1. Objectives Learn about 3 Phase vs. 1 phase F&N vs. 1 phase F&F
Electricity Pricing
Electric systems

2. Single-Phase vs. Three-Phase
For three phase:
P=√3 E I
Ref: Tao and Janis (2001)

3. 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

4. Neutral and 3-phase system
If system is well
balanced
the current
through the
neutral wire is 0

5. 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

6. Power Factor
Difference between power supplied and power that does useful work “working power”
P=E I cos θ
θ is the phase difference between current and voltage
For circuits that contain reactance (Inductor and Capacitors)
For three phase
P=√3 E I cos θ
Commercial buildings pay for power factor.

7. Electricity Billing Electrical Use (energy) Peak Demand (power)
Power factor
Which is largest portion of residential bill?
What about for commercial buildings?

8. 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, $/peak kW/month
Assume no power factor charges

9. Solution large primary service $/kW $/kWh Daily $ 12.60 $ 0.015 hours$ $
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

10. 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

11. Conductors Material Form Composition Voltage class Insulation Covering
Temperature rating
AWG American Wire Gage

12. Wiring method –Raceways
Electric tubes
Rigid conduits
Wire ways
Bus ducts
Underflow
Different variation

13. 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

14. Current-Carrying Capacity
Ref: Tao and Janis (2001)

15. Tubing size
Ref: Tao and Janis (2001)

16. 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

17. Conductor Rules Explain each of the following: In the same conduit:
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

18. 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)

19. Protective Devices What are we protecting from? Circuit breakers
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

20. Why use fuses instead of circuit breakers?
Fuses can be used multiple times
Fuses are more aesthetically pleasing
Fuses are safer
Fuses cause less damage to equipment

21. Ref: Tao and Janis (2001)

22. Emergency power systems Batteries Power generators
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

23. 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

24. Reading Assignment
Tao and Janis:
Whole chapter 11