Distribution Systems 320

This class describes the means of distributing power to motors and other electrical devices within a typical industrial shop, focusing on distribution systems within a shop.


Class Name:
Distribution Systems 320
This class describes the means of distributing power to motors and other electrical devices within a typical industrial shop, focusing on distribution systems within a shop.
Number of Lessons:
Additional Language:

Class Outline

  • Objectives
  • Power Distribution Systems
  • Types of Distribution Systems
  • Generating Stations
  • Transformers
  • Phase Connections
  • Wye Connected Systems
  • Delta Connected Systems
  • Substations
  • Switchboards
  • Switchboard Uses
  • Panelboards
  • Motor Control Centers
  • Busways
  • Distribution System Maintenance
  • Summary


  • Define power distribution systems.
  • Identify the different types of distribution systems.
  • Describe generating stations.
  • Explain the role of transformers in a distribution system.
  • Identify phase connections in distribution systems.
  • Describe wye connected systems.
  • Describe delta connected systems.
  • Explain the role of substations in a distribution system.
  • Describe the role of switchboards in distribution systems.
  • Describe the ways switchboards are used.
  • Describe panelboards in distribution systems.
  • Describe motor control centers in distribution systems.
  • Describe busways used in distribution systems.
  • Describe common maintenance practices on points in the distribution system.



Vocabulary Term Definition
branch circuit The part of a distribution system consisting of circuit conductors, between the final overcurrent protection and the outlet or load attached.
busbar Thick, rigid strips or bars of copper that serve as a common connection between several circuits. Busbars are usually used near the switchboard and can be enclosed in busways.
busway The part of a distribution system consisting of metal enclosed busbars. Busways come in premade sections that can be fitted to any shape needed.
center tap The point where all three windings are connected to each other in a wye connection. For delta connections, one of the lines must be tapped to produce a neutral.
copper loss A power loss due to current flowing through wire. Copper loss is proportional to the resistance of the wire and the square of the current.
core A component of a transformer. The iron or steel core provides a controlled path for the magnetic flux generated in the transformer by the current flowing through the windings.
delta A connection of three components where a triangular series circuit is formed. Delta connections are used in three phase power systems.
delta/delta A transformer with both the primary and the secondary connected as a delta.
delta/wye A transformer with the primary connected as a delta and the secondary connected as a wye.
distribution substation A bank of step-down transformers near the end users. The distribution substation converts voltage closer to the required usable levels.
distribution system A circuit of users linked to a generating station and substations that is typically arranged in either a radial or interconnected manner. Local distribution systems transport power within a building.
enclosure A physical barrier designed to provide mechanical or electrical protection for components used in a system.
feeder A circuit conductor between the power supply source and a final branch circuit overcurrent device.
generating station A building where a utility company houses large linked AC generators to create the massive power for widespread use.
grounded conductor A system conductor that is intentionally grounded.
grounding conductor A conductor used to connect equipment in a wiring system to ground.
interconnected system A distribution system with multiple available power sources that can loop throughout the network. If one source goes down, a different source can be activated to maintain service.
kilovolt-amp A unit used to rate transformer and generator strength. A kilovolt is a thousand volts.
laminated Sealed together in multiple layers of thin sheets to construct the core for a transformer. Lamination helps reduce resistance losses.
line voltage The voltage present between any two of the conductors in a three-phase system. It is 1.732 times higher than phase voltage in a wye connection.
load center A device found in residential locations that performs the same functions as a panelboard does in commercial settings. Load centers are not required to have many panelboard safety features because homes do not draw the same amount of current as industry.
local power distribution system The part of a distribution system located within a building. Local distribution systems are the responsibility of shop electricians and maintenance workers and fall under NEC rules.
main disconnect switch A heavy, spring-loaded switch on a factory switchboard that can cut off power to the entire building. Switchboards with more than six switches or breakers are required by the NEC to have a main disconnect.
motor control center A modular assembly specifically designed to plug in motor control units. Motor control centers are supplied by a common bus, usually straight from the switchboard.
mutual induction The ability of one coil to induce a voltage into another coil. This principle is used in transformers to step up or step down voltage.
NEC The standard for minimum safe electrical installations. The National Electrical Code is adopted in some form as law in all 50 states.
network system A distribution system where multiple power sources are connected to each user to guarantee uninterrupted power. A network system is expensive, so it is used only when constant power is vital.
open delta A transformer connection that does not complete the triangle, using only two of the three sides. An open delta is used when full power is not needed.
overcurrent Excess current that may result from overload, short circuit, or ground fault. Devices like fuses and circuit breakers protect against overcurrent.
panelboard A single panel distribution cabinet or wall cutout box that holds automatic overcurrent protection devices for lighting, heat, or power circuits.
phase symbol (Φ) The Greek letter Phi, used to represent phases, such as in a 3Φ4W system.
phase voltage The voltage between any one conductor and ground.
plug-in busway A type of feeder busway with electrical control that allows loads to be added along the bus structure.
primary winding Turns of wire on a transformer core, used to transfer voltage from the input to the core.
radial system A distribution system with only one power source. If that source goes out, power is lost to the entire circuit.
revolving field generator A generator setup where an electromagnetic field is rotated between the conductor wound armature. Most AC generators are of this design.
secondary winding Turns of wire on a transformer core, used to transfer voltage from the core to the outside load.
service-entrance The point where electricity enters a buidling. A service-entrance switchboard has metering equipment and devices for overcurrent protection and electrical control.
single-phase A continuous single alternating current cycle.
step up In electricity, a phrase used to describe voltage adjustment. To step up voltage means to increase voltage.
substation Any grouping of switchgear and transformers located near a source or user. Substations are used to alter voltage levels and provide a service point of electrical control.
switchboard A freestanding assembly of panels with metering equipment, overcurrent and other protection devices. Switchboards take a large block of power from a substation and break it down into smaller blocks for use within a building.
switchgear A combination of disconnect switches and breakers used to isolate equipment in substations.
tap A wire connected to the midpoint of a transformer secondary winding. This allows the transformer to use different turns ratios.
terminal A connecting point in a circuit where a wire is attached to create an electrical connection.
three-phase A continuous series of three overlapping AC cycles offset by 120 degrees. Three-phase power is used for all large scale distribution systems.
three-phase power The most common form of AC power for distribution. Three-phase power has three overlapping AC cycles offset by 120 degrees.
transformer A device that transfers electrical energy from one circuit to another, without changing the frequency, using electromagnetic induction. A transformer is most often used to change the line voltage.
transmission substation A bank of step up transformers near the generating station. Transmission substations increase voltage as high as 800kV for widespread distribution.
turns ratio A comparison of the number of turns in the primary versus the number of turns in the secondary of a transformer.
wye A connection of three components where one end of each component is connected at a common point. Wye connections are used in three phase power systems.
wye/delta A transformer with the primary connected as a wye and the secondary connected as a delta.
wye/wye A transformer with both the primary and the secondary connected as a wye.