There are two terms that seem to confuse designers. These terms are “diversity factor” and “demand factor.” To better understand the application of these terms when calculating the load for a service or a feeder supplying a facility, one must understand their meaning.
Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system (or part of a system) to the maximum demand of the whole system (or part of the system) under consideration. Diversity is usually more than one.
Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.
Application of diversity factor
Consider two facilities with the same maximum demand but that occur at different intervals of time. When supplied by the same feeder, the demand on such is less the sum of the two demands. In electrical design, this condition is known as diversity.
Diversity factors have been developed for main feeders supplying a number of feeders, and typically, they are 1.10 to 1.50 for lighting loads and 1.50 to 2.00 for power and lighting loads.
Diversity factor and load factor are closely related. For example, consider that a feeder supplies five users with the following load conditions: On Monday, user one reaches a maximum demand of 100 amps; on Tuesday, two reaches 95 amps; on Wednesday, three reaches 85 amps; on Thursday, four reaches 75 amps; on Friday, five reaches 65 amps. The feeder’s maximum demand is 250 amps.
The diversity factor can be determined as follows:
Diversity factor = Sum of total demands ÷ Maximum demand on feeder = 420 ÷ 250 = 1.68 × 100 = 168%
Calculate the size of a main feeder from substation switchgear that is supplying five feeders with connected loads of 400, 350, 300, 250 and 200 kilovolt-amperes (kVA) with demand factors of 95, 90, 85, 80 and 75 percent respectively. Use a diversity factor of 1.5.
Calculate demand for each feeder:
• 400 kVA × 95% = 380 kVA
• 350 kVA × 90% = 315 kVA
• 300 kVA × 85% = 255 kVA
• 250 kVA × 80% = 200 kVA
• 200 kVA × 75% = 150 kVA
• The sum of the individual demands is equal to 1,300 kVA
If the feeder were sized at unity diversity, then 1,300 kVA ÷ 1.00 = 1,300 kVA
However, using the diversity factor of 1.5, the kVA = 1,300 kVA ÷ 1.5 = 866 kVA for the feeder. Transformer supplying the main feeder plus wiring methods and equipment can be sized from this kilovolt-ampere rating.
Applying demand factors
Although feeder conductors should have an ampacity sufficient to carry the load, the ampacity needs not always be equal to the total of all loads on connected branch-circuits.
A study of the National Electrical Code (NEC) will show that a demand factor may be applied to the total load. Remember, the demand factor permits a feeder ampacity to be less than 100 percent of all the branch-circuit loads connected to it.
Keep in mind that demand factor is a percentage by which the total connected load on a service or feeder is multiplied to determine the greatest probable load it may be called on to carry.
When additional loads are connected to existing facilities having services and feeders as originally calculated per 220.87, the maximum kilovolt-ampere calculations in determining the load on existing services and feeders should be used if these conditions are met:
• If the maximum data for the demand in kVA, such as demand meter ratings, is available for a minimum of one year
• If 125 percent of the demand ratings for the period of one year added to the new load does not exceed the rating of the service; where demand factors are used, often the load as calculated will probably be less than the demand meter indications.
The Ex. to 220.87 contains requirements for where the maximum data for one year is not available. In such, the calculated load is permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter connected to the highest loaded ungrounded (phase) of the feeder or service based on the initial loading at the start of the recording.
By referencing Parts III and IV in the NEC, designers can find other useful demand factors that are applicable to specific loads.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.