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 I 1 E 1 = I 2 E 2 Change - Voltage [E] and - Current [I] Transformers Es/E p = N s /N p

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 = I 2 R= E 2 /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? ates/Commercial/index.htmhttp:// ates/Commercial/index.htm

Example: September cost of Electricity for ECJ Assume ECJ is 120,000 ft 2 and that it needs, on average, 8 W/ft 2 for 8 hours a day, 6 W/ W/ft 2 for 4 hours a day, and 4 W/ft 2 for 12 hours a day Use Austin Energy Large Primary summer service rate 1.5¢/kWh, $/peak kW/month Assume no power factor charges

Solution large primary service $/kW $/kWh Daily $ $ hourskWkWh demand $12, energy $ 7, TOTAL16320 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

Homework 5 Problem 1: Economical analyses of chilled water saving

Advantage of higher voltage 120/ / /480

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

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

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