Objectives
• Review conductor and conduit selection
• Learn about protection systems (basic rules)
• Learn to design Residential Electrical Systems
Current-Carrying Capacity
Ref: Tao and Janis (2001)
Tubing size
Ref: Tao and Janis (2001)
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.
B.
C.
D.
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)
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
Residential Systems
• 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
Procedure
1.
2.
3.
4.
5.
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
• 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
Bedroom 2
Laundry Closet
Kitchen
Bathroom
Mechanical
closet
Master
Bedroom
Family
room
Entry
Bedroom 3
Dining
Garage
NEC about receptacles
No longer approve non-polarized and
ungrounded receptacles in new installations.
Each receptacle (duplex) is rated for 1.5 A.
For 120 V: Each duplex 1.5×120 = 180W
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
Homework 5 - problem 2
• Sizing the residential electrical system
• Defined by NEC article 220-30
• Similar to the example we worked out
• You need to collect the data for several
characteristic consumers
• Includes conductor sizing