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 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 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
  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.

What is an NEC earth connection?
4
Service Entrance Panel SEP Load Center

• Master Disconnect

120/240 V Service
Service Entrance Neutral
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
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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.

• 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.

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Grounding

• Branch circuits have two different types of
  grounds.
• System
• Equipment

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 color of the equipment
ground?
7
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 component.
• Must make a complete, low resistance circuit from
  all metallic electrical devices in the system to
  the earth.

Equipment ground
What hazard is created if the equipment ground is
interrupted?
8
120 V Circuits

• 120 V circuits have 3 or 4 conductors
• one energized (hot) conductor Black or red
• one neutral conductor White
• one ground conductor. Bare or green

What does PVC stand for?
What other common building component is made from
PVC
9
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.

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Three Circuit Types

• General purpose branch circuits
• Individual branch circuits
• Motor

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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, pg 21)
• Recommended location for DCOs (Table 1-12, pg 18).

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Special Purpose Branch circuits

• Used for known specific loads
• Stationary motors
• Stationary appliances
• SPOs (Special Purpose Outlets)
• Usually used for loads greater than 20 amps240
  V.

What would be an example of an SPO in an
agriculture building?
13
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.

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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

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Motor CircuitsShort Circuit Protection

• Fuse 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
  current
• Time delay fuses 1.75 times full load current.

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Motor CircuitsShort 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
• SolutionUse 10 A breaker in the SEP and install
  a 4 to 6 amp fuse inline with the motor.

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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 on or
  off.

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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 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.

What is a snap switch?
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Motor CircuitsController

• A 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 controllers and
  timers are considered to be a controller.

Is a heater/airconditioner thermostat within
sight and 50 feet of the furnace/airconditioner?
If not, does this meet code?
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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 an serve
  as the controller.
• If a knife switch is operated by hand, it can
  serve as both the disconnecting means and the
  controller.

What is a magnetic starter?
21
Motor CircuitsControllerMagnet Starter
22
Motor CircuitsOverload protection

• Motors 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 last long enough to damage
  the motor.

When 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 design and
size of the motor.
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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.

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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
  ventilation fan in a chicken house.

Why?
25
Branch Circuit Conductors
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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).

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Sizing Conductorscont.

• 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.

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Sizing Conductorscont.

• The minimum size of an individual conductor is
  determined by two factors.
• Ampacity
• Voltage drop

What is ampacity?
What is voltage drop?
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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 the conductor (cmils).
• Expressed in an equation

A cross-sectional area in cmils (diameter in
mils)2 1mil 0.001 in
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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
31
Voltage Drop

• When 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.

What will cause excessive voltage drop?
What are some possible outcomes of a circuit with
excessive voltage drop?
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.

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33

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)

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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.
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35
Series 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 series
  circuits is different than the method for
  parallel circuits.

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36
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.

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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 have been combined.
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38
Parallel Circuit Resistance
Determine the total resistance for the circuit in
the illustration.
or
or
38
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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.

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Calculating Voltage In A Series Circuit

• What 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 current in the
  circuit
• Knowing the amount of current we can calculate
  the voltage drop.

Note circuit conductors behave like resistors in
series.
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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.
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42
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.

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43
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

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44
Determining Amperage in a Parallel Circuit--cont.
When the total resistance is known, the circuit
current (Amps) can be calculated.
Total current is
A1 12.76 A
When the circuit current (Amps) is known, the
current for each branch circuit can be calculated.
Branch current is
A2 1.9 A
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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.
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Insulation

• The common types used in Agriculture.

90o C 194o F
Electricity for Agricultural Applications, Bern
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 never wear out.
• Insulation deteriorates over time.
• Insulation reacts with oxygen, ammonia, oil,
  gasoline, salts, UV and water.

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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

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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.

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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.

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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 33-19. (Agricultural
  Mechanics)

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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.
Pg 35 Wiring handbook
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Sizing Conductors by Voltage Drop

• Voltage drop is the result of a current passing
  through a resistance.
• Example
• What is the percent voltage drop at the service
  entrance panel for the building in the
  illustration?

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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.
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Example--cont.
This circuit diagram illustrates the resistance
of the conductors.
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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 size is determined by calculating
the allowable resistance for the desired voltage
drop.
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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 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 conductor that has a lower resistance.
• Use a higher voltage.
• Of these 5 options, number 2 and 5 are usually
  the only practical solution.

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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.
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Designing For Acceptable Voltage Drop

• Because all conductors have resistance, it can
  not be eliminated from the circuit.
• Therefore, circuits are designed for a specific
  voltage drop.
• Maximum of 2 in branch circuits is common
  standard
• NEC allows up to 3 maximum in branch circuit at
  farthest power outlet
• NEC allows 5 maximum drop in feeder and branch
  circuit combined

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Conductor Size Equation

• The equation for calculating the conductor size
  (cmils) for a specified voltage drop

Note it is common practice to add 10 to the
length to account for the resistance of the
connections.
61
Conductor Size Example -1

• Using the resistivity equation, determine the
  size of conductor that should be used to power an
  grow lamp that draws 6.6 amps and is operating on
  single phase and 120 volts. The grow lamp is
  located 75 feet from the nearest source. Copper
  conductors will be directly buried and a 2
  voltage drop is acceptable.

62
Conductor Size Example-cont.
4705.39 cmil AWG 12 Ampacity 12 Voltage
Drop 12
Note 10 has been added for connections.
63
Resistivity Equations

• Because of inductance in the conductor, the
  equation for copper is usually changed for design
  purposes to

• 22 constant for copper
• I Circuit load (amp)
• l Run (distance)
• E Allowable voltage drop (V)

The resistivity equation for aluminum conductors
is changed to
Note in these equations the length is the one
way length, not the total length.
64
Conductor Size Example 2

• Using the table method, determine the size of
  copper conductor that should be used to provide
  electrical service to a livestock building that
  is located 65 feet from the source. The service
  is 120 V and the estimated load for the building
  is 35 amps. UF-B cable and a 2 voltage drop
  will be used.

First determine minimum size based on ampacity.
Second determine size based on VD
AWG 8
AWG 6
65
Conductor Size Example 3

• Determine by calculation, using the standard
  equation, the size of conductor that will be
  required to provide service to a 120 V, 1-1/2 hp
  water pump that is located 250 feet from its
  source. The conductors with be copper and
  directly buried. A 2 voltage drop is
  acceptable. The motor has a power factor of
  0.70.
• The standard equation for copper requires values
  for amperage, voltage and length.
• The load on the circuit, amperage, must be
  determined first.

• 1 hp 746 watts, but when determining conductor
  sizes for electric motors it is common practice
  to use 1,000 watts per horsepower.

66
Conductor size 3--cont.

• For this circuit with a 8.75 amp load, the
  circular mills of the conductors can be
  determined by

The conductor size is
6 26,240 cmil 8 16,510 cmil.
Which size should be used?
67
Conductor Sizing Conclusion

• For short distances, ampacity determines the
  minimum conductor size.
• For long distances, voltage drop determines the
  minimum size.

Because long and short are relative terms, both
ampacity and voltage drop must be checked when
sizing conductors.
68
Questions ?