Chapter 5: Distribution Systems
Designing Electrical Distribution Equipment
Selecting and Sizing Distribution System Components
Using Conductors in Parallel
Create a single-line diagram to industry standards.
Calculate the required amperage sizes for the distribution equipment necessary for an electrical design.
Generalize system requirements and criteria into a single-line diagram.
Interpret the grounding requirements for an electrical system and apply them into an electrical design.
All facilities have a main point of entry for the electrical service that originates at the serving utility and terminates in the main electrical switchboard cabinet(s). From this main switchboard, all power is distributed to other panelboards serving lighting and power branch circuits.
When a facility requires more than 200 A, the distribution system will be a switchboard system. A switchboard is an electrical cabinet or cabinets (depending on the electrical requirements) that has provisions for the service entrance method, utility metering, and overcurrent protective devices that serve distributions to both the panelboards and larger branch circuits that serve larger motor loads (see Figure 5-1). Switchboards are larger than the typical wall-mounted electrical services that serve distributions of 200 A or less. These cabinets typically range in size from 30 in. to 36 in. in width to 90 in. tall and house service entrance conductors, utility metering equipment, the main disconnecting means, overcurrent devices for distribution to panelboards, and larger branch circuits.
Figure 5-1: Main switchboard
All the power requirements for all components such as the general branch circuits, specialized equipment branch circuits, and lighting system are brought together to develop a single-line diagram of the distribution system. A single-line diagram (also called a one-line diagram) is a simple diagram that illustrates all the information and requirements of the main electrical distribution system (see Figure 5-2), including the following:
Service entrance equipment
System grounding methods
When single-line diagrams are used for 3-phase systems, the conductors, bus bars, and feeders are all drawn as a single line representing the phases. Transformers, circuit breakers, and fuses are shown using standard symbols.
Figure 5-2: Single-line diagram. (Note: this diagram is also included in the Student Resource CD-ROM.)
The simple format of the single-line diagram clearly conveys all the required information about the main service entrance and distribution sections that serve the panelboards, transformers, and other large loads within the facility. Single-line diagrams do not provide information for smaller branch circuits; this information is conveyed through panel schedules and raceway legends. Branch-circuit information is only included in the single-line diagram if the equipment is served directly from the main switchboard (as is the case with some large motors or machinery loads that are not derived from branch-circuit panelboards).
Single-line diagrams are not usually drawn to scale. Instead, they are drawn to an appropriate size based on the scope of the project so that they can include all the necessary information within a single plan sheet.
In a single-line diagram, the switchboard is represented as a dotted outline showing the number of electrical cabinets required. Switchboards are typically illustrated with utility metering requirements on the left and distributions such as panel-boards on the right, though this is not necessarily the order in which they will be installed. Cabinetry is manufactured as sections, and the electrical designer determines the order of these sections based on the location of the switchboard within the facility and the requirements dictated by the serving utility for the point of entry. The associated electrical equipment, including utility metering equipment and disconnecting means, is illustrated in the single-line diagram as a large rectangle adjacent to the underground section with fuse or circuit breakers marked using the appropriate symbols.
Service entrance conductors, as defined by the National Electrical Code (NEC), are the conductors from the service entry point to the service main disconnecting means. These conductors are installed from the main switchboard to a connection point dictated by the serving utility, and, depending on the serving utility requirements, they can be installed as part of an overhead-type installation or in underground conduits.
For underground installations, the electrical contractor typically provides and installs any required underground conduits to specifications provided by the serving utility. The serving utility supplies and installs the service entrance conductors.
In overhead (aboveground) installations, the designer determines the raceway and conductor sizes based on the requirements of NEC Article 230, and the electrical contractor provides and installs them.
Utilities may have a service planner who can work directly with electrical designers and contractors. The service planner may provide a detailed plan indicating raceway size, location, and burial depth requirements for underground conduit installations or point of building entry for overhead type installations.
The distribution section of the single-line diagram provides information such as style of protection and rated amperage for individual distributions from the main switchboard to the panelboards, transformers, and, if designed, large motorized equipment loads throughout the facility.
The switchboard manufacturer builds into the system the required number of appropriately sized overcurrent protective devices. If the switchboard uses a fused-based system, the size of the disconnecting means, fuse sizes, and the number of poles must be indicated. If the switchboard uses a circuit breaker-based system, the size of the circuit breaker and the number of poles must be indicated.
Designers must always consult the utility provider and local municipality during the design phase to determine the applicable requirements.
