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1、1Branch CircuitsChapter 2汽車防盜器 2IntroductionThe purpose of branch circuits is to carry the current from the service entrance panel (SEP) to the electrical devices.Three common types of current are used in agricultural buildings:120 volt (108 125)240 volt (220 250)Three phaseWhy are a range of voltag

2、es listed?What happens when the voltage drops below 108 V?What happens when the voltage goes above 124 V?3Service Entrance Panel (SEP)The service entrance panel (load center) is the entry point for the electricity into the building.The size (amp capacity) of the load center is determined by the numb

3、er of circuits and total amp load for the building.Current NEC regulations require that the load center have a master disconnect.The entrance panel must be grounded with a NEC approved earth connection.What is an NEC earth connection?4Service Entrance Panel SEP “Load Center”Master Disconnect120/240

4、V ServiceService “Hot” ConductorsMetal BoxNon-conducting baseNon-conducting Attachment barsConducting Attachment bars120 V Branch Circuit Ground (bare) Conductor240 V Circuit120 V Branch Circuit “Hot” (black) ConductorGroundNeutral120 V120 V120 V Branch Circuit Neutral (white) ConductorEarth GroundB

5、onding ScrewCircuit Neutral & Ground ConnectionsGrounding BarBreakerService Entrance Neutral5SEP-cont.The 120/240 service is attached to the master disconnect (breaker).From master breaker each hot conductor is connected to one of the conducting breaker bars.The 120/240 neutral conductor is attached

6、 to the grounding bar.A 120 volt breaker attaches by snapping onto one conducting and one non-conducting bar in the load center.For a 240 volt circuit two individual breakers may be used and the levers are pined together or a combination breaker may be used.GroundingBranch circuits have two differen

7、t types of grounds.SystemEquipment6System grounding is accomplished by one of the two current carrying conductors (white).What is another term for the system ground?What is the insulation color of the system ground?What is the insulation color of the equipment ground?Grounding - EquipmentEquipment g

8、rounding is the bonding of all non current carrying metal components back to the SEP.The equipment ground is designed to provide a low resistant circuit to the earth in the event of a short from the energized conductor to any metallic component.Must make a complete, low resistance circuit from all m

9、etallic electrical devices in the system to the earth.7Equipment groundWhat hazard is created if the equipment ground is interrupted?120 V Circuits120 V circuits have 3 or 4 conductors:one energized (hot) conductorBlack or redone neutral conductor White one ground conductor.Bare or green8What other

10、common building component is made from PVCWhat does PVC stand for?240 V Circuits240 Volt circuits have three conductors:Two hotEquipment groundNeutral circuit is not required unless both 240 and 120 circuits are supplied by the device.The 240 Volt electrical service to the SEP will have a neutral so

11、 both 240 and 120 Volt branch circuits can be used.9Three Circuit TypesGeneral purpose branch circuitsIndividual branch circuitsMotor10General Purpose Branch CircuitsDesigned for temporary loads such as lights and DCOs (Duplex Convenience Outlets) under 1500 W.Minimum 12 AWGFused at 20 ampsNo more t

12、han ten (10) DCOs or light fixtures per circuit. (Fig. 2-3, pg 21)Recommended location for DCOs (Table 1-12, pg 18).11Special Purpose Branch circuitsUsed for known specific loadsStationary motorsStationary appliancesSPOs (Special Purpose Outlets)Usually used for loads greater than 20 amps240 V.12Wha

13、t would be an example of an SPO in an agriculture building?Motor CircuitsUse 240 V whenever possible.Reduces amperage load on circuitReduces stray voltage potentialFive (5) horsepower and larger should be 3 phase.13Motor Circuitscont.Branch circuits for electric motors have four (4) requirements (Fi

14、g 2-6 through 9, pg 23-26):Branch circuit, short circuit protectionA disconnecting meansA controllerOverload protectionSummary Table 2-1, pg 2614Motor CircuitsShort Circuit ProtectionFuse or circuit breakerFor motor circuits they must have greater capacity than full load current.Motor starting load

