In general the type of occupancy (Office, Store, Bank, etc.) is first determined and categorized before the load calculation is begun. These steps proceed as follow:
1. Determine the area of the building using outside dimension, less any inhabitable area such as garage, patio, porch and the like.
2. Multiply the resulting area by the electrical load per square foot amount allowed by codes for the type of occupancy.
3. Determine the volt-amperes of continuous load (Lighting, etc) and multiply the volt amperes (VA) for these loads by a factor of 1.25.
4. Apply demand factors to any qualified loads per local codes.
(Mechanical loads, Kitchen loads and elevator loads)
5. Calculate the total adjusted general lighting load.
6. Determine the type of VA rating of any other loads, such as appliances circuits, circuits for track lighting, show windows in stores and like.
7. Add these “other loads” to the total adjusted general lighting load to obtain the total load for the project.
A 3,000square foot building is served by 208 –volt three phase service.
The building contains the following:
1. 80 duplex receptacle
2. 30 ft of show window
3. 1200VA signage
4. 20,000VA Air-conditioning unit
None continues loads
Calculate the load for 100 receptacles.
100 x 180 = 18000 VA
10,000 VA @ 100%= 10,000
14,400 VA – 10,000 = 4400 @ 50% = 2200VA
Subtotal = 12,200 VA
General lighting 3000 square ft at 3 VA per Square ft = 9000VA
Show window lighting load 30 ft at 200 VA per ft = 6000VA
Outside signage = 1200VA
Subtotal continuous load at 1.25% = 20,250 VA
Air conditioning motor = 20,000VA
Total load = 52,450VA @ 208V 3 phase = 145 A
145A x .25% for future load = 181 A
The next higher standard size is 200A