For example, in the sample single-line diagram in Figure 5-2, the distribution to A/C 1 is referenced as 60AS/40ATDF, 3-pole. This designation indicates that this disconnecting means is to be supplied with a three-pole 60-A disconnect switch containing 40-A time delay fuses.
Panelboard, Transformer, and Branch-Circuit Distributions
The panelboard, transformer, and branch-circuit distribution section of the single-line diagram indicates the following information:
Equipment being served
Overcurrent protective device size and type
Raceway quantity and type
Conductor quantity, size, and insulation type
Distance to the equipment
Panelboard size (in amperage), phase, and voltage
Unless the owner of the facility has special identification requirements, panelboard identification is left to the discretion of the designer. Typically, power panelboards are designated P1. P2 and so forth; panelboards that supply only lighting loads are designated L1 L2 and so forth; and panelboards that supply both lighting and power are designated LP1 LP2, and so forth. If a switchboard will supply a separate cabinet designed as a motor control center, the designation is typically MCC.
The distribution transformers section of the single-line diagram includes:
Transformer title (e.g., T)
Transformer size (in kilovolt-amperes [kVA])
Transformer primary and secondary voltages
Size of the transformer(s) primary and secondary protection (in amperes)
Raceway size(s) for the primary and secondary sides of the transformer
Conductor quantity and size for the primary and secondary sides of the transformer
Distance of conductors serving the transformers
System grounding electrode conductor size and grounding method
Voltage drop may also be indicated on the single-line diagram (next to the distance to the load) to indicate that voltage drop on the feeder circuit(s) was calculated and that the design was adjusted accordingly.
System Grounding Methods
The system grounding methods section of the single-line diagram indicates the following information:
Size of the grounding electrode conductor(s) for the main switchboard
Bonding of the equipment
Designers should always confer with the local building department authorities, who may have specific regulations about which grounding methods are to be employed. If the jurisdiction is unknown at the time of the design, then the print should include a general note stating that the system grounding methods must conform to the requirements of the authority having jurisdiction. A note may also be added to the print stating, “Ground as per all applicable methods, based on NEC 250″ to ensure that all applicable NEC requirements are met.
Designers can contact local officials during the design process to inquire about which system grounding methods are approved for the intended jurisdiction.
Designing Electrical Distribution Equipment
All distribution assemblies require a standard amount of physical space based on their voltage and amperage capacities. Electrical designers cannot arbitrarily choose a location for the main electrical distribution equipment. The serving utility may have location requirements, and any locations with electrical equipment must be designed with sufficient physical wall, ceiling, and floor space to meet the NEC working clearance requirements [110.26 and 110.30]. The switchboard manufacturer determines the amount of space required for the designed distributions and how many sections (cabinets) are required. Once this information is known, the electrical designer must ensure that the area where the cabinetry will be located is of sufficient size to meet the requirements of the NEC.
NEC Article 110 outlines the amount of working space required in areas where electrical equipment is located.
The width of the cabinets limits the quantity of distributions that can be derived from any one cabinet. If all the distributions are between 30 A and 100 A, then the switchboard could possibly be manufactured as a single cabinet. For distributions greater than 100 A, the switchboard may need to be manufactured with multiple cabinets because of the physical size of all distributions required.
Designers should always contact the switchboard manufacturer to obtain physical dimensions for required cabinetry.
Selecting and Sizing Distribution System Components
The first step in the development of a single-line diagram is for the designer to make an overall determination of how many panelboards are required to serve all the branch circuits designed for general electrical needs, specialty equipment, and lighting. Panelboards are capable of serving approximately 42 branch circuits. Although large numbers of branch circuits can be served from any one panelboard, designs usually incorporate several panelboards. This is common practice because panelboards should be located within the approximate area where the loads are being served and because branch circuits serving general office areas and computers should be isolated from branch circuits that may serve motorized equipment loads. The starting and stopping of motorized equipment loads can sometimes cause momentary voltage surges, and if motorized equipment loads are supplied through the same panelboard as the one serving computer equipment and lighting circuits in office areas, these momentary voltage surges can cause problems with the computers or noticeable fluctuations in the lighting systems.
Sometimes facilities require multiple voltages, such as 480 and 208 volts for equipment and 120 volts for general office loads. The utility will not serve both voltage systems to such a facility. If, for example, the facility is served with a 480-volt system and it also requires a 120/208-volt system, the utility will serve the higher potential and the designer will have to provide for step-down transformers to obtain the additional voltage. In such situations, a panelboard cannot serve both 480 and 208 volts; therefore, additional panelboards are required to supply the lower voltage potential.