15、is higher than the running loadSCP devices must be able to handle temporary overload.Inverse time breakerTime delay fusesMaximum sizeInverse time breaker = 2.50 times full load currentTime delay fuses = 1.75 times full load current.15Motor CircuitsShort Circuit Protection-exampleDetermine the requir

16、ed SCP for a 120 V circuit for a horsepower, single phase motor.16Determine the required SCP for a 240 V circuit for a 1/6 horsepower, single phase motor.Smallest breaker is 10 ASolutionUse 10 A breaker in the SEP and install a 4 to 6 amp fuse inline with the motor.Motor CircuitsDisconnecting MeansE

17、ach motor or motor circuit must have an individual disconnecting means.The disconnecting means must disconnect all hot wires.The DM must clearly indicate whether it is on or off.17Motor CircuitsDisconnecting Means-cont.Must be located within sight and within 50 feet of the controller and the motor.D

18、isconnecting MeansStationary motors can use the circuit switch as long as correct size.Portable motors the plug and receptacle is acceptable.The circuit switch can be a snap switch as long as the motor is 2 hp or less and its capacity is equal to or 1.25 times greater than the motor full load rating

19、.18What is a snap switch?Motor CircuitsControllerA controller is a device used to automatically start and stop a motor.Only required to open enough conductors to stop the motor.One wire for 120 & 240 V single phase.Must be located within sight and 50 feet of the motor.Thermostats, variable speed con

20、trollers and timers are considered to be a controller.19Is a heater/airconditioner thermostat within sight and 50 feet of the furnace/airconditioner?If not, does this meet code?Motor CircuitsControllercont.Current rating must be greater than or equal to motor full load rating, or a magnetic starter

21、must be used.For 1/3 hp and less portable motors the plug and receptacle can function as the controller.If motor is 2 hp or less, a snap switch an serve as the controller.If a knife switch is operated by hand, it can serve as both the disconnecting means and the controller.20What is a magnetic start

22、er?Motor CircuitsControllerMagnet Starter21Motor CircuitsOverload protectionMotors and conductors must be protected from overloads.Because motors draw more current for starting that running, the overload protection device must allow temporary overload on the circuit but not allow the overload to las

23、t long enough to damage the motor.22When magnetic starters are used the overload protection is usually included in the starter.Common practice to use a heater device to trip the controller before the conductors or motor overheats. One hp and larger motors have specific requirements based on the desi

24、gn and size of the motor.Motor CircuitsOverload protectioncont.For motors less than 1 hp, and manually started, the circuit breaker or fuse can serve as the OPD.Smaller motors may include a built in overload protection switch.23Motor CircuitsOverload protectioncont.Critical issue is if a manual rest

25、art or automatic restart is used.Manual restart is usually used unless the motor operates a critical function such as a ventilation fan in a chicken house.24Why?Branch Circuit Conductors2526Sizing Conductors Conductors are usually considered single wires. Cables are multiple conductors in the same s

26、heathing.Conductor are sized using two systemsAmerican Wire Gauge (AWG) circular mills (cmil).27Sizing Conductorscont.AWGNumbers run from 40 to 0000AWG numbers only apply to non-ferrous metals.The larger the number-the smaller the diameter of the wire.cmilsCircular-mils (cmils) is a unit used to des

27、cribe the cross-sectional area of wire.A mil = 0.001 inchAWG sizes greater than 0000 are sized in thousands of circular mils (kcmil)AWG #8 and higher are usually multiple strands.The diameter of multiple strand wire in cmils is the cmils of each strand times the number of strands.Sizing Conductorsco

28、nt.The minimum size of an individual conductor is determined by two factors.AmpacityVoltage drop28What is ampacity?What is voltage drop?29Ampacity-ResistivityAll materials will conduct electricity. Good conducting materials have low resistance.The resistance of a conductor depends on the physical pr

29、operties of the material (), the length (ft) of the conductor and the cross-sectional area of the conductor (cmils).Expressed in an equation:A = cross-sectional area in cmils = (diameter in mils)21mil = 0.001 in30Example-ResistanceWhat is the resistivity of a 1/2 inch steel rod that is 12 feet long?

