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

Next class QUIZ Circuits Chapters 11 and 13

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)

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

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

Ref: Tao and Janis (2001)

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

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

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

Electric panel - 3 wire 1 phase 120/240

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

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

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)

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

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

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

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

Electric panel - 3 wire 1 phase 120/240

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

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

Homework 4 - problem 2 Sizing the residential electrical system Defined by NEC article 220-30 (BOOK chapter 13) Similar to the example we worked out You need to collect the data for several characteristic consumers Includes conductor sizing

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 220-30 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

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)

Circuit problems (for practicing) Start with problems that Yune provided: http://www.ce.utexas.edu/prof/Novoselac/classes/ARE346N/Handouts.html

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

Example problem 2

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

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.