For most commercial applications, one panelboard is installed to serve the office area within a facility and one or more additional panelboards are installed in the manufacturing area based on the number of branch circuits required and their required operating voltages.
For a single-line diagram, the designer must determine the required number of panelboards based on the branch-circuit requirements, and then draw a basic outline of the distribution methods and the required equipment. For example, for a facility served with a 277/480-volt, 3-phase system and with the following requirements, a minimum of three panelboards should be designed:
480-volt, single- and/or 3-phase equipment loads
280-volt, single- and/or 3-phase equipment loads
120-volt general office loads
The panelboards would be assigned as follows:
One panelboard is located in the manufacturing area for the 480-volt equipment loads.
One panelboard is located in the manufacturing area for the 208-volt equipment loads.
One panelboard is located in the office area for the 120-volt general office loads.
Based on the minimum quantity of branch circuits required for each of the preceding applications, additional panelboards might be necessary.
With the number of panelboards and their branch circuits assigned, the designer can determine the amperage size of the panelboards and the conductor size of the feeders that supply them. Next, the designer creates a basic outline of the single-line diagram illustrating the distributions to the known panelboards. The designer will calculate the amperage of the panelboards and the size of the feeder conductors that serve them later in the design process.
Panelboards and Feeders
After a basic outline of the single-line diagram has been completed, the next step in designing the distribution system is for the designer to determine the appropriate feeder conductors and raceway sizes that will serve the panelboards. In the completed panel schedules, the total amperage listed at the bottom of the panel schedule can be referenced to standard National Electrical Manufacturers Association (NEMA) panelboard sizes (see Table 5-1). At a minimum, panelboards must have the amperage capacity to serve the designed loads determined by the panel schedules. Feeder conductors should then be sized based on the capacity of the panelboards.
As per the NEC, feeder conductors that serve panelboards are required to be rated at a capacity not less than the load to be served plus 125 percent for any part of the load that will be serving continuous loads [215.2(A)(1)]. If a panelboard will serve both noncontinuous and continuous loads (as is some-times the case with general purpose receptacle and commercial lighting loads served by the same panelboard), then the panelboard and feeders must be sized to serve the noncontinuous load at 100 percent plus the continuous lighting load at 125 percent. The designer adds up all the loads and adds an additional 25 percent for any loads that have been determined to be continuous loads.
When a transformer is installed to change one serving voltage to another to serve loads with different voltage requirements, the transformer must be sized appropriately to serve the associated loads. The size of the transformer is based on the total capacity of the panelboard being served, not the total calculated load determined by the panel schedule. Table 5-2 lists standard NEMA manufactured sizes for 3-phase transformers.
As per the NEC, transformers must have over-current protection on the primary side [450.3(B)]. When transformers serve branch-circuit panel-boards, they are required to have overcurrent protection on secondary side [408.36(B)]. Therefore, in commercial installations, both primary and secondary protection is required when transformers are used to reduce a source voltage of 480 volts to a lower 120/208 voltage to serve panelboards serving loads with lower voltage requirements.
The proper grounding of an alternating current system is of utmost importance to protect personnel and equipment from the dangers associated with electrical shock. The NEC is very detailed and specific about the associated components required for proper grounding and how the process is to be completed for all devices and equipment [Article 250].
For any system that is required to be grounded, an effective grounding path starts with the proper grounding of the electrical service equipment, which also includes the grounding of any transformers installed in a facility. When all the required grounding components are properly designed and installed to the designed specifications, a designer can feel confident that the system adheres to the requirements and that a high degree of safety is present in the electrical system.
To design a panelboard that serves a combination of continuous lighting and noncontinuous loads with the following specifications:
277/480-volt, 3-phase, 4-wire panelboard
28,306-VA load contribution for the lighting branch circuits
205,310-VA load contribution for other noncontinuous loads
Calculate the required additional 25 percent for the long continuous load by multiplying the long continuous load served by the panelboard by 25 percent:
Sum all loads plus the long continuous load volt-amperes:
Noncontinuous load volt-amperes
Continuous load volt-amperes
25% long continuous load factor
Total connected load
Open table as spreadsheet
Use Ohm’s law to calculate the total connected load in amperes:
Reference manufacturer catalogs to select the correct size panelboard:
The closest NEMA sized panelboards rated at 480 volts are 300 A and 400 A. For this application with a total calculated value of 290 A, a 300-A panelboard meets all the requirements. Therefore, this application can be served with the 300-A panelboard. However, the calculated value of 290 A for this application is close to the maximum allowable amperage for a 300-A panelboard; therefore, the next higher size of panelboard can be used so that the facility has future capacity.
