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.
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.
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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
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
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= (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
Example 1 – Chart
Summary of Demand Loads
Equipment
Lighting
Receptacle Outlets
Lathe
Air Compressor
Fire Pump
TOTAL
15
HP
10
20
15
kW
5.0
22.5
7.5
15.0
11.25
61.25 kW
D.F.
1.0
0.10
0.33
0.50
0.0
Demand KW
5.0
2.25
2.46
7.46
0.0
.
17.17 kW
Connected KW Vs Demand KW
Customer's Connected KW Vs Actual Demand KW
Project Name
Customer
Connected KW
Actual Metered
Demand KW
Demand Factor
Sam's Club
1,477
773
52
HEB Grocery
2,135
1,060
50
116
27
23
IKEA Store
3,409
1259
37
Stone Oak Elementary School
1,557
554
36
539
118
22
Sundance Office Building
1952
1115
57
Texas A&M Health Science Bldg
2272
930
41
HEB Gas & Car Wash Site
Holiday Inn
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.
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Air Conditioning 1 unit
several small units
bldg. core cooling in winter
Chiller systems (A/C) entire
Heating 1 unit
several small units
aux. heat for heat pumps
Interior lighting under 100 fixtures
over 100 fixtures
w/large storage & conference areas, etc.
External lighting (off day, on evening)
summer
winter, closed at night
winter, open at night
Sign lighting sign on during day
summer
winter, closed at night
winter, open at night
Water heating
Water circulation pumps
Process equipment continuous
non-continuous
Air Compressor
Computers
main frame
Welders
CAT or X-RAY
Laundry & Dry Cleaning Equipment commercial
Refrigeration
Miscellaneous Motors gas pump
pneumatic tubes
elevators
Commercial cooking fast foods
other applications
100%
85-95%
40-60%
65%
80%
50-80%
40%
100%
95%
90%
0%
0%
100%
100%
0%
0%
100%
50-60%
90%
70-80%
40-50%
40-60%
90-100%
5-10%
0-5%
50-60%
60%
10-20%
10%
20%
60-70%
40%
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.
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Watts Per Square Foot
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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
Watts Per Square Foot Example
Typical Office Building
TABLE 1
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
[summer & winter]
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.
TOTAL= 8 watts/sq. ft.
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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.