Electrical System

The Hagerty Library is powered by a 13.2 KV source provided by PECO. This electricity is brought through a 15 KV switch that safely allows control over the system. In order to use this source power, it must be stepped down to a lower usable voltage through a transformer. The following information can be seen below in an image of the single line diagram for the Hagerty library containing all the components explained through the page. The majority of large commercial buildings run on 480/227V 3-phase 4-wire distributions. This is largely due to the demand of large equipment in commercial buildings. Most mechanical equipment that can properly and efficiently handle the loads of commercial buildings, require a 480V 3-phase supply. Alternatively, 480/277V also works for single-phase loads such as lighting. 277V in a 4-wire system simply connects from phase to neutral. In 3-wire systems containing 3 hot wires, single-phase loads can be connected from phase to phase, however, 4-wire systems works better with imbalanced loads by using and additional neutral wire to carry the imbalances.
Click on the link to enlarge the Single Line Diagram: Hagerty Library SLD
Since a voltage reduction is necessary, the library has a 1000/1333 KVA transformer between the 15 KV switch and the main circuit breaker (MCB) that carries power to all of the building. This transformer is taking the primary 13.2 KV 3-phase, 3-wire power and stepping it down to its secondary 480/277V 3-phase, 4-wire power. From the MCB there are several circuits going out to different mechanical equipment and panels throughout the building. Each circuit is sized according to the standards in the National Electrical Code (NEC). For all the lighting and large equipment (i.e. air-handling units, chillers, etc.) 480/227V power is appropriate. However, there are receptacle loads and smaller equipment, such as motors, that require another step down in supply voltage. These transformers can be found throughout the single-line diagram along the MCB. The primary power in these transformers would be 480/277V 3-phase, 4-wire and step down to a secondary voltage of 208/120V 3-phase, 4-wire. This works very well since receptacle loads require 120 V single-phase power.
All of the described system attributes so far are for the normal power distribution of the library. As far as the emergency power, the library is backed up by a 450 KW generator sized appropriately to hold the loads of al the emergency equipment and panels. This does not include all the system, rather only the essential components to keep the building operating during an emergency situation. In the event of an emergency, the life safety equipment needs to be transferred over to the generator to ensure that the occupants are safe. This is done by using a 600A auto transfer switch at the 7th circuit containing the life safety system, including the elevators. When the MCB loses power, the auto transfer switch switches over to the emergency generator feed and draws power form the generator. When this happens, the emergency power control relay takes over for system control until power is returned to the MCB.
When designing these major components in electrical distribution systems, there are several load calculations and sizing that needs to be done to ensure that the system can handle all the different loads. This starts at the most basic level of finding a room or area’s lighting and power loads, using the total amps to size a panel, and selecting the proper conductor sizes to ensure proper distribution. This process follows up the single-line diagram (SLD), which can be found further down on this page. Refer to the SLD for clarification of any of the explained components. Below is an example of the basic methods used to size components of the system. In this example, the sizing for the loads of panel PN1 on the main entrance level will be demonstrated. You can find the excel sheet for all load calculations including this example here: Electrical Load Calculations
Panel PN1 serves a small portion of the north area on the main level. Most of the load for this panel comes from the receptacle loads throughout the spaces. Since this is a 208/120V power panel, no lighting is added. The first step in panel, transformer, and bus sizing is to calculate the load. In PN1 there are 42 receptacles and 2 heaters that were able to be identified on the drawings. This number is by no means finite since information was limited. The NEC rates, 120V receptacles as contributing a 180VA load. By multiplying the 180VA by the number of receptacles, the general load is calculated as 7560 VA. The NEC 220.44 also mandates that the first 10 KVA is to have a demand factor of 100% while the remaining can be a lower 50%. In PN1’s case, all of the load falls under the first 10 KVA. Using Ohm’s law to convert to amps, the total connected load is 21A for general receptacles.
The 2 heaters were rated at 16.7 amps each. By multiplying the amps by the voltage (120V) the load for each of the heaters is 2004 VA. However, NEC indicates that the highest-rated motor must be taken at 125% of its load. This means one heater must  be 2505 VA, ending in a combined 4509 VA. Using Ohm’s law again results in a connected load of 13A for the two heaters.
The combined load for PN1 including both receptacles and the heaters is 34A. Using a table of standard National Electrical Manufacturers Association (NEMA) panel boards, there are two reasonable options, a 60A or a 125A panel. A 60A panel would suffice for the load calculated here, however, there is clearly equipment of some sort not clearly indicated on the drawings. This is due to lack of knowledge on our part as students receiving this information. Since the original drawings of the Hagerty Library indicate that panel PN1 is a 125A panel, it is clear that some information is missing and that the load was slightly higher than what we have calculated here. If the load was even 50A it would be wise to upsize since the amperage is starting to get close to the max.
From this point, NEC Table 310.60 , contains minimum wire sizes for certain amperage ratings. By using 125A, it is clear that the appropriate conductor size to handle the panel is a 1 AWG. This is why the engineers originally selected this wire size for the panel. The next point up on the SLD is the 225 KV transformer to convert between 480/227V and 208/120V. The bus line that the transformer is feeding is 800A. Looking at standard full load amp (FLA) ratings of transformers, a 225 KVA is the most appropriate transformer to handle that amount of amperage. knowing the transformer size and connected load of the bus line, the proper circuit breaker size can be selected. In the case of this distribution section, a 400 AF/375AT fused circuit breaker is the most efficient option based on the size of the load.
This sizing method is how the original engineers decided on what equipment and parts were needed in order to manage all of the loads for the building. After all of the loads were considered, we came up with total electrical load of 974 KW and connected load of 1419 A. This resulted in an annual cost of $36300 which is moderate for the building size and purpose. In order to find these loads, we used ASHRAE and IEEE standard prescriptive load per square foot values. These values are general watts per square foot that the industry has come up with to use for preliminary design. For example, according to IEEE 1990, when designing large school buildings, the designer can typically assume 1 watt per square foot for general receptacle loads. IEEE was also used for large equipment loads while ASHRAE 1989 design methods were used for lighting. The 1989 and 1990 versions of the code were used due to the fact that they were the oldest code that was close to 1981 that we were able to access.
With a 1419 A connected load, a MCB large enough to safely handle this load is required. This is why the original engineers decided to use a 2000 A MCB seen in the SLD. This circuit breaker has an trip rating of 1800A and a frame size of 2000A. This means that a load of 1800A would cause the circuit breaker to trip.  Based on our limited information and lack of the current code of the time, we believe our calculated load is low. Certain assumption such as the design 1.84 w/sqft for lighting is most likely too low. Looking at information of energy audits of older buildings, many spaces at the time of Hagerty Libraries construction were above 2 w/sqft and even pushing the limit at 3 w/sqft. Knowing that the load is most likely higher than our calculations, a 1800 AT is the most sensible option. It seems a little over sized, however, based on standard bus line sizes, head room safety codes, and the fact that the load was probably higher, the 1800 AT MCB is appropriate.