Reference NEC Table 310.16 to properly size the feeder conductors that will serve the panelboard: If a 300-A panelboard is used, then per Table 310.16, the minimum size thermoplastic high water-resistant nylon-coated (Type THWN) copper conductor rated at 300 A = 350 kcmil.
If a 400-A panelboard is used, then per Table 310.16, the minimum size Type THWN copper conductor rated at 400 amperes = 600 kcmil.
Answer: A panelboard sized at 300 A would be fed with 350 kcmil feeder conductors. A panelboard sized at 400 A would be fed with 600 kcmil feeder conductors.
To calculate amperage size of a panelboard, transformer, and secondary feeder conductors for a system with the following specifications:
Primary voltage of 480 volts
Secondary 3-phase voltage of 120/208 volts
Calculated secondary load of 165 A at 120/208-volt, 3-phase, 4-wire potential
Refer to NEC Table 310.16 to determine the amperage size of the panelboard sufficient for the load to be served:
For a calculated load of 165 A, the minimum required size panelboard is 200 A.
Properly size the transformer to serve the load:
For this application, the calculated load is determined to be 165 A, so a 200-A panelboard is used. The transformer must be rated to serve 200 A, and the secondary feeder conductors that serve the panelboard must be rated at 200 A.
Properly size the secondary feeder conductors:
A 200-A panelboard must be served with feeder conductors capable of supporting the 200-A panel-board. As per Table 310.16, size 3/0, copper Type THWN secondary feeder conductors are required.
Answer: A 75-kVA transformer with a maximum secondary amperage of 208 A at 120/208 volts supporting a 200-A panelboard with size 3/0 Type THWN copper conductors are necessary for this application.
Grounding Distribution Transformers
When distribution transformers are installed to change a voltage to a higher or lower potential, the new potential then becomes a separately derived system and must conform to the system grounding requirements as dictated by the NEC [250.20]. In typical distribution transformers, the secondary voltages are provided through the principle of magnetism and the primary windings and secondary windings are not electrically connected. In a system where all grounding requirements are adhered to, but because the secondary side of a transformer derives its potential through magnetic principles and is no longer connected to the primary side of the transformer, any grounding that may have taken place on the primary side is lost. This new potential on the secondary side is classified as a separately derived system, and this new system must be grounded [250.30].
To ground a system properly, a continuous, non-spliced, grounding electrode conductor is installed between the electrical service equipment and the first grounding electrode (also called the primary ground) (see Figure 5-3). From the point of connection at the primary grounding electrode, an additional bonding jumper of equal circular mil area should be installed to the second grounding electrode (and third, if required). It must be understood that the primary grounding electrode conductor must not be spliced between the electrical service and the primary grounding electrode and must be attached to the grounding electrode using approved methods [250.64]. From the point of connection at the primary ground, a bonding jumper of the same American Wire Gauge (AWG) size as the grounding electrode conductor must be connected between the primary ground and the secondary ground. The bonding jumper is allowed to be a separate conductor from the primary grounding electrode conductor, but the grounding path is more consistent and reliable when both are installed as one complete conductor.
Figure 5-3: Grounding illustration
To size the grounding electrode conductor (using the 75 kVA transformer and 200-A panelboard):
Refer to NEC Table 250.66, which illustrates the required grounding electrode conductor sizes for alternating current (AC) systems based on the size of the largest ungrounded conductor serving the system.
Determine the secondary feeder conductor sizes. (The new 120/208-volt, 3-phase system is a separately derived system and all the grounding requirements must be based on secondary values.) For this example, size 3/0 Type THWN copper conductors are used as secondary feeder conductors.
Answer: NEC Table 250.66 requires that a size 4 copper grounding electrode conductor be used for a secondary feeder size of size 3/0 copper.
NEC table 250.66 Grounding Electrode Conductor for Alternating-Current Systems
Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors[a] (AWG/kcmil)
Size of Grounding Electrode Conductor (AWG/kcmil)
Aluminum or Copper-Clad Aluminum
Aluminum or Copper-Clad Aluminum[b]
2 or smaller
1/0 or smaller
1 or 1/0
2/0 or 3/0
2/0 or 3/0
4/0 or 250
Over 3/0 through 350
Over 250 through 500
Over 350 through 600
Over 500 through 900
Over 600 through 1100
Over 900 through 1750
Source: NEC® Handbook, NFPA, Quincy, MA, 2008, Table 250.66
[a]This table also applies to the derived conductors of separately derived ac systems.
