DEMAND VS DIVERSITY FACTOR

Damand Vs. Diversity Factor

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%
Given
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.

Solution
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.

Equipment Load Factors, Use Factors and Diversity Factors As Well as a General Discussion of Energy Audit Procedures

To do a good job on an energy audit, the energy auditor must understand the areas of equipment load factor, use factor and diversity factor.

Definitions: First, let's define these terms.

a)         Load 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 motor load factor is then 15/20 = 75%.

b)        Use (or utilization) factor - the ratio of the time that a piece of equipment is in use to the total time that it could be in use.

For example, the motor above may only be used for eight hours a day, 50 weeks a year. The hours of operation would then be 2000 hours, and the motor use factor for a base of 8760 hours per year would be 2000/8760 = 22.83%. With a base of 2000 hours per year, the motor use factor would be 100%. The bottom line is that the use factor is applied to get the correct number of hours that the motor is in use.

c)         Diversity factor - the probability that a particular piece of equipment will come on at the time of the facility's peak load.

The diversity factor is the most complicated of these factors. For example, we might have ten air conditioning units that are 20 tons each at a facility. In Florida we typically assume that the average full load equivalent operating hours for the units are 2000 hours per year. However, since the units are each thermostatically controlled, we do not know exactly when each unit turns on. If the ten units are substantially bigger than the facility's actual peak A/C load, then fewer than all ten units will likely come on at once. Thus, even though each unit runs a total of 2000 hours a year, they do not all come on at the same time to affect the facility's peak load. The diversity factor gives us a correction factor to use, which results in a lower total kW load for the ten A/C units. If the energy balance we do for this facility comes out within reason, but the demand balance shows far too many kW for the peak load, then we can use the diversity factor to bring the kW into line with the facility's true peak load. The diversity factor does not affect the kWh; it only affects the kW.

Motor load, use and diversity factors: Sometimes motor load factors that are too low are chosen because the auditor has not properly determined the hours of use of the motors - i.e. the auditor uses an incorrect use factor. For example, just because a facility has a production shift that is 2000 hours per year it does not mean that all of the production-related motors in that facility are operated for 2000 hours per year. Some motors - or machines - might only be used one day a week rather than every day. Other motors might be used every day, but for only half the day, i.e. 4 hours per day. Other motors might be in use throughout the day so that their use really is 2000 hours per year.

The auditor must collect data on the use factor - or hours of use - for every motor in the facility during site visit. For each machine, line, process or operation, ask "How many hours a day does this machine (line, process or operation) operate?" This data then needs to be entered into the energy balance. Motor load factors in many facilities are more in the range of 40% - 50%, than in the range of 80% that had been a standard assumption for many years of doing audits. Rarely do you find a motor running at 100% load factor.

However, not all motors at a facility are running at the same load factors. Ventilating fans that come from a supplier as a packaged unit with a fan and a motor are most often assumed to be operating at near full load. You should probably use a load factor of 80% here, since the manufacturer of the ventilating fans should have reasonably matched these loads. Other motors may also be in this category - some engineering judgment and common sense are required to determine which other motors these are.

Motors with variable loads are going to have the lowest load factors in general. A dust collector fan motor will normally have quite a variable load, and would often be expected to have a low load factor. Other examples are saws, presses, milling machines, sanders and grinders, waste grinders, water pumps, hydraulic pumps, etc.

If a group of motors do not all operate together all of the time, then using a diversity factor is appropriate. This is the case with a number of separate air conditioning units (considering the motors for the compressors) that are individually thermostatically controlled.   It could also be the case for a group of production motors if some of the motors are not in use all of the time. You should use a diversity factor in your motor calculations, since it is not often the case that a facility has all of the motors on at the same time.

Reconciling the energy balance: When you perform an energy balance, do not use the motor load factor as the first and only adjustment made to reconcile the estimated energy use (energy balance) with the energy bills. Making this adjustment too quickly results in failure to pick up other things that have been overlooked.

