The Nature of Loads II
Load Study
qIf all the connected load are knowns in details, like in the previous
examples, then it is possible to determine the total connected load
per class and use demand and diversity factors to estimate the
total maximum diversified demand which can be used in sizing the
feeder, transformer, and distribution board equipment.
qHowever, in many cases it is not possible to know the details of
each possible connected load, which makes the above method
difficult to implement.
Is there other ways of simple load study? Yes, there are.
Load Study: Other Method
qFor simple load study and calculation model, we can use the
load densities defined by Standards and Codes for different
types of loads and building occupancies.
q However, it is sometimes necessary to confine our
calculation taking into account the following six features:
1. Building placement
2. Building Structure
3. Level of comfort
4. Air conditioning option
5. Technical characteristic
6. Building automation/technical building management
Load Study: Other Method
Another Load Study Procedure
Define the type of
occupancy (School, Use Load density provided by standard/codes (IEC, NEC,
Mall, House, Hospital, PEC,…) to estimate the load capacity for each class
Stadium,…)
Use diversity
Classify Loads Use related demand
factor/Simultaneity
(Lighting, air factors to estimate
factors to estimate the
conditioning, General maximum demand
total maximum
appliances,…) for each class
(coincident) demand
Determine Building / Calculate total Size your main feeder,
Fucntional Areas maximum non- transformer and
(m^2) for each class coincident demand distribution board
equipment
Load Study: Other Method
qLoad Classification: Non-Industrial Loads
1. Lighting and Illumination: Includes indoor and outdoor
lighting. This includes normal and emergency lighting.
2. Small Appliances: general purpose sockets and office
appliances, TV and refrigeration.
3. Space conditioning: Heating ventilation and Air
conditioning.
4. Water pumping sewage, fire fighting and water heaters
5. Light loads: Alarms and telephones
6. Dynamic loads: lifts, elevator, and escalator
Load Study: Other Method
qLoad Classification: Industrial Loads
1. Heavy Load
2. Light Load
3. Moderate Load
Note: Load study for industry requires a bit information about
the type of loads
Load estimate is based on experience and occupancy per m^2 (per area).
This stage is very important in licensing , estimation of appropriate
transformer, and sizing of space required for electrical components.
Load Study: Other Method
Some Typical Load Densities
qUtilities Supply a broad range of loads, from rural areas with
load densities of 10kVA/m2 to urban areas with 300kVA/m2.
qSome typical load density values are as follows:
vFor buildings:
• Lighting: 10 – 25 W/m2
• Air conditioning: 1 – 3 kW/equipment
• Office building: 100W/m2, 2kVA per workplace
• Lifts: 10 – 50 kVA / lift
• Hotels: 3 – 4 kVA / room
Load Study: Other Method
Some Typical Load Densities
vFor industrial and trading Center
• Repair workshops, automatic lathes, weaving and spinning
mills: 50 – 100 W/m2
• Machine tool manufacture, mechanical workshops and welding
plants: 70 – 300 kW /m2
• Press shops, hardening, steel melting and rolling mills: 200 –
500 kW/m2
Codes for General Lighting Loads According to
Occupancy
qIllumination represents from 20 – 50% of total electrical load.
qRanges from 3W/m2 as in storage spaces to 50 W/m2 as in stadiums
and varies according to standard used.
qThe range for W/m2 has changed over the years dues to the use of
energy saving lamps.
Examples of Codes for General Lighting Loads According to Occupancy
NEC 220.14 PEC
General Purpose Sockets
qMany Methods are used for estimating the loads on general
purpose sockets.
qFor example:
vIt is estimated that each socket carry up to 180 VA
vOr each socket carries 1.5 A (120V)
vOr using table according to the load itself as in table in next slide.
qGenerally, we can say that each socket must tolerate 100W,
unless the socket is designed for a specific load such as 500W
for an electric water heater socket.
General Purpose Sockets
qIn general, the number of general purpose sockets
connected on one radial is between 2-5.
qThe presence of range in the load density is due to
electronic and computer load increase in the recent years.
qThis type of load increase with time.
General Purpose Appliances
qThere are appliances for general use like lifts, escalator, and
water pump.
qThis type of load is selected by mechanical engineers.
vThe typical value for the lifts are 15 – 25 kW depending on
the building height and number of user.
vTypical values for water pumps are 5kW.
General Air conditioning load according to occupancy
IEC Standard
qExample 1:
vConsider a 4-floors office building with a floor area of 2000
ft2.
vCalculate the total preliminary electrical for the transformer
feeding this building by using the following tables from IEEE
Standard.
vThe preliminary electrical load is based on a transformer
load level of 70%.
qExample:
IEEE STANDARD
qExample:
IEEE STANDARD
qExample:
IEEE STANDARD
qExample:
Solution
Total 4-floors office
Building Area:
4x2000 = 8000 ft^2
Lighting Load
Density 1.81W/ft^2
qExample:
Solution
Office Building
Average General
Receptacle Load
Density
1 VA/ft^2
qExample:
Solution
Office Building AC
Load Density
6 VA/ft^2
qExample:
Solution
1) The total gross are of the building. (4 x 2000 = 8000 ft^2)
2) The building type or usage is office.
