Load Factor

7/31/2019 Load Factor

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You use demand factors to adjust the rating of the main service for a building based on the

expectation that at any given time, not all of the electrical equipment will be drawing at the

full rated load. You can specify one or more demand factors for lighting, power, HVAC, orother systems in your project based on system load. In addition to the predefined demandfactors, you can also create your own.

For example, for the load classification Kitchen, with a demand factor of 1.00, if you have10 lights at 60 VA each, the load is 60 VA x 10 (lights) = 600 VA. Therefore, the lights for

the load of this load category would be 600 VA. This value is accurate only if all the lightsare turned on at the same time. To account for only half of the lights being turned on at atime, you would specify the demand factor as a constant value set to 0.5 (50%). Theestimated load is then 600 VA X 0.5 (demand factor unit) = 300 VA.

Building codes determine what values to use for demand loads. For example, if the total

receptacle on a panel is 20,000 VA, the demand load could be 15,000 VA depending on thelocal code.

Demand factors are assigned to load classifications, and load classifications are assigned to

device connectors. The estimated load for a device is calculated by multiplying the load bythe demand factor. SeeLoad CalculationsandDemand Load Calculation.

The estimated demand load is displayed in the panels instance properties (in theProperties

Palette) and in the panel schedule. The panel schedule can also display the load for eachload classification. SeePanel Schedules.

You can specify a demand factor to calculate the estimated demand load on a circuit. Thedemand factor can be determined by:

a constant value

the quantity of connected objects

load

The load classification values are:

For a load classification on a panel

Total connected load = the sum of all loads attached to the panel (directly or to subpanels) from

connectors assigned to that load classification

Demand Load = the result of the specified demand factor being applied to the sum of all loads

attached to the panel (directly or to subpanels) from connectors assigned to that loadclassification

NoteThe demand factor is specified in the Load Classifications dialog. SeeCreating a Load

Classification.

For all loads on a panel
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Total connected load = the sum of all loads attached to the panel (directly or to subpanels)

Demand Load = the sum of all demand loads (for all load classifications) on the panel


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LC Bob

Total Connected Load = (1+2)

Total Demand Load = DFBob * (1+2)

LC Mary

Total Connected Load = 3

Total Demand = DFMary * (5+9)

LC Frank

Total Connected Load = 0

Total Demand Load = 0

Total Connected Load (Panel A) = (1+2+3)

Total Demand Load (Panel A) = DFBob * (1+2) + DFMary * 3

For Panel MCB

LC Bob

Total Connected Load = (1+2+4+8)

Total Demand Load = DFBob * (1+2+4+8)

LC Mary

Total Connected Load = (3+5+9)

Total Demand = DFMary * (3+5+9)

LC Frank

Total Connected Load = (6+7+10)

Total Demand Load = DFFrank * (6+7+10)

Total Connected Load (Panel MCB) = (1+2+3+4+5+6+7+8+9+10)

Total Demand Load (Panel MCB) = DFBob * (1+2+4+8) + DFMary * (3+5+9) + DFFrank *

(6+7+10)

Calculations are performed per panel. There are two types of load: Total Connected, and Total

Estimated Demand. For each of these types, the desired calculations can be per panel (all loads),

or per load classification.


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For Total Connected Load on a panel, all loads of the same load classification connected below

that panel (to either the panel directly, or to any child or grand-child panels connected up to thepanel) are summed.

Diversity Vs. DemandbyJames G. StallcupPublished: March 2009

Dont be confused by these terms

There are two terms that seem to confuse designers. These terms are diversity factor and demandfactor. To better understand the application of these terms when calculating the load for a service or afeeder 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 asystem (or part of a system) to the maximum demand of the whole system (or part of the system) underconsideration. 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 thetotal connected load on the system (or part of the system) under consideration. Demand factor is alwaysless than one.

Application of diversity factor

Consider two facilities with the same maximum demand but that occur at different intervals of time. Whensupplied by the same feeder, the demand on such is less the sum of the two demands. In electricaldesign, this condition is known as diversity.

Diversity factors have been developed for main feeders supplying a number of feeders, and typically, theyare 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 userswith the following load conditions: On Monday, user one reaches a maximum demand of 100 amps; onTuesday, two reaches 95 amps; on Wednesday, three reaches 85 amps; on Thursday, four reaches 75amps; on Friday, five reaches 65 amps. The feeders 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%

Given

Calculate the size of a main feeder from substation switchgear that is supplying five feeders withconnected 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.

Solution
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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 kVAIf 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 notalways 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 totalload. Remember, the demand factor permits a feeder ampacity to be less than 100 percent of all thebranch-circuit loads connected to it.

