1、1,Branch CircuitsChapter 2,汽車(chē)防盜器 ,2,Introduction,The 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 phase,Why are a range

2、 of voltages listed?,What happens when the voltage drops below 108 V?,What happens when the voltage goes above 124 V?,3,Service 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 determ

3、ined by the number 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?,4,Service Entrance Panel SEP “Load Center”,Mas

4、ter Disconnect,120/240 V Service,Service “Hot” Conductors,Metal Box,Non-conducting base,Non-conducting Attachment bars,Conducting Attachment bars,120 V Branch Circuit Ground (bare) Conductor,240 V Circuit,120 V Branch Circuit “Hot” (black) Conductor,Ground,Neutral,120 V,120 V,120 V Branch Circuit Ne

5、utral (white) Conductor,Earth Ground,Bonding Screw,Circuit Neutral & Ground Connections,Grounding Bar,Breaker,Service Entrance Neutral,5,SEP-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

6、bars. The 120/240 neutral conductor is attached 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 b

7、e used.,Grounding,Branch circuits have two different types of grounds. System Equipment,6,System 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 colo

8、r of the equipment ground?,Grounding - Equipment,Equipment grounding 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 comp

9、onent. Must make a complete, low resistance circuit from all metallic electrical devices in the system to the earth.,7,Equipment ground,What hazard is created if the equipment ground is interrupted?,120 V Circuits,120 V circuits have 3 or 4 conductors: one energized (hot) conductorBlack or red one n

10、eutral conductor White one ground conductor.Bare or green,8,What other common building component is made from PVC,What does PVC stand for?,240 V Circuits,240 Volt circuits have three conductors: Two hot Equipment ground Neutral circuit is not required unless both 240 and 120 circuits are supplied by

11、 the device. The 240 Volt electrical service to the SEP will have a neutral so both 240 and 120 Volt branch circuits can be used.,9,Three Circuit Types,General purpose branch circuits Individual branch circuits Motor,10,General Purpose Branch Circuits,Designed for temporary loads such as lights and

12、DCOs (Duplex Convenience Outlets) under 1500 W. Minimum 12 AWG Fused at 20 amps No more than ten (10) DCOs or light fixtures per circuit. (Fig. 2-3, pg 21) Recommended location for DCOs (Table 1-12, pg 18).,11,Special Purpose Branch circuits,Used for known specific loads Stationary motors Stationary

13、 appliances SPOs (Special Purpose Outlets) Usually used for loads greater than 20 amps240 V.,12,What would be an example of an SPO in an agriculture building?,Motor Circuits,Use 240 V whenever possible. Reduces amperage load on circuit Reduces stray voltage potential Five (5) horsepower and larger s

14、hould be 3 phase.,13,Motor Circuitscont.,Branch circuits for electric motors have four (4) requirements (Fig 2-6 through 9, pg 23-26): Branch circuit, short circuit protection A disconnecting means A controller Overload protection Summary Table 2-1, pg 26,14,Motor CircuitsShort Circuit Protection,Fu

15、se or circuit breaker For motor circuits they must have greater capacity than full load current. Motor starting load is higher than the running loadSCP devices must be able to handle temporary overload. Inverse time breaker Time delay fuses Maximum size Inverse time breaker = 2.50 times full load cu

16、rrent Time delay fuses = 1.75 times full load current.,15,Motor CircuitsShort Circuit Protection-example,Determine the required SCP for a 120 V circuit for a horsepower, single phase motor.,16,Determine the required SCP for a 240 V circuit for a 1/6 horsepower, single phase motor.,Smallest breaker i

17、s 10 A SolutionUse 10 A breaker in the SEP and install a 4 to 6 amp fuse inline with the motor.,Motor CircuitsDisconnecting Means,Each 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

18、 on or off.,17,Motor CircuitsDisconnecting Means-cont.,Must be located within sight and within 50 feet of the controller and the motor. Disconnecting Means Stationary motors can use the circuit switch as long as correct size. Portable motors the plug and receptacle is acceptable. The circuit switch