3) From the above IEEE tables, the load densities will be as
follows:
a) Lighting = 1.81 W/ft^2
b) Small Appliances = 1 VA/ft^2
c) Air conditioning = 6 VA/ft^2
qExample:
Solution
4) The subtotal estimated electrical loads will be as follows:
a) Lighting = 1.81 W/ft^2 x 8000 = 14,480 = 14,480VA
b) Small Appliances = 1 VA/ft^2 x 8000 = 8,000VA
c) Air conditioning = 6 VA/ft^2 x 8000 = 48,000VA
5) The total estimated electrical load for building :
14,480 + 8,000 + 48,000 = 70,480 VA = 70.5 KVA
6) Based on t he probl em statement , we k now t hat t he
transformer maximum load level should not exceed 70% of its
rating. Therefore, the transformer power rating for the building:
70.5kVA/0.7 = 100.7 kVA
qExample 2:
vFor high-rise office building with shopping arcade, the
power supply has to be planned for a 10-story (12 floors)
building with a floor area of approximately 1350 m2.
vThere is a 3-level car parking for customers (Level-1 to -3)
vThere are 2 utility rooms for technical equipment that are
located on roof level and have an approximate area 0f 105m2
each.
vCalculate the main electrical service size for this building.
(Transformer rating based on 0.85 pf and 70% maximum
loading)
q Example 2:
q Example 2:
vSolution
§ Consider first case: availability of grouped load density for each space.
§ Hence, calculation will be as follows:
1. The building will be divided to the following spaces:
• Parking Garage / Utilities areas (including roof area)
• Shopping center / bank
• Offices
q Example 2:
vSolution
2. The gross of each space will be as follow
§ Parking Garage / Utilities areas (including roof area)
• Basement level-1 to -3 with 1,350 m2 each + utilities areas
105 m2 each
• Area = (3 x 1,350 m2) + (2 x 105 m2) = 4,260 m2
§ Shopping center / bank
• Area = 1 x 1,350m2 = 1,350m2
§ Offices level-1 to 10 with 1,350m2 each
• Area = 10 x 1,350m2 = 13, 500m2
q Example 2:
vSolution
3. Determine the grouped load
density for each space (from
table) as follow:
§ Parking Garage / Utilities
areas (including roof area)
Assumed average demand:
10W/m2
§ Shopping center / bank
Assumed average demand:
50W/m2
§ Offices
Assumed average demand:
50W/m2
q Example 2:
vSolution
4. Multiply each gross interior area by its grouped load density to
get the estimated electrical load for this space as follows:
§ Parking Garage / Utilities areas (including roof area)
• Estimated Electrical Load = 4,260 m2 x 10W/m2 = 42.6 kW
§ Shopping center / bank
• Estimated Electrical Load = 1,350m2 x 50W/m2 = 81 kW
§ Offices
• Estimated Electrical Load = 13, 500m2 x 50W/m2 = 675 kW
q Example 2:
vSolution
5. Sum all the estimated electrical loads for all spaces to get the
total preliminary electrical load (Maximum Diversified Demand)
for the whole building
vTotal preliminary electrical load = 42.6 + 81 + 675 = 798.6 kW
6. Required Transformer Output:
vThe established total power demand determines the required
transformer output. The determination is based on a pf = 0.85
and a transformer load level of 70%.
vTransformer Output rating = 798.6kW / (0.7*0.85) = 1,324 kVA
qExample 3:
vConsider a similar building in the previous example to be
solved if individual load densities are known for each load
type as follow
Space Load Load Density (W/m^2)
Parking Garage / Utilities Lighting 7
areas (including Roof Area) Small Appliances 3
Shopping Center / Bank Lighting 28
Small Appliances 20
Offices Lighting 28
Small Appliances 20
vNote that, the load densities are given all in W/m2.
vCalculate the main electrical service size for this building.
qExample 3:
vSolution
§ The Following table summarizes all the calculation steps
Space Area (m2) Load Load Density (W/m2) Estimated Load Estimated
(W) Electrical Load for
Space
Parking Garage / 4,260 Lighting 7 29,820 42,600W
Utilities Areas Small Appliances 3 12,780
(including roof area)
Shopping Center / 1,350 Lighting 28 37,800 64,800W
Bank Small Appliances 20 27,000
Offices 13,500 Lighting 28 378,000 640,800W
Small Appliances 20 270,000
Total Estimated Electrical Load 755,400W
Application of pf (0.85) 888,706 VA
Application of Transformer Loading Level (0.7) 1,269,580 VA
Transformer Output rating 1,270 kVA
