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branch circuit
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The part of a distribution system consisting of circuit conductors, between the final overcurrent protection and the outlet or load attached.
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busbar
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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.
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busway
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The part of a distribution system consisting of metal enclosed busbars. Busways come in premade sections that can be fitted to any shape needed.
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center tap
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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.
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copper loss
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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.
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core
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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.
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delta
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A connection of three components where a triangular series circuit is formed. Delta connections are used in three phase power systems.
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delta/delta
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A transformer with both the primary and the secondary connected as a delta.
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delta/wye
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A transformer with the primary connected as a delta and the secondary connected as a wye.
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distribution substation
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A bank of step-down transformers near the end users. The distribution substation converts voltage closer to the required usable levels.
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distribution system
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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.
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enclosure
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A physical barrier designed to provide mechanical or electrical protection for components used in a system.
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feeder
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A circuit conductor between the power supply source and a final branch circuit overcurrent device.
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generating station
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A building where a utility company houses large linked AC generators to create the massive power for widespread use.
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grounded conductor
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A system conductor that is intentionally grounded.
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grounding conductor
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A conductor used to connect equipment in a wiring system to ground.
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interconnected system
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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.
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kilovolt-amp
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A unit used to rate transformer and generator strength. A kilovolt is a thousand volts.
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laminated
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Sealed together in multiple layers of thin sheets to construct the core for a transformer. Lamination helps reduce resistance losses.
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line voltage
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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.
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load center
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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.
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local power distribution system
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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.
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main disconnect switch
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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.
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motor control center
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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.
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mutual induction
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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.
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NEC
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The standard for minimum safe electrical installations. The National Electrical Code is adopted in some form as law in all 50 states.
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network system
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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.
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open delta
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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.
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overcurrent
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Excess current that may result from overload, short circuit, or ground fault. Devices like fuses and circuit breakers protect against overcurrent.
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panelboard
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A single panel distribution cabinet or wall cutout box that holds automatic overcurrent protection devices for lighting, heat, or power circuits.
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phase symbol
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(Φ) The Greek letter Phi, used to represent phases, such as in a 3Φ4W system.
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phase voltage
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The voltage between any one conductor and ground.
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plug-in busway
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A type of feeder busway with electrical control that allows loads to be added along the bus structure.
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primary winding
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Turns of wire on a transformer core, used to transfer voltage from the input to the core.
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radial system
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A distribution system with only one power source. If that source goes out, power is lost to the entire circuit.
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revolving field generator
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A generator setup where an electromagnetic field is rotated between the conductor wound armature. Most AC generators are of this design.
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secondary winding
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Turns of wire on a transformer core, used to transfer voltage from the core to the outside load.
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service-entrance
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The point where electricity enters a buidling. A service-entrance switchboard has metering equipment and devices for overcurrent protection and electrical control.
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single-phase
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A continuous single alternating current cycle.
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step up
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In electricity, a phrase used to describe voltage adjustment. To step up voltage means to increase voltage.
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substation
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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.
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switchboard
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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.
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switchgear
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A combination of disconnect switches and breakers used to isolate equipment in substations.
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tap
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A wire connected to the midpoint of a transformer secondary winding. This allows the transformer to use different turns ratios.
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terminal
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A connecting point in a circuit where a wire is attached to create an electrical connection.
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three-phase
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A continuous series of three overlapping AC cycles offset by 120 degrees. Three-phase power is used for all large scale distribution systems.
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three-phase power
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The most common form of AC power for distribution. Three-phase power has three overlapping AC cycles offset by 120 degrees.
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transformer
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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.
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transmission substation
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A bank of step up transformers near the generating station. Transmission substations increase voltage as high as 800kV for widespread distribution.
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turns ratio
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A comparison of the number of turns in the primary versus the number of turns in the secondary of a transformer.
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wye
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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.
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wye/delta
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A transformer with the primary connected as a wye and the secondary connected as a delta.
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wye/wye
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A transformer with both the primary and the secondary connected as a wye.
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