objectives review electric circuits and electricity pricing - introduce hw5 select conductor and...
TRANSCRIPT
Objectives
• Review electric circuits and electricity pricing
- Introduce HW5
• Select conductor and conduit
• List electrical devices
Voltage (potential difference)
•Units?
•Symbols: V, E or U
•Current (electron flow)
•Units?
•Symbol I
Voltage and Current
E or V [V] R [Ω]
I [A]
Across a transformer
I1E1 = I2E2
Change
- Voltage [E] and
- Current [I]
Transformers
Es/Ep = Ns/Np
Ref: Tao and Janis (2001)
Single-Phase vs. Three-Phase
Ref: Tao and Janis (2001)
For three phase:
P=√3 E I
Why Three-Phase?
• Larger loads
• Smaller wire sizes (because higher voltages)• P = EI = I2R= E2/R (E = IR)• More efficient use of neutral wire
• 3 needed for efficient operation of equipment
Electricity Billing
• Electrical Use (energy)• Peak Demand (power)• Power factor
• Which is largest portion of residential bill?• What about for commercial buildings?
• http://www.austinenergy.com/About%20Us/Rates/Commercial/index.htm
Example: September cost of Electricity for ECJ
• Assume ECJ is 120,000 ft2 and that it needs, on average, 8 W/ft2 for 8 hours a day, 6 W/ W/ft2 for 4 hours a day, and 4 W/ft2 for 12 hours a day
• Use Austin Energy Large Primary summer service rate• 1.5¢/kWh, 12.60 $/peak kW/month
• Assume no power factor charges
Solution
large primary service
$/kW $/kWh
Daily $ 12.60 $ 0.015
hours kW kWh
8 960 7680 demand $12,096.00
4 720 2880 energy $ 7,344.00
12 480 5760
TOTAL 16320 Total $19,440.00
Other Pricing Strategies
• Time of use pricing• Becoming more common for residential and
commercial• Electricity cost related to actual cost• Requires meter
• Interruptible pricing• Utility can shut off electricity for periods of time
Other Pricing Strategies
• Time of use pricing• Becoming more common for residential and commercial• Electricity cost related to actual cost• Requires meter
• Interruptible pricing• Utility can shut off electricity for periods of time
For Austinhttp://www.austinenergy.com/About%20Us/Rates/Commercial/index.htm
Homework 5 Problem 1: Economical analyses of chilled water saving
Advantage of higher voltage
120/208
220/380
277/480
EIP 3
Neutral and 3-phase system
• If system is well
balanced
the current
through the
neutral wire is 0
Grounding
• What is electrical ground?• Why do we ground electrical devices/systems?
• Protect equipment• Lightening strike• Protection of people
• Sometimes need ungrounded power• If the hot side touches ground, it can put out of
service our circuit• Isolate ungrounded systems
Conductors
• Material
• Form
• Composition
• Voltage class
• Insulation
• Covering
• Temperature rating
AWG American Wire Gage
Wiring method –Raceways
• Electric tubes
• Rigid conduits
• Wire ways
• Bus ducts
• Underflow• Different variation
www.hhrobertson.com/
Design issues with conductors
• Material (copper/aluminum)• Size of conductor (pg. 365)• Conduit requirements
• Location• Residential or Commercial• With or without raceways• Compression fittings or set screws• Threaded connections
• NEC and local codes
Current-Carrying Capacity
Ref: Tao and Janis (2001)
Ref: Tao and Janis (2001)
Tubing size
Other Issues
• In general, no more than 40% of raceway can be filled with wiring• Why?
• To prevent extensive heat up • To prevent physical installation of wires
Conductor Rules
• Explain each of the following:• No more than 4 90 ° bends are allowed between
pull boxes
• In the same conduit:• No mixing of high- and low-voltage conductors• No mixing control and power conductors• No mixing phone and power conductors• Do place all three phases in the same conduit
Receptacles and switches
• Receptacles (duplexes)• Number
• Shape
• Voltage rating
• Current Rating
• Number of poles and wires
• Switches• Type (NEC rating)
• Contact method
• Speed of operation
• Voltage rating
• Number of poles
• Method of operation
• Enclosure
• Duty
• Other (dimming)
Protective Devices• What are we protecting from?
• Overcurrent• Overvoltage
• Circuit breakers• Switch that responds to thermal or short circuit loads
• Can be bimetal, magnetic, or electronic• Reusable, remote control, compact, can be used as disconnect switch
• Fuses• Melting metal• Self destructive, larger
Why use fuses instead of circuit breakers?
A. Fuses can be used multiple times
B. Fuses are more aesthetically pleasing
C. Fuses are safer
D. Fuses cause less damage to equipment
Ref: Tao and Janis (2001)
Motor starters• ON-OFF switches are NOT for motors
• Motor circuit-rated switches• Reduce the current during the start
until the motor rich full speed• Protect the motor form overload
•Emergency power systems Batteries Power generators
Summary
• Describe role of electrical system components
• Calculate billing for an electrical system given the rate structure
• Size conduit and conductors given current requirements and conductor type
Reading Assignment
Tao and Janis:
Whole chapter 11