What is Demand Factor and Diversity Factor? Quick Concepts

The generally asked question during electrical design “What is Demand Factor and Diversity Factor”. We can define the Demand Factor as the ratio of the maximum demand or the maximum running load to the system-rated or connected load.

What is Demand Factor and Diversity Factor?

The maximum demand of each load differs according to the load; hence, the demand factor also varies with the maximum demand. The rated capacity or the connected load is constant, so the demand factor only depends upon the maximum demand that the load would be taking on the spot.

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As the maximum demand is always less than the total connected load or the capacity, therefore the demand factor tends to be less than unity.

Let’s take a few examples which will enhance our understanding of the demand factor.

Examples of the Demand Factor

For example, there is a pump having a capacity of 40KW. When we turn it on it consumes 35KW of power instantaneously. As the maximum demand is around 35KW, the demand factor will be around 35/40= 0.875. So, the value of the demand factor comes out to be less than unity. In percentage, we can say it is around 87.5%. We can denote demand factor in terms of percentage as well as in comparison with unity.

The demand factor does not always remain the same. It varies continuously with the passage of time. For example, if there is a three-phase motor connected in a circuit. When we switch on the power supply, the motor takes a lot of inrush current. Which is almost 3 times larger than its rated capacity.

This happens for a very short duration and the load stabilizes as it reaches its rated/normal speed. So, depending on the operation of the motor, the demand factor varies. If there is no load, the motor will draw less power.. Hence demand factor is low and the reverse happens when the motor is on load.

If the demand factor is less then to power up the load we require less capacity of the generation system. A house has 10 no. of Fans and each one rates at 100 watts. Let’s assume if only 5 of them turn on at a time then the demand factor will be 500/1000= 0.5 or 50%.

This means the installed capacity is 1000 watts but only 500 watts turn on at a time. Thus demand factor helps us to understand the actual demand on the load or consumer side. So, by keeping in view the real scenario, one can decide the breaker size for the circuit. And also the selection of cable becomes easy.

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As in electrical engineering design, we must also keep an eye on the economical side of the design.

Applications of Demand Factor

As we have seen above, the demand factor varies with the load. So, the conductors supplying power to the loads should have enough amperage capacity so that the voltage drop and losses are less. The amperage capacity also varies and it is not necessary that total amperage capacity is equivalent to generation end amperage capacity as the load and demand factor is variable.

Each type of load has been defined by IEC and NEC standards and each load has a different demand factor. For example, for residential buildings demand is around 0.6 while for commercial buildings it is around 0.7 to 0.75. Similarly, as the number of motors in a vicinity increase, the demand factor tends to reduce. Up to 10 no. of motors demand factor is 0.75 while up to 30 no. of motors it is around 0.6. There are NEC standards for demand factor for various loads and each type of load has different demand factor. For example according to article 220 of NEC lighting loads have demand factors as below.

These value in above table are in Volt-Amperes which is the Apparent power of the load. Read more about active reactive and apparent power here in our previous article. First 3000 volt-amperes or 3KVA load is on 100% demand factor. Then the demand factor gradually decrease. Similarly NEC 220 also defines the demand factors for general lighting according to the type of occupancy. In below figure there is a direct method for each type of occupancy against which we can select value as volt-amperes/square-meter.

Then comes the category of kitchen equipment including electric ranges, wall-mounted ovens and other household cooking appliances, for which demand factor is according to the number of appliances and their rated capacity.

There is a separate category for schools feeder and demand factors apply according to the connected load.

For new restaurants getting new connections, the demand factors apply according to article 220 of NEC as below.

What is Diversity Factor?

The diversity factor we can describe as the ratio between the maximum separate demands of all of the components connected to the circuit and the maximum demand of the entire circuit in a specified period of time.

We can also say that the diversity factor is the ratio of the loads installed to the running loads which is in fact the maximum demand of the system in a specified period of time. Diversity factor is always greater than unity. The opposite of diversity factor we refer as the coincidence factor which is always less than unity.

Let’s say there two feeders having the same load with the same maximum demand but this maximum demand does not occur at the same time. Now, when these feeders will be supplied by the same feeder, their sum of individual maximum demands is greater than the maximum demand of the two feeders. That’s what we call diversity and it is just like the demand but deals in individual maximum demands to the overall system maximum demand.

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In fact we can say if a load has diversity factor of 80%, it means that the load operates or functions on its maximum demand level almost 80% of time. Diversity factor gives us an overview of the load behavior that how much time it will be operating on its maximum demand level.

As the sum of individual maximum demands is larger than the maximum demand of the system, so the diversity factor is always greater than > unity and we denote in in terms of %age or the fraction. If it represents in terms of fractions then we divide the diversity factor with load and on the other hand if its in %age then we multiply it with the load.

Diversity factor varies with the loads behavior and before installations of feeders the ground survey of loads and the requirements of maximum demands must be accounted in. Otherwise, the cable selections might end on the higher end increasing the costs. Diversity factor apply on each node of the distribution system and on each distribution panel.

Calculations of Diversity Factor

Let’s say there are three feeders which rate at the maximum demands as 200KW, 210KW and 190KW. The main feeder supplying these three feeders has a maximum demand of 500KW. So, the sum of individual maximum demands will be 200+210+190= 600KW. The diversity factor we can calculate as 600/500= 1.2 which is higher than unity.

Application of Diversity Factor

In Practice most commonly diversity factor is used in calculations and is used as a replacement of the coincidence factor or the simultaneous factor. Thus the value we take as less than one in calculations. As a common rule the diversity factor apply on each type of distribution board separately. While there are different local standards according to which diversity factor we take as according to the number of downstream consumers.

Now, let’s assume there is three floor building with 20 consumers and each floor has total installed load of around 5KW. Ground floor has 7, first floor has 5 and second floor has 8 consumers. So, each floor has installed load as 35KW, 25KW, and 40KW respectively.

So, total installed load of the building is 35+25+40= 100KW. If power factor is around 0.9 then it is equivalent to 111KVA. So, the total requirement of apparent power of the building will be 111x 0.49= 54.44KVA. In the same way we can also calculate the high rise buildings currents like on the ground floor the current is equal to (111x 0.49×1000)/400×1.732= 78.57A. Similarly we calculate the current entering the second floor.

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Firstly convert KW to KVA as 38KVA, 27KVA and 44KVA. To calculate the current entering into second floor we will add consumers of ground and first which comes out to be 12 and diversity factor equals to 0.63. So current entering to second floor will be as ((38+27)x0.63×1000)/400×1.732= 59A.

Diversity Factor For Switchboards

the IEC standard 61439-1 and 2 defines the switchboards diversity factor comprehensively which is lower than or equal to unity. This standard stipulates that if there is no clarity of loading circuits and not sure of outgoing circuits currents then following diversity factors may apply.

Difference between Diversity and Demand Factor

As we have discussed both factors in detail above, diversity factor is employed in calculating the distribution transformer sizes and mainly used to estimate the actual loading of the system in a specific period of time. While we use demand factor to estimate the peak loading of the system in any specific period of time.

Therefore demand factor is mainly to size the feeder sizes after estimating the peak demands of the systems. Electrical low and high voltages designers employ both factors to estimate the actual loading of the system. Therefore, they can estimate the size of cables, transformers and protection equipment in a comprehensive way.

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