Air conditioning is lovely in the summer, and heating is desirable in the winter.
However, there are instances where heating and cooling may operate simultaneously, and instances where they may not. To power and feed both heating and cooling systems with high demand, recognizing the latter argument as an example of noncoincidental loads is important. This is a key factor to properly sizing an electrical distribution system to serve both loads, which in turn can be beneficial to the end-user in cost savings and space allocation.
What is it?
According to the National Electrical Code (NEC), Article 220.60, noncoincident loads are described as: “Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of feeder or service. Where a motor is part of the noncoincident load and is not the largest of the noncoincident loads, 125 percent of the motor load shall be used in the calculation if it is the largest motor” (Earley, NEC). What this means is, multiple loads within a building that run at separate times, then the largest grouping of those loads shall be used to size the distribution equipment.
Why Do We Use It?
When sizing electrical distribution equipment, the sum of loads that will run simultaneously has the largest impact on the sizing approach. If there are 10 loads that will be on the distribution equipment, but only 4 of those loads will run at one time, the amperage of these 4 loads should be used to properly size the distribution equipment This approach could be the difference between having a 1000A distribution panelboard or having a 600A distribution panelboard. Sizing a panelboard using the noncoincident load method can lower the cost to the end user in regard to panelboards, conduit, and wire size, versus sizing a panelboard without this method. The sizes of wire and conduit would be much smaller, which shaves cost off the budget that owners and/or clients love to hear. This means potentially the panelboard will be smaller and will use less space, which results in more real estate for space design options and fewer costs for electrical rooms.
Calculating the Noncoincident Load
Understanding how to calculate noncoincidental loads is important. When there is more than one group of loads, the largest load will be used to calculate the total load. For example, there are two groups of motor loads A, and B. Group A has 3 loads consisting of the following ratings: 10 amps, 50A, and 200A. Group B has 2 loads comprising the following ratings: 30A and 75A. Group A will be the deciding factor for the noncoincident load since it has a total of 260A, which is greater than Group B’s total of 105A. Since these are motor loads, a 125% demand factor must be applied to the largest motor per NEC Article 220.18(A). The largest motor load from Group A (200A) is multiplied by the demand factor (1.25) resulting in 250A. The noncoincident load of Group A is the sum of the largest load and the rest of the loads. With that, the sum comes to a total of 310A.
Sizing Distribution Equipment
To size switchgear (i.e. panelboards, distribution boards and switchboards), a recommended 125% factor for spare capacity is applied to the load. From the example above, the total would come out to be 388A which results in a 400A panelboard. If the noncoincidental load approach was not used, the total load would have been 415A, and with the spare capacity factor the distribution panelboard would have required a 600A panelboard from a total of 519A. Using a noncoincidental calculation, the 200A decrease in size of panelboard as well as conduit and wire saves the owner/client money.
Using the noncoincidental load method to determine distribution equipment sizes helps reduce the cost to the end-user by decreasing panelboards, conduits, and wire sizes. It also allows for smaller distribution equipment, which means less space to be occupied due to limiting the size of distribution equipment. This all adds up to increased efficiency. The end user pays for their needs, which in this case saves money and can be very attractive to an owner. When available as an option, this is a very useful method to use to provide exemplary service to your client. As the owner’s engineer, we desire to keep our clients happy, and understanding how to apply the NEC helps us accomplish that goal.
Electrical Designer Engineer