30、Steel = 100 ohm-cmil/footElectricity for Agricultural Applications, BernMaterialResistivity ()Silver9.55Copper10.67Gold14.7Aluminum17.01Tungsten33.10Platinum66.9Steel100Lead129Cast iron360Mercury577Voltage DropWhen electricity passes through a resistance heat is generated. Heat is energyThe loss ene

31、rgy shows up as voltage drop.All conductors have resistance = all conductors have voltage drop.What must be avoided is excessive voltage drop.31What are some possible outcomes of a circuit with excessive voltage drop?What will cause excessive voltage drop?3232Voltage Drop-Cont.When there is no curre

32、nt flow, there is no voltage at the load.A 2 % voltage drop is considered normal.If the voltage drop is more than 2% the circuit will overheat. 3333Three Ways of Wiring CircuitsThe loads and electrical components in a circuit can be connected in three different ways:Series Parallel Series-parallel (

33、not included)3434Series CircuitIn a series circuit the electricity has no alternative paths, all of the electricity must pass through all of the components.The total circuit resistance is the sum of the individual resistances.For these calculations assume no resistance in the conductors or connectio

34、ns.Determine the total resistance for the circuit in the illustration.3535Series Circuit-cont.To the power source, a series circuit appears as one resistance.=In all circuits a voltage drop occurs as electricity passes through each resistance in the circuit.The method for calculating voltage drop in

35、 series circuits is different than the method for parallel circuits.3636Parallel CircuitsIn parallel circuits the electricity has alternative paths.The amount of current in each path is determined by the resistance of that path. “Electricity follows the path of least resistance”Because there are alt

36、ernative paths, the total resistance of the circuit is not the sum of the individual resistances.In a parallel circuit: The inverse of the total resistance is equal to sum of the inverse of each individual resistance.3737Parallel Circuits-cont.An alternative equation is:When a circuit has more than

37、two resistors, select any two and reduce them to their equivalent resistance and then combine that resistance with another one in the circuit until all of the resistors have been combined.3838Parallel Circuit ResistanceDetermine the total resistance for the circuit in the illustration.oror3939Circui

38、ts SummaryWhen the source voltage, and the total resistance of the circuit is known, amperages and voltages can be determine for any part of a circuit.In a series circuit the amperage is the same at all points in the circuit, but the voltage changes with the resistance.In a parallel circuit the ampe

39、rage changes with the resistance, but the voltage is the same throughout the circuit.4040Calculating Voltage In A Series CircuitWhat would V1 read in the illustration?Ohms Law states: Therefore:At this point there is insufficient data because I (amp) is unknown.Using Ohms Law to solve for the curren

40、t in the circuit:Knowing the amount of current we can calculate the voltage drop.Note: circuit conductors behave like resistors in series.4141Determining Voltage In A Parallel CircuitAssuming no resistance in the conductors, the two volt meters in the illustration will have the same value-source vol

41、tage.4242Determining Amperage In A Series CircuitDetermine the readings for A1 and A2 in the illustration. In a series circuit the electricity has no alternative paths, therefore the amperage is the same at every point in the circuit.The current in the circuit is determined by dividing the voltage b

42、y the circuit resistance.4343Determining Amperage in a Parallel CircuitDetermine the readings for amp meters A1 and A2 in the circuit.In a parallel circuit the amperage varies with the resistance.In the illustration, A1 will measure the total circuit amperage, but A2 will only measure the amperage f

43、lowing through the 6.3 Ohm resistor.To determine circuit amperage the total resistance of the circuit must be calculated:4444Determining Amperage in a Parallel Circuit-cont.When the total resistance is known, the circuit current (Amps) can be calculated.Branch current is:A2 = 1.9 AA1= 12.76 A Total

44、current is:When the circuit current (Amps) is known, the current for each branch circuit can be calculated.45Conductor SizeThe conductor size is determined by seven (7) factors.the load on the circuit the voltage of the circuit the distance from the load to the sourcethe circuit power factorthe type

45、 of current (phases) the ampacity of the conductor the allowable voltage dropThe type of insulation is determined by the environment.46InsulationThe common types used in Agriculture.Electricity for Agricultural Applications, Bern90o C = 194o F47Environment-cont.The selection of insulation is very im

