Branch Circuits Chapter 2

1. Branch CircuitsChapter 2

2. Introduction • The purpose of branch circuits is to carry the current from the service entrance panel (SEP) to the electrical device. • Three common types: • 120 volt (108 – 125) • 240 volt (220 – 250) • Three phase

3. Service Entrance Panel (SEP) • The 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 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.

4. Service Entrance Panel [SEP] “Load Center” 120/240 V Service Service Entrance Neutral Master Disconnect Service “Hot” Conductors Metal Box Breaker Non-conducting base Grounding Bar Non-conducting Attachment bars Circuit Neutral & Ground Connections Conducting Attachment bars Ground Bonding Screw Neutral 240 V Circuit 120 V 120 V 120 V Branch Circuit “Hot” (black) Conductor Earth Ground 120 V Branch Circuit Ground (bare) Conductor 120 V Branch Circuit Neutral (white) Conductor

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 bars. • The 120/240 neutral conductor is attached to the grounding bar. • The grounding bar must be attached to an approved earth ground. • 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.

6. Grounding • Branch circuits have two different types of grounds. • System • Equipment • Equipment grounding is the bonding of all non current carrying metal components back to the SEP. • Grounding conductors are designed to carry current only when there is damage or defect in the equipment or wiring. • Must make a complete, low resistance circuit from all metallic electrical devices in the system to the SEP. • System grounding is accomplished by one of the two current carrying conductors (white). • Also called system neutral • Carry current in AC circuits.

7. 120 V Circuits • 120 V circuits have 3 or 4 conductors: • one hot conductor Black or red • one neutral conductor White • one ground conductor. Bare or green • The neutral and ground conductors must not be connected together until the SEP. • Do not install fuses or breakers in the neutral or ground conductor • Switches are only installed in the hot conductor. • Double pole • Switch loop

8. 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 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 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) • Recommended location for DCOs (Table 1-12).

11. Special Purpose Branch circuits • Used for known specific loads • Stationary motors • Stationary appliances • SPOs (Special Purpose Outlets) • Usually used for loads greater than 20 amps—240 V.

12. Motor Circuits • Use 240 V whenever possible. • Reduces amperage load on circuit • Reduces stray voltage potential • Five (5) horsepower and larger should be 3 phase.

13. Motor Circuits—cont. • Branch circuits for electric motors have four (4) requirements (Fig 2-6 through 9): • Branch circuit short circuit protection • A disconnecting means • A controller • Overload protection • Summary Table 2-1

14. Motor Circuits—Short Circuit Protection • Fuse or circuit breaker • For motor circuits must have greater capacity than full load current. • Maximum size • Inverse time breaker = 2.50 times full load current • Time delay fuses = 1.75 times full load current. • Motor starting load is higher than the running load—SCP must be designed to handle temporary overload. • Inverse time breaker • Time delay fuses

15. Motor Circuits—Short Circuit Protection--example • Determine the required SCP for a 120 V circuit for a ½ horsepower, single phase motor. • Determine the required SCP for a 240 V circuit for a 1/6 horsepower, single phase motor. • Smallest breaker is 10 A • Solution—Use 10 A breaker in the SEP and install a 3 to 4 amp fuse inline with the motor.

16. Motor Circuits—Disconnecting 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 on or off.

17. Motor Circuits—Disconnecting Means-cont. • Must be located within sight and within 50 feet of the controller and the motor. • Disconnecting Means • Stationary motors the circuit switch is acceptable 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.

18. Motor Circuits—Controller • A controller is a device used to 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 controllers and timers are considered to be a controller.

19. Motor Circuits—Controller—cont. • 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 an serve as the controller. • If a knife switch is operated by hand, it can serve as both the disconnecting means and the controller.

20. Motor Circuits—Overload protection • Because a motor draws 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. • Common practice to use a heater device to trip the controller when motor overheats. • One hp and larger motors have specific requirements based on the design and size of the motor. • For motors less than 1 hp and manually started the circuit breaker or fuse can serve as the OPD.

21. Branch Circuit Conductors

22. Circuit Analysis • To understand conductor sizing it is useful to understand how resistance affects amperage and voltage in circuits.

23. 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).

24. Sizing conductors • Conductor usually refers to a single wire. • Cable is used to describe multiple wires in the same sheath. • Conductors are sized using two systems: • American Wire Gauge (AWG) • Circular mils (cmil)

25. Sizing Conductors—cont. • 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 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.

26. Sizing Conductors—cont. • The size of an individual conductor is determined by two factors. • Ampacity • Determined by the resistivity of the conductor. • Voltage drop • Determined by the load on the circuit.

27. Ampacity--Resistivity • All materials will conduct some 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 the conductor (cmils). • Expressed in an equation: A = cross-sectional area in cmils = (diameter in mils)2 1mil = 0.001 in

28. 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

29. Voltage Drop • Voltage drop occurs because when electricity passes through a resistance heat is generated. • Heat represents loss energy • The energy loss is expressed as less voltage. • Using a conductor that is too small for the load causes excessive voltage drop.

30. Voltage Drop--Cont. • When there is no current flow, there is no voltage at the load. • A 2 % voltage drop is considered normal. • 3% under some conditions. • A voltage drop of more than 2% is excessive and the circuit will not function properly. 30

31. Three Ways of Wiring Circuits • The loads and electrical components in a circuit can be connected in three different ways: • Series • Parallel • Series-parallel (not included) 31

32. 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 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. 32

33. Series Circuit-cont. • To the power source, the a series circuit appears as one resistance. = • A characteristic of all circuits is that there is a voltage drop across each resistance in the circuit. • The method for calculating voltage drop in series circuits is different than the method for parallel circuits. 33

34. Parallel Circuits • In 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 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 the inverse of each individual resistance. 34

35. 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 have been combined. 35

36. Parallel Circuit Resistance Determine the total resistance for the circuit in the illustration. or or 36

37. 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 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 amperage changes with the resistance, but the voltage is the same throughout the circuit. 37

38. Calculating Voltage In A Series Circuit • What would V1 read in the illustration? • Ohm’s Law states: • Therefore: • At this point there is insufficient data because I (amp) is unknown. • Using Ohm’s 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. 38

39. Determining 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. 39

40. Determining Amperage In A Series Circuit • Determine 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 by the circuit resistance. 40

41. Determining Amperage in a Parallel Circuit • Determine 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 flowing through the 6.3 Ohm resistor. • To determine circuit amperage the total resistance of the circuit must be calculated: 41

42. Determining Amperage in a Parallel Circuit--cont. When the total resistance is known, the circuit current (Amp’s) can be calculated. Total current is: A1= 12.76 A When the circuit current (Amp’s) is known, the current for each branch circuit can be calculated. Branch current is: A2 = 1.9 A 42

43. Conductor Size 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 the environment.

44. Insulation The common types used in Agriculture are : Electricity for Agricultural Applications, Bern

45. 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 never wear out. • Insulation deteriorates over time. • Insulation reacts with oxygen, ammonia, oil, gasoline, salts and water.

46. 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 from 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

47. Determining Conductor Size--cont. • Circuit load • The circuit load is the amperage used by the electrical device, 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 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.

48. Determining Conductor Size--cont. • Once 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 the electricity passing through the resistance of the conductors.

49. 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 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 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 32-19.

50. 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 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.