Trystan Casey
Electrical feeder schedules are an important aspect of electrical design and installation, providing essential information for professionals in the field. Feeder conductors are responsible for delivering electricity within a house, taking over after the service entrance to supply power to the branch circuits. Before installation, a feeder diagram is often required, detailing the load, demand factors, size, type of conductors, and rating of overcurrent protective devices. Understanding how to read a feeder schedule is crucial for ensuring proper sizing of cables and conductors, considering voltage drop and equipment grounding. It also aids in quick identification of phase and voltage by qualified personnel. Creating a custom schedule with relevant parameters can be automated using tools like Revit MEP, aiding in efficient electrical feeder scheduling.
| Characteristics | Values |
|---|---|
| Purpose | To provide fundamental information for electrical design tasks and calculations |
| Information Included | Feeder sizes, overcurrent protective device ratings, wire sizes, voltage drop, equipment grounding conductor size, feeder type and load |
| Use | Allows qualified persons to quickly identify the phase and voltage of feeder conductors |
| Creation | Can be created using electrical add-ins for Revit MEP |
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What You'll Learn
Understand the difference between service and branch circuit conductors and feeder conductors
Conductors are wires that carry electricity. Feeder conductors are those that are neither service conductors nor branch circuit conductors. They deliver electricity within a house, taking over from the service entrance, which brings power from the utility pole, and supplying power to the branch circuits.
Service conductors are those between the utility service point and the service disconnecting means. They are not feeder conductors, and special service conductor rules apply because these conductors do not have short-circuit or overload protection.
Branch circuit conductors are those between the final overcurrent device protecting the circuit and the outlet(s). They have a smaller current flow than feeder conductors.
Feeder conductors are the "middle set" of conductors in the power distribution scheme. They are the circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. They are sized differently from branch circuit conductors. With branch circuits, the OCPD size is known, and the conductor is sized accordingly. With feeders, the load is determined, and then the conductor and OCPD are sized accordingly.
Feeder circuits must include an equipment grounding conductor (EGC) of a type listed in Sec. 250.118. The EGC must be installed in a way that allows branch-circuit EGCs to connect to it.
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Know how to identify the phase and voltage of feeder conductors
Feeder conductors are circuit conductors that deliver electricity within a house. They are distinct from service conductors and branch circuit conductors. Feeder conductors take over after the service entrance, which brings power from the utility pole, and they supply power to the branch circuits.
To identify the phase and voltage of feeder conductors, it is important to refer to the feeder identification method used and posted at each feeder panel board. This information should be readily available to those who will service the electrical system.
The National Electrical Code (NEC) guidelines outline specific identification methods for ungrounded conductors, which can be identified by phase or line and system at all termination, connection, and splice points. Identification methods for ungrounded AC system conductors may include colour coding, marking tape, tagging, or other approved means. For example, the NEC mandates the use of the colour orange to identify the high-leg of a 4-wire delta-connected system. Electricians often use a colour system for power and lighting conductor identification, such as black, red, and white for 120/240V, single-phase.
Additionally, it is important to consider the voltage drop in feeder conductors to ensure it remains within acceptable limits. Voltage drop calculations are essential, especially for long runs, and larger conductors may be necessary if the voltage drop exceeds the recommended limits.
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Learn how to create a feeder diagram
To create a feeder diagram, you will first need to create a custom diagram template. This template should be configured with specific diagram rules.
Firstly, a Set Starting Point rule must be set to specify the feeder as the starting point for the Trace rule. Secondly, a Set Root Junction rule must be set to specify the feeder as the root junction for the Tree layout. Thirdly, a Trace rule must be configured to force the automatic execution of a subnetwork trace from the input feeder. Finally, a Smart Tree layout or Mainline Tree layout should be chosen to automatically lay out your diagram content.
The Mainline Tree Layout algorithm will give you direction options to determine the direction of the main line, for example, left to right.
To learn about creating diagram templates in detail, you can refer to the Configure Your Network Diagrams section, which provides advice on using geoprocessing models to manage diagram template rules and layout definitions.
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Understand how to calculate voltage drop
Voltage drop calculations are essential in electrical design to ensure equipment functions properly and efficiently. Voltage drop occurs when an electrical current passes through a conductor, such as a wire, and the current flow generates resistance, causing a decrease in voltage. The longer the conductor and the larger the current, the greater the voltage drop.
The National Electrical Code (NEC) provides guidelines, formulas, and tables to calculate voltage drop based on the type and size of conductor and the length of the circuit. While the NEC does not enforce requirements on voltage drop, it is important to consider as it can waste power and, if the drop is too high, can cause equipment malfunction or failure.
To calculate voltage drop, you can use one of the following methods:
- Multiply the allowed voltage-drop percentage by the nominal system voltage. For example, in a 120/208 volt system, the allowed voltage drop for a single-phase 120-volt branch circuit load is (120 0.03) = 3.6 volts.
- Use the formula: Vd = (1.73 x Z x I x L) / 1000 for three-phase systems, where Vd is the voltage drop, Z is the impedance of the conductor per 1000 ft, I is the load current in amperes, and L is the length in feet.
- Compare the one-way cable length in feet to the applied voltage. If the cable length is almost or more than the applied voltage, a voltage drop calculation is recommended.
It is important to note that the calculation is based on the design load, which is the calculated maximum demand load on the circuit per electrical codes. For feeders, this is typically the maximum demand load on the circuit, and for branch circuits, it is either the branch circuit rating for receptacle loads or the 100% load of a specific load such as a motor.
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Learn how to use a feeder schedule to look up information
A feeder schedule is a quick-reference tool that provides fundamental information for electrical design tasks and calculations. It is particularly useful for looking up information related to electrical feeder design and installation.
Before installing feeder conductors, a feeder diagram is often required by the relevant authority. This diagram includes details such as the total calculated load on the feeder, any demand factors influencing the sizing of the feeder conductors, the size and type of feeder conductors, and the rating of the feeder overcurrent protective devices. This information is crucial for understanding the electrical feeder system and can be found on the feeder schedule.
For example, if you need to determine the appropriate feeder size for a specific application, you can refer to the feeder schedule. It provides feeder sizes based on the rating of the overcurrent protective device. By matching the device rating with the corresponding feeder size, you can ensure you select the correct feeder for your electrical design.
Additionally, the feeder schedule can assist in addressing voltage drop issues. It provides guidance on whether to upsize the conductors to compensate for voltage drop. However, it's important to note that using the feeder schedule for upsizing may result in an incorrect equipment grounding conductor (EGC) size. In such cases, a manual calculation may be necessary to determine the accurate EGC size.
Overall, the feeder schedule serves as a valuable resource for electrical professionals, providing quick access to essential information related to feeder design, sizing, and installation. By referring to the feeder schedule, professionals can make informed decisions and ensure the safe and effective implementation of electrical feeder systems.
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Frequently asked questions
Feeder conductors deliver electricity within a house. They are different from service and branch circuit conductors. Feeder conductors take over after the service entrance, which brings power from the utility pole, and supply power to the branch circuits.
A feeder schedule is a tool that provides fundamental information for electrical design tasks and calculations. It includes the size and type of feeder conductors, as well as the rating of the feeder overcurrent protective devices.
You can create a custom schedule that displays the wiring parameters, but this may include all branch circuits and feeders. To automate this process and only display feeders, you can use electrical add-ins for Revit MEP to create a shared parameter attached to the electrical equipment that displays the feeder size.
A feeder diagram should include the total calculated load on the feeder and any demand factors used in sizing the feeder conductors. It should also include the size and type of feeder conductors and the rating of the feeder overcurrent protective devices.
