22 watt/m2
Total Lighting Load requirement= 100 X 22 = 2200W ~~ 2.2KW
Standard receptacle load= 40-60watt/m2 = Assuming a value of 50watt/m2
Standard receptacle load= 100 X 50 = 5000W ~~ 5.0KW
Standard mechanical load= Assuming a value of 50watt/m2
Standard mechanical load= 100 X 50 = 5000W ~~ 5.0KW
As the project does not use any of the conveyance services
We now proceed in taking the sum of the calculated load so that the future load requirement can be derived.
Building total load= 2.2KW + 5.0KW + 5.0KW
Building total load= 12.2KW
Future load requirement= Assuming a value of 20% = 2.44KW
Building total load requirement = Building total load + Future load requirement
Building total load requirement = 2.2KW + 5.0KW + 5.0KW+ 2.44KW
Building total load requirement = 14.64KW
Thus the building Total Power Requirements is 14.64KW.