46、portant because the life of the conductor is usually determined by the life of the insulation.Conductors never wear out.Insulation deteriorates over time.Insulation reacts with oxygen, ammonia, oil, gasoline, salts, UV and water.48Determining Conductor SizeThe first step is to determine answers for

47、five of the seven factors. These are:the load on the circuit the voltage of the circuit the distance from the load to the sourcethe circuit power factorthe type of current (phases) Once these are known, the remaining two factors are used to determine the conductor size.the ampacity of the conductor

48、the allowable voltage drop49Circuit loadThe circuit load is the amperage used by the electrical device, or the size of over current protection device that will be used. Circuit voltageCircuit voltage is the source voltage.Distance from sourceThe distance between the source and the load is not used a

49、s often as the run. The run is the total amount of conductor that is used to connect the load to the source.Power factorThe power factor for reactive loads is less than one.The power factor for resistance loads is equal to one.The number or phases must be know.Three phase current can use smaller dia

50、meter wires.Determining Conductor Size-cont.50Once values are known for the first five factors, the last two are used to determine the minimum conductor size.Ampacity is the largest load that a conductor is designed to carry regardless of length.Voltage drop is the amount of energy that is lost from

51、 the electricity passing through the resistance of the conductors.Determining Conductor Size-cont.51AmpacityAmpacity refers to the current carrying ability of the conductor.Ampacity is dependent on the conductor resistance, the allowable operating temperature of the insulation and the heat dissipati

52、on ability of the conductor.Ampacity increases with conductor size.Ampacity for copper is higher than the ampacity for aluminum.Ampacity is higher for conductors which have higher temperature ratings.Exceeding the ampacity rating increases the heat of the insulation.The amount of damage that occurs

53、is a function of the amount of overload and the duration of the overload.Ampacity ratings for conductors can be determined from tables such as 33-19. (Agricultural Mechanics)52Ampacity-cont.Ampacity can be calculated, but tables present this information.Example: what is minimum size of conductor wit

54、h THWN insulation in conduit, operating on 120 volts that should be used to carry 15 amps?AWG 14 ?Note: Based on ampacity, #14 is sufficient, but according to the NEC #12 is smallest size of wire that can be used under any conditions using 120 V.Pg 35 Wiring handbook53Voltage drop is the result of a

55、 current passing through a resistance.Example:What is the percent voltage drop at the service entrance panel for the building in the illustration?Sizing Conductors by Voltage Drop54Example-cont.The first step is to determine the total resistance of the circuit. CopperAluminumSize (AWG)Area (cmil)Num

56、ber of StrandsWeight (lb/1000 ft)Resistance (ohms/1000 ft)Weight (lb/1000 ft)Resistance (ohms/1000 ft)144110112.53.073.785.06126530119.81.936.013.181010380131.431.219.5562.00816510149.980.76415.201.26626240779.440.49124.150.8084417407126.30.30838.410.5083526207159.30.24548.430.40326636072050.30862.3

57、0.319In this example the resistance for each conductor is determined separately.55Example-cont.This circuit diagram illustrates the resistance of the conductors.56Example-cont.The next step is to determine the voltage drop and the percentage drop.Voltage drop is:Percent voltage drop is:This is an un

58、acceptable voltage drop.Picking the wire size first must not be the best way.The conductor size is determined by calculating the allowable resistance for the desired voltage drop.57Voltage Drop Example-cont.A voltage drop of 4.09% is excessive.Results of excessive voltage drop.The heat output of a r

59、esistance heater will decrease more than 8% because power output is proportional to the square of the voltage.The useable light from an incandescent lamp will drop about 10%.Five (5) possible solutions:Decrease the load.Use larger conductors.Reduce the distance between the load and the source.Use a

60、conductor that has a lower resistance.Use a higher voltage.Of these 5 options, number 2 and 5 are usually the only practical solution.58Voltage Drop Example-cont.If the voltage is increased to 240 V, what will be the percent voltage drop?2.04% is an acceptable loss for this electrical service.59Desi