[b]See installation restrictions in 250.64(A).
The size of the grounding electrode conductor is based on the size of the largest ungrounded service entrance conductor—or, in the case of a transformer, the secondary feeder conductors—used to serve the panelboard. These form a new separately derived system and therefore should be treated as service conductors. The size of the service grounding conductor is based on the sizes found in NEC Table 250.66.
Grounding Service Entrance Equipment
Service entrance equipment is grounded in the same manner as the distribution transformers are. As with transformers, an electrical service supplied through a utility transformer is classified as a separately derived system. The grounding electrode conductors for the main service are sized according to NEC Table 250.66 and, for the grounding electrode conductor for the main switchboard, the largest service entrance conductor. Grounding of service entrance equipment can differ somewhat from transformers if the utility serves the facility through an underground system. When a facility is served with an underground type distribution, the service entrance conductors are typically supplied and installed by the utility. Therefore, the designer may not know the actual size of the conductors. When this is the case, the designer selects the grounding electrode conductor based on the size of the largest service entrance conductor required for the main electrical service based on the size of the main overcurrent device.
Using Conductors in Parallel
When currents 400 A or greater are required by service equipment, panelboards, and other equipment, the conductors to serve them can be very large in kcmil size. This can be costly because of conductor and raceways size and difficulty of installation. Conductors may be connected in parallel (electrically joined at both ends) to reduce the size of conductors [310.4]. This follows the basic Ohm’s law principle for parallel circuits, which states that currents divide in parallel circuits. If each path has equal resistance and length, the currents divide equally. As per the NEC, conductors to be connected in parallel must be [310.4]:
For sizing grounding electrode conductors in parallel, the required size listed in NEC Table 250.66 is based on the equivalent circular mil area of the parallel service entrance conductors.
Sizes 1/0 and larger
Equal in length and circular mil area
Of the same material and insulation type
Terminated in the same manner
Paralleling of conductors is not limited to two conductors per phase; when equipment requires large amounts of current, three or more conductors may be installed for each phase in parallel groups in multiple raceways as long as the minimum size of the paralleled conductors is greater than size 1/0 AWG/kcmil.
To parallel conductors for a system with a 400-A load with two parallel conductors:
Divide the total load in amperage by the number of conductors to determine the minimum amperage of each paralleled conductor:
Refer to NEC Table 310.16 to find that a 200-A copper Type THWN conductor should be size 3/0 AWG.
Ensure that the parallel conductor sizes are greater than size 1/0 AWG:
A size 3/0 conductor is larger than a size 1/0 conductor.
Answer: For this application, two parallel size 3/0 conductors could be utilized as an option to using single 600-kcmil conductors. Please note that the paralleling of conductors is not limited to two, but can be three or more if the amperage of the load(s) is greater and the size of the paralleled conductors is not less than a size 1/0 conductor.
Wrap Up: Master Concepts
Designers develop single-line diagrams to outline the electrical service entrance equipment and how the electrical system is distributed throughout a facility.
Generally transformers are required to provide for a change in voltage from the source voltage (at the service entrance point) for general and equipment electrical loads that operate at different voltages.
Electrical service entrance equipment and transformers must be properly grounded to protect personnel and equipment.
continuous load Any load where the maximum current is expected to continue for 3 hours or more .
effective grounding path A grounding path of low resistance that ensures, either through raceway methods or additional wiring methods, that the operation of protective devices will occur to isolate a faulted system and thus protect personnel from the dangers of electricalshock or explosion.
grounding electrode A conducting object through which a direct connection to earth is established .
grounding electrode conductor A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or a point on the grounding electrode system .
main disconnecting means The main device that disconnects the supply conductors from all sources of supply.
separately derived system A premises wiring system whose power is derived from a source of electrical energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system .
service entrance conductors The conductors from the service entry point to the service main disconnecting means. These can be installed as part of an overhead-type installation or in underground conduits.
single-line diagram A simple diagram (also called one-line diagram) that illustrates all the information and requirements of an electrical distribution system.
switchboard An electrical cabinet, or cabinets (depending on the electrical requirements), that has provisions for the service entrance method, utility metering, and overcurrent protective devices that serve distributions to equipment.
utility metering equipment The components that make up the parts of an electrical cabinet used solely for the purposes of utility metering, such as kilowatt-hour/ demand meters, meter testing points, and current transformers.
