Electrical Contractor readers requested an article explaining a simplified method for calculating a load for a commercial or industrial facility. Let’s arrange these loads in systematic order so that it is easier to derive a total load.
Commercial and industrial facilities, such as offices, banks, restaurants, refineries and similar occupancies, are equipped with diverse loads. These loads are considered continuous or noncontinuous with or without demand factors. A continuous load is any load that is capable of operating for three hours or more with demand factors that are considered less than one.
Loads must be calculated based on the type of occupancy and the requirements of the equipment supplied. Either the standard or optional calculation is used to calculate the loads in volt-amperes (VA) or amperes (A) to size a feeder or the service equipment. How the lighting, receptacle and equipment loads are used determines how they are grouped. Such loads can be grouped into seven loads, and the National Electrical Code (NEC) requirements are applied to each of these loads based on their use. The lighting load is the first load and is grouped as follows:
• General lighting load per Table 220.12
• Sign lighting load per 220.14(F)
• Outside lighting load per 220.18(B)
• Show window load per 220.43(A)
• Lighting track load per 220.43(B)
These loads must be calculated for continuous or noncontinuous duty with demand factors where applicable. For example, the general lighting load for a 3,000-square-foot office building, including the receptacles per Table 220.12, is 13,500 VA (3,000 4.5 VA = 13,500 VA). The remaining four loads are calculated per Table 220.12, 220.14(A) through (L) or 220.18(B).
The receptacle load is the second load to be calculated. This load is divided into two subgroups as follows:
• General purpose outlets
• Multioutlet assemblies
For example, the load for 100 continuous outlets is 18,000 VA (100 180 VA = 18,000 VA) per 220.14(I) and 230.42(A)(1). If the 100 receptacles are noncontinuous, the load is 18,000 VA per 220.14(I). With demand factors applied per Table 220.44, the answer is determined as follows:
100 outlets 180 = 18,000 VA
• Table 220.44
18,000 VA – 10,000 VA = 8,000 VA
8,000 VA 50% = 4,000 VA
10,000 VA + 4,000 VA = 14,000 VA
Multioutlet assemblies are calculated at 180 VA for each foot or 180 VA for every 5 feet of assembly as required per 220.14(H). If the load is continuous, the 125 percent rule applies.
Special appliance loads
Special appliance loads are the third load. This load is determined by either calculating it at continuous or noncontinuous duty or demand factors where applicable per Article 220. For example, a continuous process appliance of 2,400 VA is calculated at 125 percent times 2,400 VA. That is equal to 3,000 VA per 220.14(A), 215.2(A)(1) and 230.42(A)(1).
Compressors are the fourth load, and the full load volt-amperes of the compressor is increased by 125 percent per 440.32 and 440.34 where more than one unit is present. For example, if the volt-amperes of the compressor is 37,395, increase by 125 percent. The result is 46,743.75 VA.
Motors are the fifth group of loads to calculate. The volt-ampere rating of each motor is converted from the full load amps (FLA) to VA by multiplying the FLA, from Table 430.248 for single-phase or Table 430.250 for three-phase motors, by the supply voltage.
For example, a 460-volt (V), three-phase, 50 horsepower motor per Table 430.250 has a FLA of 65A and is converted to volt-amperes by multiplying 65A by 460V times 1.732 as 430.6(A)(1) requires. The result, 51,786.8 VA, is added to the calculation of seven loads. Where there is more than one motor per 430.24, the motors are calculated in this manner.
Largest between heating and AC load
Heating or an air conditioning (AC) load is the sixth group of loads to be calculated. The most volt-amperes between the heating and AC load must be selected when using the seven-load method. The larger between the two loads is selected and used, and the smaller load is dropped. For example, a 10,000 VA heating unit is chosen over an AC unit having a load rating of 3,500 VA per 220.60.
Largest motor load
The largest motor load in volt-amperes is the seventh to be calculated. The largest motor load is selected from one of the motor loads in the fourth, fifth or sixth loads. As outlined in 220.50 and 430.24, the volt-ampere rating of the largest motor is calculated by multiplying the motor’s VA by the voltage times 25 percent or 125 percent (designer’s choice).
This calculation procedure, using these seven load steps, is a dependable method that can be used to determine the total amps for selecting the elements of a feeder or service.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.