For example, if the energy use does not balance with the energy bills, the first step is to check to see that all of the equipment and uses have been accounted for.

Do the items on the energy balance spreadsheet match your recollection of the equipment you saw in the facility?

Does anything appear to be missing?

Are the utility bills for total energy use and peak kW recorded correctly?

The next step is to check the hours of use for lights and other equipment to see if it matches your knowledge of the facility's operation. Remember that each motor - as well as each other piece of equipment - does not necessarily operate the same number of hours each day or year. Finally, if some of the equipment does not come on at the same time as the facility peaks in kW use, then utilize the diversity factor to account for this.

Adjusting the motor load factors should probably be the last thing you do to reconcile the energy and demand balances. Now, if all other information and all other factors are correct to the best of your knowledge, then adjust the load factors. While motor load factors are not often in the range of 80-100%, you should be equally suspect of very low motor load factors. If you get motor load factors in the range of 20-30%, it is more likely that you have the hours of use wrong than that you have a facility which is using motors that are an average of four times too big for the job they are doing. Lumber mills and wood products facilities using lots of saws may have these low load factors. Most other places should have motors with a higher load factor.

Basic motor load measurements should be taken at the plant visit. The electrical person at the facility is generally willing to measure the current being drawn by a motor of interest. Air compressors are ones that are usually easy to do, and you should ask the plant personnel to do this for you. Let them open the motor controller or switch box and connect a clamp-on ammeter to see what the current for the motor is. You then need to know the full load current from the nameplate of the motor. The ratio of the actual current to the full load current is the approximate load factor on the motor at that time. This procedure works as long as the current is greater than or equal to about 50% of the full load current. Try to take this measurement for each of the large motors in the facility - i.e. motors of 50 hp and above; or even 20 hp or above if the facility does not have a lot of big motors. If you have not received formal electrical safety training, you should not make these electrical measurements yourself. If the facility electrician does not want to make these for you, then let it go at that.

Air handlers—use factor: Air handlers use motors and are subject to all of the comments made in the motor section. In addition, you may be able to get a better handle on the hours of use for the air handlers by knowing how the A/C system works. Ask if the air handlers run constantly when the facility is occupied. They might if the facility wants the ventilation, even though the compressors might not come on except to periodically provide some temperature reduction or moisture removal. If this is the case, then the use factor for these air handler motors should reflect an hours-of -use that matches the offices or other area that the air handlers supply. In addition, the hours-of-use must also consider the compressor run hours. Thus the total hours for the air handlers must be at least the same as the compressor hours, and may be higher if the A/C unit is left on during periods that the facility is not occupied, or if ventilation is provided.

If the air handlers only come on when the thermostat orders cooling, then the hours-of -use must be the same as the hours-of-use of the compressors.

It is important to get adequate information on the operation of the air conditioning system. To get complete data on the air handler motors for an air-conditioned facility, you will need all of the standard information - size, maker, single or three phase, etc - together with the operating basis for the air handlers discussed above.

You should also collect data on the drive belt system for air handlers. Record the number of belts, the lengths, and the types of belts. Ask about motor and drive lubrication and cleaning. Also check the A/C filters to see if they are reasonably clean.

Sometimes a visual inspection will show some real problems. Ask the maintenance person to open up one of the air handlers - or just look into it (SAFELY) if it is accessible - and see if the belt is tight, slack, or really loose. Do not stick your hand into an air handler that is off at the moment, and may come back on when the thermostat kicks in. Have the maintenance person turn the air handler motor off with the circuit breaker or control box. Do not put your finger on a moving drive belt.

Is the belt in good shape?

Is it frayed, cracked or coming apart?

Does it look like the pulleys for the motor and the fan are lined up?

Ask the electrician to measure the current that the air handler motor is drawing to see what its load factor is while driving the fan. It should be very near full load - but you never know. Maybe the original motor burned out and was replaced with a bigger one to "make sure it did not burn out again." Remember to take the full load current off the nameplate to find the load factor.