Check Your Knowledge
Which of the following is NOT illustrated on a single-line diagram?
How many electrical switchboard cabinets are required for a project?
For most typical underground service entrances, the minimum number of 10.2 cm (4-in.) raceway(s) required for every 400 A of electrical service is:
What is the minimum copper Type THWN primary conductor size required for a 112.5-kVA, 3-phase transformer with a primary voltage of 480 volts and a secondary voltage of 120/208 volts?
What is the minimum copper Type THWN secondary conductor size required for the transformer described in question 4?
What is the minimum required copper grounding conductor size for the 112.5-kVA transformer described in question 4?
What are the maximum allowable sizes for the primary and secondary overcurrent protective devices for the 112.5-kVA transformer described in question 4?
Feeder conductors supplied from the terminals of a transformer must be provided with overcurrent protection within the first:
Which of the following is NOT a standard NEMA 3-phase panelboard?
What is the required minimum copper grounding electrode conductor size for an electrical service fed with a parallel group of two 350-kcmil copper conductors for each of the phases?
A 200-A panelboard with a net computed load of 147 A must be fed with_____Type THWN copper conductors.
You are the Designer
Apply the knowledge you have gained from this previous chapter to your own electrical design. In this section you will:
Create and draw a single-line diagram with the following components:
– Service entrance conductors
– Panelboard, transformer, and large branch-circuit distributions
– System grounding
Size the required components for the distribution system:
– Panelboard and feeders
– Distribution transformers
Design a grounding system for the following components:
– Service entrance equipment
– Conductors in parallel
About Your Project
To complete your task, you must know the following details about your project:
Quantity of panelboards (previously determined)
Which panelboards (1) will be served directly from the main switchboard and are supplied by 480 V or (2) will be served from distribution transformers to supply a 120/208-volt, 3-phase, 4-wire system
Which large branch-circuit loads will be served directly from the main switchboard
To develop this part of your design, you need the following resources:
Copies of all the panel schedules you have developed
Single-line diagram on plan sheet E3 on the Student Resource CD-ROM
Get to Work
In this part of the design, you bring together several components to develop the single-line diagram that forms the foundation for the overall designed load and the required size for the electrical service that will serve the facility.
Start by entering all required information into your panel schedules, beginning with the size of the overcurrent protective device that will serve each designed branch circuit. In commercial and industrial applications, general purpose receptacle branch circuits and many small specialty equipment loads (with volt-ampere ratings of 1800 VA or less) are typically served by single-pole 20-A circuit breakers. Use the designation 20/1 for the panel schedule to indicate that the branch circuit is served by a 20-A single-pole circuit breaker (see completed panel schedule P3). For each panel schedule you have developed, determine which loads are to be served with these types of circuit breakers and enter the information in the panel schedule.
Next, calculate the size of the circuit breaker that will serve equipment loads served by panelboards and the size of overcurrent protection. Enter the amperes over the number of poles into the panel schedule (e.g., 80/3 for a 3-pole device). (See completed panel schedule P1 for an example.) With the total calculated amperage at the bottom of your panel schedules, reference Table 5-1 to determine the minimum size panelboard required and enter that information in the panel schedule next to the heading Main Breaker.
To determine the size of a circuit breaker serving an equipment load with the following specifications:
10-hp, 208-volt, 3-phase motor
Obtain the amperage of the motor based on voltage and horsepower. Use NEC Table 430.248 for single-phase motors and NEC Table 430.250 for 3-phase motors:
Multiply the full-load current of the motor by 250 percent (per NEC Table 430.52):
Reference NEC 240.6 for the standard size circuit breakers and size the over-current protection accordingly. Remember that if the value of the overcurrent device does not calculate to a standard value, you can use the next higher standard:
Next higher standard overcurrent protective device = 80 A
If the equipment to be served will be protected by a fused-based device, then a time delay type fuse should be used. For these applications, use the same procedure as described in this example, but multiply the full-load amperage by 175 percent, as allowed in NEC Table 430.52.
When your design requires a distribution transformer to change the incoming 480 volts to a 120/208-volt, 3-phase, 4-wire system, you need to determine the size of the following components:
Transformer (in kVA)
Primary and secondary overcurrent protection devices
Primary and secondary conductors
Raceway types and sizes for the feeders for the primary and secondary conductors
Grounding electrode conductor
As previously illustrated, the size of the transformer is determined by the amperage size of the panelboard it will serve. Start by identifying which panel schedules you have completed that will require a distribution transformer; these are all the panel schedules that operate at a 120/208-volt, 3-phase, 4-wire voltage.