19、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.,18,What is a snap switch?,Motor CircuitsController,A controller is a device used to automatically start and stop a motor. Only required to open enough conduct

20、ors 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 controllers and timers are considered to be a controller.,19,Is a heater/airconditioner thermostat within sight and 50 feet of the furnace/airconditioner?,

21、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 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 a

22、n serve as the controller. If a knife switch is operated by hand, it can serve as both the disconnecting means and the controller.,20,What is a magnetic starter?,Motor CircuitsControllerMagnet Starter,21,Motor CircuitsOverload protection,Motors and conductors must be protected from overloads. Becaus

23、e 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 last long enough to damage the motor.,22,When magnetic starters are used the overload protection is usually included in the starter.,Common

24、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 design and size of the motor.,Motor CircuitsOverload protectioncont.,For motors less than 1 hp, and manually started, the circuit breaker

25、or fuse can serve as the OPD. Smaller motors may include a built in overload protection switch.,23,Motor CircuitsOverload protectioncont.,Critical issue is if a manual restart or automatic restart is used. Manual restart is usually used unless the motor operates a critical function such as a ventila

26、tion fan in a chicken house.,24,Why?,Branch Circuit Conductors,25,26,Sizing Conductors,Conductors are usually considered single wires. Cables are multiple conductors in the same sheathing. Conductor are sized using two systems American Wire Gauge (AWG) circular mills (cmil).,27,Sizing Conductorscont

27、.,AWG Numbers run from 40 to 0000 AWG numbers only apply to non-ferrous metals. The larger the number-the smaller the diameter of the wire. cmils Circular-mils (cmils) is a unit used to describe the cross-sectional area of wire. A mil = 0.001 inch AWG sizes greater than 0000 are sized in thousands o

28、f 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 Conductorscont.,The minimum size of an individual conductor is determined by two factors. Ampacity Voltage drop,28,Wha

29、t is ampacity?,What is voltage drop?,29,Ampacity-Resistivity,All materials will conduct electricity. Good conducting materials have low resistance. The resistance of a conductor depends on the physical properties of the material (), the length (ft) of the conductor and the cross-sectional area of th

30、e conductor (cmils). Expressed in an equation:,A = cross-sectional area in cmils = (diameter in mils)2 1mil = 0.001 in,30,Example-Resistance,What is the resistivity of a 1/2 inch steel rod that is 12 feet long? Steel = 100 ohm-cmil/foot,Electricity for Agricultural Applications, Bern,Voltage Drop,Wh

31、en electricity passes through a resistance heat is generated. Heat is energy The loss energy shows up as voltage drop. All conductors have resistance = all conductors have voltage drop. What must be avoided is excessive voltage drop.,31,What are some possible outcomes of a circuit with excessive vol

32、tage drop?,What will cause excessive voltage drop?,32,32,Voltage Drop-Cont.,When there is no current 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.,33,33,Three Ways of Wiring Circuits,The loads and electr

33、ical components in a circuit can be connected in three different ways: Series Parallel Series-parallel (not included),34,34,Series Circuit,In 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

34、 sum of the individual resistances.,For these calculations assume no resistance in the conductors or connections.,Determine the total resistance for the circuit in the illustration.,35,35,Series Circuit-cont.,To the power source, a series circuit appears as one resistance.,=,In all circuits a voltag

35、e drop occurs as electricity passes through each resistance in the circuit. The method for calculating voltage drop in series circuits is different than the method for parallel circuits.,36,36,Parallel Circuits,In parallel circuits the electricity has alternative paths. The amount of current in each

36、 path is determined by the resistance of that path. “Electricity follows the path of least resistance” Because there are alternative 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

37、the inverse of each individual resistance.,37,37,Parallel Circuits-cont.,An alternative equation is:,When a circuit has more than 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 h

38、ave been combined.,38,38,Parallel Circuit Resistance,Determine the total resistance for the circuit in the illustration.,or,or,39,39,Circuits Summary,When 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 ser

39、ies circuit the amperage is the same at all points in the circuit, but the voltage changes with the resistance. In a parallel circuit the amperage changes with the resistance, but the voltage is the same throughout the circuit.,40,40,Calculating Voltage In A Series Circuit,What would V1 read in the

