1. Objectives Finish with Practice circuits problems for
Electric Systems (protective devices)
HW4 (part 2)
Practice circuits problems for
Next Class Quiz
Exams 2 (next Tuesday)
FE exam

2. Next class QUIZ
Circuits
Chapters 11 and 13

3. Exam 2 Tuesday (April 30th)
In class (same as the previous one)
Circuits (chapter 10 & 11 & class examples)
AC vs. DC; single phase vs. 3 phase
Pricing (HW4 part 1)
Electric systems and wiring (chapter 11 &13 HW4 part 2)

4. 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

5. 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

6. Ref: Tao and Janis (2001)

7. 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
Reading Assignment: Chapter 11

8. Residential Systems (NEC Article 220-Branch Circuit, Feeder and Service Calculations)
Very little variation between different houses
Many similarities to commercial buildings
Particularly as you get close to end uses
Need to be watchful for unusual circumstances

9. Wiring from panel to: receptacles, lighting, ceiling fan,…..
Feeders and conduit
House branch circuits
Electric panels

10. Electric panel - 3 wire 1 phase 120/240

11. Procedure Analyze building needs Determine electrical loads
Select electrical systems
Coordinate with other design decisions
Prepare plans and specifications

12. List of needs Appliances (incl. HVAC) Receptacles Lighting
Ground fault protected (GFI/GFCI)
Switched
Lighting
Switches
Ceiling Fans

13. Example House Master Bedroom Bathrooms
Entry and Outdoor (AC compressor)
Bedrooms
Dining and Mechanical Closet (resistance heat)
Kitchen (appliance circuits, range, oven)
Family
Garage/Laundry/Closet (resistance water heater, dryer)

14. Outdoor
Bathroom
Closet
Bedroom 2
Laundry
Kitchen
Bathroom
Mechanical
closet
Family room
Master
Bedroom
Entry
Bedroom 3
Dining
Garage

15. Residential system sizing
Typically 3 wire 1 phase system 120/240 V
Procedure defined by NEC article 220:
Identify characteristic electrical loads
Add lighting (3W/ft2)
Add two 20 Amps circuits for kitchen
Add one 20 Amps circuit for laundry
Electricity for HVAC equipment based on requirement for heating or cooling

17. Summary of Power Sizing
Include enough capacity for heating and cooling
Work through kitchen and then other rooms
Include any items that are unusual
Take first 10 kVA and then 40% of remaining load

18. Panel Sizing Find panel power usage and neutral usage
Size panel conductors and associated conduit and switches

19. Electric panel - 3 wire 1 phase 120/240

20. Wiring from panel to: receptacles, lighting, ceiling fan,…..
Feeders and conduit
House circuits
Electric panels

21. Outdoor
Bathroom
Closet
Bedroom 2
Laundry
Kitchen
Bathroom
Mechanical
closet
Family room
Master
Bedroom
Entry
Bedroom 3
Dining
Garage

22. Homework 4 - problem 2 Sizing the residential electrical system
Defined by NEC article (BOOK chapter 13)
Similar to the example we worked out
You need to collect the data for several characteristic consumers
Includes conductor sizing

23. Item
Watts
Comments
Air conditioner (larger than 65% of furnace)
xyz W/ton + 2 × 900 W (2 fans of air handlers)
lighting
yyW/sq. ft.
Outdoor lighting
xxW × 15
Appliance Circuits
Separate loop required by NEC article
Laundry circuit
Dishwasher
Electric oven (w/ natural gas stovetop)
30 cu ft. refrigerator
on appliance circuit of kitchen
Electric dryer
Required on separate loop
Washing machine
on appliance circuit of laundry
50 gallon electric water heater
Garage door opener
3 ceiling fans
300W × 3
Duplex outlets
2 floors × 20 duplex × 1.5 A × 120 V
Gross Load
Net load
10kW + 40% remaining load =10+0.4×32200
Current
Amps = Net load / 240 V
Panel Size
First standard larger than calculated is 100Amps

24. HW4 (problem 1) Total a) Total b) Total c)
Cost for electric energy for
no cooled water storage tank
electricity demand for COP=4
cooled water storage tank
(chiller consumption is constant)
cooled water storage
optional time-of-use rate
Hour
Cooling load [kW]
Power
[kW]
Price [$/kwh]
Cost
[$] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Total a)
Total b)
Total c)

25. Circuit problems (for practicing) Start with problems that Yune provided:

26. Example problem 1
What is the potential difference (voltage) across the 20-Ω resistor?
a. 3.2 V
b. 7.8 V
c. 11 V
5.0 V
8.6 V

27. Example problem 2

28. Example problem 3
NOTE:
After t = 5t, the capacitor acts like an open circuit

29. Example problem 4
NOTE: Time constant of the circuit is t = RC = 15 ms. Time constant is the time to charge capacitor to 63%. [1- e-1]. To charge more (80%) you need more time.