To determine the required distribution transformer size, reference your panel schedule under the Main Breaker section. The value you entered here determines the minimum kilovolt-ampere size for the transformer that will serve the panelboard. For example, the completed panel schedule P2 is protected by a 200-A main breaker; therefore, based on the transformer sizes in Table 5-2, a 75-kVA transformer is necessary.
kVA ratings for transformers must have the capacity to serve the rating of the panelboard they supply, even though the calculated loads in a panel schedule may be less.
When sizing the primary and secondary protection for transformers, you can use two methods based on NEC Table 450.3(B), which lists the maximum allowable primary and secondary protection sizes for transformers 600 volts or less. For a transformer with current greater than 9 A and that is protected on both the primary and secondary sides, the primary protection can be based on 250 percent of the full-load current and 125 percent of the full-load current of the device to be served (in this case, a panelboard). You use these maximum values typically only when a load that is to be served may have very high inrush currents that require the protections to be based on values above the actual rating of the device. For example, if the 200-A panelboard in the preceding example were to be protected at these higher values, then the primary and secondary protection sizes would be as illustrated in “Sizing Transformer Primary and Secondary Protections” below.
To size transformer primary and secondary protections with the following specifications:
75-kVA, 3-phase transformer
Calculate the maximum value for the primary by multiplying the full-load amperes by 250 percent:
Reference NEC 240.6 to determine standard size protection:
Calculate the maximum value for the secondary by multiplying the full-load amperes by 250 percent:
Reference NEC 240.6 to determine standard size protection:
Answer: If you use the maximum values permitted, this transformer could be protected on the primary side at 225 A and on the secondary side at 300 A.
Commercial and industrial applications typically do not require transformers to be protected at these maximum values because of the types of loads being served. To size protection for general use, distribution transformers are often protected at values based on the full load of the device to be served. For this application, the load to be served in a 200-A panelboard and the primary and secondary overcurrent protection sizes are determined. For example, for a 75-kVA, 3-phase transformer with a 480-volt primary and a 208-volt secondary, Table 5-2 shows that the full load at the primary is 90.2 A and at the secondary is 208 A, which—based on NEC 240.6 for a 200-A panelboard—would be sized at 200 A.
Transformer primary and secondary protection is typically provided based on a transformer’s full load amperages. These values may be increased to the maximum value permitted by NEC Table 450.3(B) when the loads have significant inrush amperages. Large loads or loads with several motors require increased protection.
Because conductors are required to be sized to the load being served, the primary and secondary feeders for transformers are based on the full-load primary and secondary amperages. For example, for the same transformer, NEC Table 310.16 shows that Type THWN conductor for 90.2 full-load primary amperes would be a size 3 AWG conductor. For the 208 full-load secondary amperes it would be size 4/0 AWG. If this transformer was installed with an electrical metallic tubing (Type EMT) raceway, it would require three size 3 AWG feeders in a 2.5 cm (1-in.) Type EMT for the primary wire with four size 4/0 AWG Type THWN conductors for secondary feeders in a 5.1 cm (2-in.) Type EMT for a 120/208 system [Annex C, Table C.1]. The grounding electrode conductor is also sized based on the transformer secondary conductor sizes. As per NEC Table 250.66, the minimum copper grounding electrode conductor size would be size 2 AWG.
The grounding electrode conductor is based on the secondary feeder conductors for the new, separately derived system, not the primary conductor sizes.
Distribution System Raceways
Once you have determined and sized the panelboard and distribution transformers, the next step is to determine the proper raceway sizes for all distribution equipment. You have already located panelboards throughout the facility in previous phases of the design, so now you must designthe raceway system that will serve them. Reference the completed power plan (E1) and the completed distribution diagram (E3). Note that transformer T1 is located adjacent to the panelboard P2, following the guideline that, when possible, it is best to locate the transformer as close as possible to the panelboard being served. Also note that the primary-side conductors are smaller (in AWG size) and carry lower amperage values than the secondary conductors. When panelboards are located in close proximity to the transformer serving them, the larger secondary conductors are much shorter in length than the primary conductors. This design helps with any voltage drop concerns caused by the length of a conductor and the amperages it must supply. Because primary currents are less, it is best to travel the distance to the panelboard with a primary conductor that serves less amperage and to supply the transformer secondary conductors with the higher amperages to the panelboard with a shorter-length conductor. Smaller conductors are also less expensive than larger conductors, so this design is more cost-effective (see Figure 5-4).