40、illustration? Ohms Law states: Therefore:,At this point there is insufficient data because I (amp) is unknown. Using Ohms Law to solve for the current in the circuit: Knowing the amount of current we can calculate the voltage drop.,Note: circuit conductors behave like resistors in series.,41,41,Dete

41、rmining Voltage In A Parallel Circuit,Assuming no resistance in the conductors, the two volt meters in the illustration will have the same value-source voltage.,42,42,Determining Amperage In A Series Circuit,Determine the readings for A1 and A2 in the illustration. In a series circuit the electricit

42、y 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 by the circuit resistance.,43,43,Determining Amperage in a Parallel Circuit,Determine the readings for amp meters A1 and A2 in the circuit.,I

43、n 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 flowing through the 6.3 Ohm resistor. To determine circuit amperage the total resistance of the circuit must be calculated:,44,44,Deter

44、mining 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 A,A1= 12.76 A,Total current is:,When the circuit current (Amps) is known, the current for each branch circuit can be calculated.,45,Conductor Size

45、,The 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 source the circuit power factor the type of current (phases) the ampacity of the conductor the allowable voltage drop,The type of insulation is determined by t

46、he environment.,46,Insulation,The common types used in Agriculture.,Electricity for Agricultural Applications, Bern,90o C = 194o F,47,Environment-cont.,The selection of insulation is very important because the life of the conductor is usually determined by the life of the insulation. Conductors neve

47、r wear out. Insulation deteriorates over time. Insulation reacts with oxygen, ammonia, oil, gasoline, salts, UV and water.,48,Determining Conductor Size,The first step is to determine answers for five of the seven factors. These are: the load on the circuit the voltage of the circuit the distance fr

48、om the load to the source the circuit power factor the type of current (phases) Once these are known, the remaining two factors are used to determine the conductor size. the ampacity of the conductor the allowable voltage drop,49,Circuit load The circuit load is the amperage used by the electrical d

49、evice, or the size of over current protection device that will be used. Circuit voltage Circuit voltage is the source voltage. Distance from source The distance between the source and the load is not used as often as the run. The run is the total amount of conductor that is used to connect the load

50、to the source. Power factor The 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 diameter wires.,Determining Conductor Size-cont.,50,Once values are known for the first five

51、 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 the electricity passing through the resistance of the conductors.,Determining Condu

52、ctor Size-cont.,51,Ampacity,Ampacity 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 dissipation ability of the conductor. Ampacity increases with conductor size. Ampacity f

53、or 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 is a function of the amount of overload and the duration of the overload.

54、Ampacity ratings for conductors can be determined from tables such as 33-19. (Agricultural Mechanics),52,Ampacity-cont.,Ampacity can be calculated, but tables present this information. Example: what is minimum size of conductor with THWN insulation in conduit, operating on 120 volts that should be u

55、sed 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 handbook,53,Voltage drop is the result of a current passing through a resistance. Example: What is the perc

56、ent voltage drop at the service entrance panel for the building in the illustration?,Sizing Conductors by Voltage Drop,54,Example-cont.,The first step is to determine the total resistance of the circuit.,In this example the resistance for each conductor is determined separately.,55,Example-cont.,Thi

57、s circuit diagram illustrates the resistance of the conductors.,56,Example-cont.,The next step is to determine the voltage drop and the percentage drop. Voltage drop is: Percent voltage drop is:,This is an unacceptable voltage drop. Picking the wire size first must not be the best way.,The conductor

58、 size is determined by calculating the allowable resistance for the desired voltage drop.,57,Voltage Drop Example-cont.,A voltage drop of 4.09% is excessive. Results of excessive voltage drop. The heat output of a resistance heater will decrease more than 8% because power output is proportional to t

59、he 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 conductor that has a lower resistance. Use a higher voltage. Of these 5 options, number 2 and 5 are usually the only practical solution.,58,Voltage 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.,59,Designing For Acceptable Voltage Drop,Because all co