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Objectives • Review electric circuits and electricity pricing - Introduce HW5 • Select conductor and conduit • List electrical devices Voltage and Current I [A] E or V [V] Voltage (potential difference) •Units? •Symbols: V, E or U •Current (electron flow) •Units? •Symbol I R [Ω] Transformers Change - Voltage [E] and - Current [I] Across a transformer I1E1 = I2E2 Es/Ep = Ns/Np Ref: Tao and Janis (2001) Single-Phase vs. Three-Phase For three phase: P=√3 E I Ref: Tao and Janis (2001) 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/R ates/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 hours 8 4 12 Daily kW 960 720 480 TOTAL large primary service $/kW $/kWh $ 12.60 $ 0.015 kWh 7680 2880 5760 16320 demand energy $12,096.00 $ 7,344.00 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 Austin http://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 P 3EI 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) Tubing size Ref: Tao and Janis (2001) 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. B. C. D. Fuses can be used multiple times Fuses are more aesthetically pleasing Fuses are safer 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