Branch Circuits Section 11 Unit 33

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Branch CircuitsSection 11Unit 33
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Introduction

• For an electrical system to be as safe as
  possible, all conductors, devices and connections
  must follow the current National Electrical Code
  (NEC).
• Buildings without finished interior walls have
  special requirements.

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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.

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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.
• 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.

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Electrical Boxes

• All electrical connections must occur inside an
  enclosure.
• Electrical boxes are the most common type of
  enclosure.
• Hold conductors and conduit so stress cannot be
  applied to the connections.
• Attached to building for switches, fixtures and
  other electrical devices.
• Prevent mice and other vermin from chewing on and
  nesting around electrical connections.

Boxes are available in plastic or steel,
rectangular or octagonal shapes, and different
widths and depths.
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Electrical Box Wiring Requirements

• Boxes must be securely fastened to building.
• Cable or conduct must be clamped to the box.
• Cables (conductors) must run inside walls, floors
  and ceilings and be attached close to the box and
  periodically along the length of the cable.
• Metal boxes must be grounded.
• Wires must be connected to each other using
  solderless connectors.
• No more than one wire per terminal on switches,
  receptacle and other electrical devices. Fig
  33-4 not acceptable everywhere
• Hot wires are attached to brass colored screws.
• Neutral wires are attached to aluminum colored
  screws.
• Ground wires are attached to green colored
  screws.
• When the white (neutral) conductor is used as a
  hot conductor (switch loop) it must be marked
  with black paint or tape. Fig 33-5

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Sizing Conductors

• When discussing the differences between series
  and parallel circuits the assumption was made
  that the conductors had no resistance.
• This assumption can not be made when determining
  conductor size.
• Conductor size is based on voltage drop and
  voltage drop is caused by resistance.

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Conductor Sizing

• Two systems are used to define the sizes of a
  conductor
• AWG
• cmils
• 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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Conducting Materials

• 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
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Example--Circular mills (cmils)

• What is the diameter (in) for a AWG 12 wire?

Electricity for Agricultural Applications, Bern
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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
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Insulating Materials

• The required characteristics of electrical
  insulation are
• environmentally durable
• high dielectric strength.

Electricity for Agricultural Applications, Bern

• Electrical insulating materials are rated on
  their dielectric strength.
• Dielectric strength voltage per 0.001 inch of
  thickness at which their resistance reaches a
  breakdown point and they become conductors.

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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 are

Electricity for Agricultural Applications, Bern
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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.

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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 32-19.

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Ampacity-cont.

• Ampacity can be calculated, but tables such as
  32-19 present this information in the left side
  of the table.
• Example what is minimum size of conductor with
  TW insulation in conduit, operating on 120 volts
  that should be used to carry 15 amps?

AWG 12
Note According to the NEC, 12 is smallest size
of wire that can be used under any conditions
using 120 V.
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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
• Equation for calculating the conductor size for a
  specified voltage drop

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

• Using the restivity 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.
• The first step is to determine the minimum size
  based on ampacity.
• AWG 12 minimum recommended for all circuits.
• The next step is to determine the size based on
  voltage drop by using the resistivity equation.

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Conductor Size Example-cont.
4705.39 cmil AWG 12 Ampacity 12 Voltage
Drop 12
Note 10 has been added for connections.
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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.
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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 70 feet from the source. The service
  is 120 V and the estimated load for the building
  is 35 amps. THW in conduct will be used.

Ampacity
8
Voltage drop
6
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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 value for
  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.
• This accounts for motor efficiency.

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Conductor size 3--cont.

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

The conductor size is
AWG 6 26,240 cmil 8 16,510 cmil. 26,240
cmil gt 22,057 cmil gt 16,510 cmil Minimum
ampacity AWG 12 VD AWG 6 should be used.
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Conductor Sizing Example 4

• Determine the size of conductor that will be
  required to supply a 40 amp load when TW wire in
  conduct, will be used. The load is 25 feet from
  the source. A 2 voltage drop is desired.
• Based on ampacity

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Voltage drop
8
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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.
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Questions ?