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

I1E1 = I2E2

Change

- Voltage [E] and

- Current [I]

Transformers

Es/Ep = Ns/Np

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 = 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/Rates/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

large primary service

$/kW $/kWh

Daily $ 12.60 $ 0.015

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

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 Austinhttp://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

EIP 3

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)

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