Keep in mind that demand factor is a percentage by which the total connected load on a service or feederis 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 originallycalculated per 220.87, the maximum kilovolt-ampere calculations in determining the load on existingservices 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 minimumof one year

If 125 percent of the demand ratings for the period of one year added to the new load does not exceedthe rating of the service; where demand factors are used, often the load as calculated will probably beless than the demand meter indications.

The Ex. to 220.87 contains requirements for where the maximum data for one year is not available. Insuch, the calculated load is permitted to be based on the maximum demand (measure of average powerdemand over a 15-minute period) continuously recorded over a minimum 30-day period using a recordingammeter connected to the highest loaded ungrounded (phase) of the feeder or service based on theinitial loading at the start of the recording.

By referencing Parts III and IV in the NEC, designers can find other useful demand factors that areapplicable to specific loads.


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STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electricalindustry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contacthim at 817.581.2206.

8/10/2006

Diversity and Demand factor..etcDEMAND FACTOR, DIVERSITY FACTOR AND TRANSFORMER SIZE CALCULATION

a) Demand factor - the ratio of the load that a piece of equipment actually draws when it is in operation to

the load it could draw (which we call full load).

For example, an oversized motor - 20 HP - drives a constant 15 HP load whenever it is on. The motordemand factor is then 15/20 = 75 %.

Demand factor= Maximum demand/ Connected load.

This is generally confused with diversity factor. Demand factors should be ideally applied to individual

loads and diversity factor to a group of loads. When you talk about diversity, there is naturally more than

one (many, in real case scenarios) loads involved.

......................//..............................//....................................//.......................

Modified for further clarity and as per reader's comments: 06-09/2011

....................................................................................................................

b) Diversity factor -Gives an indication about the simultaneous maximum demad of a system considering

various loads operating at a time and the highest ' coincident' Maximum demand of all those loads.

Diversity factor =

Sum of individual maximum demands of various sub-loads / Maximum demand of the total system.

This will be less than 1.

Note: Reciprocal of the above ratio ( will be more than 1) also is used in some other countries.

c) Where to use which factor!

Demand factor can be applied to calculate the size of the sub-main which is feeding a Sub panel or a

fixed load like a motor etc.

If the panel (Say SSB 1) total load is 250 kVA , considering a Demand factor of 0.8, we can size the

feeder cable for 250 x 0.8= 200 kVA, which is equal to 280 Ampere on 415 V 3 phase supply.

Diverity factor can be used to estimate the total load required for a facility or to size the Trnasformer


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required.

Simultaneous maximum demand = (Sum individual maximum demands of different loads) x Div. Factor

( Note: if a value greater than 1 is used, divide with that factor)

Assuming there are 4 such SSBs as per the chart below,

PANEL LOAD Demand Factor% Max Demand

SSB 1 250 kVA 80 200

SSB2 200 kVA 80 160

SSB3 150 kVA 75 112.5

SSB4 400 kVA 85 340

Total 812.50 kVA

The Sum of individual maximum demands is 812.5 kVA as given above.

To calculate the Main incoming capacity and Transformer size, we can use the Diversity factor which will

take into account the fact that all the four SSBs above wont attain their individual maximum

simultaneously.

Assuming a Diversity factor of 0.7, the simultaneous maximum demand would be

812.5 x 0.7 = 568.75 kVA

Transformer could be sized after considering a loading factor of 80 % and future requirements of another

20 % extra.

Transformer size= ( 568.75 / 0.8 ) x 1.2 = 853 kVA ( Say 1000 kVA , the nearest available standard size)

Okay, here we go.

Demand factor:The word demand says it all.

The net power consumed at a specific period of time for a given load divided by the amount of

power that would have been consumed if the entire load would have been connected.

Example: Maximum power demand at a point of time is 35 KW in your house as you are running

,but the maximum power that can be consumed at your residence if you switch on all the loadwill be 70 KW

so the Demand factor of your house at a particular point of time when you have 35 KW load is

35/70 = 0.5.

Demand factor is always less than 1.

.

Now Diversity factor:Diversity factor is an extended version of demand factor.It deals with

maximum demand of different units at a time/Maximum demand of the entire system.


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Example.

You and your neighbor have demands at a time as 35 KW and 42 KW ,but the maximum

demand of you and your neighbor when seen individually is 70 KW

Now your combined demand at a time is 77KW.

If you connect all loads the maximum demand will be 70 Kw and so is for your neighbor.

So Diversity factor for you and your neighbor at a time is77/70 =1.1.

Diversity factor can shoot up above 1.

.

Finally system loading,

It's the process of supply of power and balancing of load on a system on a particular point of

time.

In the above 2 definitions you have seen that the system loading for you was 35 KW and that for

your neighbor was 42 KW so combined load was 77 KW.

System loading in other words is also known as load dispatch to the system under a safe limit.

.

Hope that helps.Please ask if you don't understand any point.

ETW.

Read more:What is demand factor, diversity factor, and system loading in Electrical system? |

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