Electric Load Estimating 2011 SWEDE Conference

Customer’s Connected Load & Electric Utility’s Demand Load• Customers provide the utility company with connected load based on

the National Electric Code requirements.

• This means everything is turned on and running at instantaneous peak loading. The code may even require them to add safety factors that increase the load over 100%.

• The electric utility company will determine the diversified demand load based on the customer’s total connected load information. The utility company does not have to use the NEC requirements for sizing its facilities.

• The customer, electrician and engineer will try to get you to match their connected load for sizing transformers and services, but don’t let them. Demand load will typically range from 25% to 75% of their total connected load.

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Electric Load Estimating

One of the most important responsibilities of the Utility Project Manager is to accurately prepare a commercial or industrial load estimate for service to the customer. This load estimate influences the design & installation cost of the electric utility facilities. It becomes a factor in determining the cost to the customer for electric service.

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Demand Meters

• For all commercial and industrial customers the demand is metered to be the highest average demand in any 15 minute period during each billing cycle month.

• Average is the key word here. It is not the maximum KW during the 15 minute period!

• The meters reset the demand to zero every 15 minutes, keeping only the highest.

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Demand Factor

• How does the load data from the customer get estimated into metered demand? This is done by using a term called demand factor.

• Demand factor is the ratio of the peak demand kW of a system or load to the total connected kW of a system or load. It can be written as follows:

• Demand Factor [D.F.] = peak demand kW [metered]

total connected kW

• If you are calculating the peak metered demand, the equation may be rewritten as follows:

• Peak demand kW [metered] = total connected kW X D.F.

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OBTAINING LOAD INFORMATION

• To calculate the peak meter demand of a load you must know the total connected kW and the demand factor.

• It is the responsibility of the customer to provide you with proper connected electrical load data, not the main disconnect size, panel size, or service amps.

• The customer should provide connected kW information for each piece of equipment as well as phase and voltage.

• This information can be obtained by having the customer complete your Customer Load Requirements Form or by obtaining a copy of the electrical and mechanical plans of the project.

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Motor Loads

• Motor horsepower [hp] - For motor loads, hp is the common way for the customer to indicate connected load. For large motors requiring a motor start calculation you also need to secure the starting code of the motor. To convert this information into the desired kW, the following equation is used.

• connected kW = connected hp X .75

• This is derived from the fact that there are 746 watts/hp. So, there is .746kW/hp and it is simply rounded off to .75

• Since this is just for estimating purposes, motor efficiency is not taken into consideration.

Motor Load Example

• The typical Demand Factor for an elevator motor is 20%.

• This is because the elevator is often starting, stopping and idle during a 15 minute period.

• A 30 hp motor would only have a 4.5 kW demand.

• 30 hp x .75 (kW/hp) x .20 (D.F.) = 4.5 kW

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Air Conditioning Load

• Air conditioning - to properly determine air conditioning connected kW the Energy Efficiency Rating [EER] of the air conditioning unit and the rated tonnage of the unit must be known.

• Equation connected kW = tons X 12

EER

• This equation is derived from the fact that:

EER is in units of BTUH/watt

kW = watts

1000

1 ton=12,000 BTUH

• This equation calculates connected load. Then, a demand factor is utilized to reduce it to the demand load.

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Demand and Diversity Factors

• Once you have secured proper load information from the customer you can determine the connected kW for each load.

• After the connected kW is determined, the peak metered demand may be calculated if the demand factor is known.

• To determine the appropriate demand factor for a connected load the Demand Interval Factor and the Diversity Factor need to be determined.

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Demand Interval Factor

The demand interval factor is the percentage of operational run time of the load during a 15 minute demand interval.

For example a motor cycles 10 minutes on and 5 minutes off during a 15 minute demand interval. Therefore the demand interval factor would be:

Demand Interval Factor = 10 minute run time / 15 minutes = .67

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Diversity Factor

• The diversity factor is defined as the probability that a load will be operating during the period when the demand meter records the high peak demand.

• Typically we use a diversity factor of 100% if the load is expected to operate during the peak period and 0% if the load is expected to operate only outside the peak period (Off-Peak Load).

• For two or more loads of the same type, the diversity factor will reflect the probability that these same types of loads will all operate during the peak period. The range of diversity factors for these same type of loads could be anywhere from 0% to 100%.

• For example the probability that all the receptacle outlets in a building will all be used during the peak period is fairly low and is typically given a diversity factor of 10%.

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Example 1:

• Tom's Machine Shop is a building that has 5 kW of fluorescent fixtures, 15 receptacle outlets at 1500w each, 1-10 hp lathe, 1-20 hp air compressor and 1-15 hp fire pump.

• After questioning the customer about the various loads, the information is further deciphered as follows:

• The shop lights are on only during the hours of 8 a.m. to 5 p.m.

• The receptacle outlets are in the office only, and will have computers and other small loads plugged into them.

• The lathe is fully loaded for 5 minutes periods. The rest of the time is setup time. This procedure repeats every 15 minutes.

• The air compressor supplies air to air tools and cycles off and on about half the time.

• The fire pump only runs for 30 minutes when tested which is once a month after hours.