Figure 5-4: To help eliminate voltage drop and reduce installation costs, locate panelboards in close proximity to the distribution transformer serving them
When a panelboard supplied by a transformer is fed from another panelboard, the load (in VA) must be included in the primary panelboard to properly reflect the load being by the primary panelboard (see completed panel schedule P1.
The completed single-line diagram also shows that panelboard P2 is supplied by panelboard P1 through a 75-kVA transformer with maximum allowable primary of 90.2 A. The total volt-amperage values from panel schedule P2 are included in panel schedule P1 because panelboard P1serves both equipment loads and panelboard P2. Before the addition of the 75-kVA transformer loads, it was determined that panelboard P1 was also to serve the 75-kVA transformer and that the added load would not substantially increase the size of panelboard P1; therefore, P1 could also serve panelboard P2. Panel-board P2 could have been served directly from the main switchboard, but this would have made it necessary to run the primary feeder conductors for transformer T1 from the main switchboard, resulting in greater lengths and a possible increase in the primary feeder conductor sizes (because of possible voltage drop). The disadvantage to serving panelboard P2 through panelboard P1 is that if panelboard P1 is de-energized for repair or service, panelboard P2 is also de-energized.
It is important to remember that when feeder conductors are connected to the secondary terminals of a transformer, protection must be provided in the first 10 ft [240.21(C)(2)]. This requirement is met when a panelboard is located within 10 ft of a transformer and the transformer has secondary protection (as is the case with panelboard P2 having a main breaker). If the panelboard will be located more than 10 ft from the secondary of a transformer, an overcurrent device must be installed within the first 10 ft. Figure 5-5 illustrates both options.
Figure 5-5: In the installation method on the left, the larger secondary feeder conductors carrying greater amperages are much longer in length, leading to voltage drop and greater installation costs. Overcurrent protection must be provided within 10 feet as per NEC 240.21(B)(1). In the illustration on the right, the primary feeder conductors carrying lower current amounts are carried the greater length, eliminating voltage drop concerns and helping lower installation costs. The main breaker in the panelboard also provides for the required transformer overcurrent to be within 10 feet of the transformer
For your design, determine whether any panelboards served by transformers would be better served by supplying the panelboard through another panelboard. If you find one, revise your panel schedule file for the panelboard that will be serving a transformer and enter the volt-amperage values from the panelboard being served into the panel schedule for the panelboard serving the transformer (see completed E3 for an example). If you alter your design, you must reevaluate the volt-amperage totals for the panelboard serving the transformer in case its size must be increased to accommodate the additional load. If so, you will also need to resize the main breaker, feeder conductors, and the raceway system serving the panelboard that serves the transformer.
To complete the panel schedules, ensure that they include a panelboard title, location (e.g., office or manufacturing area), mounting method, voltage/phase, bus (same as panelboard size), and main breaker size.
Equipment Loads Served Directly from the Main Switchboard
When equipment loads are directly served from a main switchboard, you must list the following information:
Equipment title (e.g., A/C 1)
Branch-circuit conductor sizes
Equipment grounding conductor sizes (if applicable)
Overcurrent protective device size and number of poles
Additional disconnects (if applicable)
Horsepower and/or full-load amperage ratings
Distances from main distribution system to loads
When installing branch circuits to air-conditioning units, additional disconnects are required at the air-conditioning unit locations for disconnection of power for servicing the units. When these disconnects are provided, they are not required to have additional overcurrent protective devices because overcurrent protection is provided at the source of supply. When installed, the devices are installed as nonfused devices and are illustrated on the single-line diagram as “nonfused disconnect” or “NFD.”
Serving larger, motorized loads directly from the main service eliminates the need to increase the size or add additional panelboards to serve these loads.
Completing the Single-Line Diagram
At this point in your design, you can now complete all the required information about your distribution system. Use completed E3 as a reference and illustrate the following information on your plan:
Panelboard distribution layout
Panelboard feeder sizes
Titles (e.g., T1)
Size (in kVA)
Primary and secondary voltage
Primary and secondary feeder sizes
Loads (in amperage, located at bottom of panelboard)
Most plan errors occur in single-line diagrams. Review the single-line diagram to ensure that all necessary information is included and correct.