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Example 1 – Summary

• Lighting Demand Factor = Demand Interval Factor x Diversity Factor = (15 minute run time/ 15 minutes) x 1.0

= 1.0 Lighting Demand Load = 5 kW x 1.0 = 5 kW

• Receptacle Outlet Demand Factor = Demand Interval Factor x Diversity Factor = (15 minute run time / 15 minutes) x 0.1

= 0.1 Receptacle Outlet Demand Load = 15 x 1500 watts x 0.1 = 2.25

kW• Lathe Demand Factor = Demand Interval Factor x Diversity Factor

= (5 minute run time / 15 minutes) x 1.0 = .33 Lathe Demand Load = 10 hp x .746 x .33 = 2.46 kW

• Air Compressor Demand Factor = Demand Interval Factor x Diversity Factor = (7.5 minute run time / 15 minutes) x 1.0 = .5

Air Compressor Demand Load = 20 hp x .746 x .5 = 7.46 kW• Fire Pump Demand Factor = Demand Interval Factor x Diversity Factor

= (15 minute run time/ 15 minutes) x 0.0 = 0.0 Fire Pump Demand Load = 15 hp x .746 x 0.0 = 0.0 kW

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Example 1 – Chart

Summary of Demand Loads

Equipment HP kW D.F. Demand KW

Lighting 5.0 1.0 5.0

Receptacle Outlets 22.5 0.10 2.25

Lathe 10 7.5 0.33 2.46

Air Compressor 20 15.0 0.50 7.46

Fire Pump 15 11.25 0.0 0.0 .

TOTAL 61.25 kW 17.17 kW

Connected KW Vs Demand KW

Customer's Connected KW Vs Actual Demand KW

Project NameCustomer

Connected KWActual Metered

Demand KW Demand Factor

Sam's Club 1,477 773 52

HEB Grocery 2,135 1,060 50

HEB Gas & Car Wash Site 116 27 23

IKEA Store 3,409 1259 37

Stone Oak Elementary School 1,557 554 36

Holiday Inn 539 118 22

Sundance Office Building 1952 1115 57

Texas A&M Health Science Bldg 2272 930 41

Typical Demand Factors

• The following factors can be used as a guideline to check the calculated demand factor. Good judgment must be used. Do not use this information as correct in all situations because it is not. The demand factor and diversity factor determination must be answered to ensure correct demand factor assignment.

• Air Conditioning 1 unit 100%• several small units 85-95%• bldg. core cooling in winter 40-60%• Chiller systems (A/C) entire 65%• Heating 1 unit 80%• several small units 50-80%• aux. heat for heat pumps 40%• Interior lighting under 100 fixtures 100%• over 100 fixtures 95%• w/large storage & conference areas, etc. 90%• External lighting (off day, on evening)• summer 0%• winter, closed at night 0%• winter, open at night 100%• Sign lighting sign on during day 100%• summer 0%• winter, closed at night 0%• winter, open at night 100%• Water heating 50-60%• Water circulation pumps 90%• Process equipment continuous 70-80%• non-continuous 40-50% • Air Compressor 40-60%• Computers main frame 90-100%• Welders 5-10%• CAT or X-RAY 0-5%• Laundry & Dry Cleaning Equipment commercial 50-60%• Refrigeration 60%• Miscellaneous Motors gas pump 10-20%• pneumatic tubes 10%• elevators 20%• Commercial cooking fast foods 60-70%• other applications 40%

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Watts Per Square Foot

• Sometimes a situation exists in which obtaining accurate load data is not possible.

• An example of this situation is a strip shopping center where the developer does not know what type of tenants will lease the spaces.

• Table 1 lists the watts/sq. ft. estimates for situations where the load is not available such as speculative lease spaces.

• This table can also be used as a comparison tool to validate your calculated demand load.

Watts Per Square Foot

• TABLE 1 (This Information is 20 Years Old!)• LOAD INFORMATION – WATTS per SQ. FT. ESTIMATES• These may be used when actual load data is not available.• Watts Per Square Foot• Air Conditioning comfort cooling 4 watts/sq ft.• computer rooms 5 watts/sq. ft.• Heating 6 watts/sq. ft.• Office Lighting 3 watts/sq. ft.• Warehouse Lighting 1 watt/sq. ft.• Office miscellaneous 1 watt/sq. ft.• Warehouse miscellaneous 1/2 watt/sq. ft.• Air Conditioning – By Tons• A/C (at 8.0 SEER) 1.5 kW/ton• Chiller units (including auxiliary equip.)

1.0 kW/ton• Fluorescent Lighting• 2' x 4' lay-in 2 lamp (4') 100 watts/fixture• 2' x 4' lay-in 4 lamp (4') 200 watts/fixture• 2' x 8' lay-in 2 lamp (8') 200 watts/fixture• Other Guidelines• 1 phase or 3 phase motor 750 watts/ horsepower• Swimming pool pumps (inefficient) 1.2 kW/horsepower• Refrigeration compressors 1.4 kW/ton

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Watts Per Square Foot ExampleTypical Office Building

TABLE 1LOAD INFORMATION – WATTS per SQ. FT. ESTIMATES

These may be used when actual load data is not available.

Watts Per Square Foot

Air Conditioning

comfort cooling 4 watts/sq ft.

computer rooms 5 watts/sq. ft.[summer & winter]

Heating 6 watts/sq. ft.

Office Lighting 3 watts/sq. ft.Warehouse Lighting 1 watt/sq. ft.

Office miscellaneous 1 watt/sq. ft.Warehouse miscellaneous 1/2 watt/sq. ft.

TOTAL= 8 watts/sq. ft.

Do You Know Jack?

• Another way to check your Demand KW estimate is by comparing it to an existing facility on your distribution system.

• You are probably not serving the first “Jack in the Box” restaurant on your electric distribution system.

• If you are serving a customer that has other existing facilities located on the distribution system – check your meter information system for the Demand KW of the existing locations.