Electrical Systems: Power and Service Equipment

• Electrical System is the network of conductors and equipment designed to safely carry, distribute, and convert electrical power from delivery or generation to building loads.

Power Generation and Transmission

• Electricity comes from utility-owned power stations.
• Power stations are facilities housing electrical energy generation equipment.
• Generators convert mechanical energy into electrical energy by rotating an armature within a magnetic field.
• A turbine (rotary engine) drives the generator's armature shaft.
• Power substations near housing areas reduce transmission voltages to avoid the dangers of high-voltage transmission through populated areas.
• Power substations are small facilities with transformers and switches that lower transmission voltages for safer distribution.

Building Electrical Service Equipment

• Service Entrance Equipment carries power from the transformer through a metering device to the building's service disconnects.

• Service entrance conductors, except for the neutral wire in overhead installations, must be insulated.

• Two types of service entrance equipment:

  • Underground (service lateral)
  • Overhead (service drop from a distribution transformer)

• Single phase service drop uses 2-3 wires, whereas a three phase service drop uses 3-4 wires.

• Service Entrance connects the utility's wiring to the service disconnect, excluding the utility's metering equipment.

• Service entrance equipment includes metering equipment, a main power disconnect, and overcurrent protection devices like circuit breakers or fuses.

• Electric meter measures and records electrical energy consumption; feed-through meters are used in building services rated up to around 400 A.

• Service disconnect allows electrical service to be switched off from the utility company; it can be a set of fuses or a circuit breaker.

• Switchboards are large metal cabinets housing disconnecting switches, overcurrent protection devices, and instruments to divide large electrical current into smaller amounts.

• Typically floor mounted, switchboards have current levels of 1200-6000 amperes (A) and voltages below 600 volts (V).

• Panelboards are metal cabinets that act as a single unit, containing buses and overcurrent protection devices, with or without switches.

• A panelboard is a cabinet set in or against a wall that can be accessed from the front.

• Panelboards can also be referred to as power panels, load centers, distribution centers, or main power panels

• Load centers are panelboards with a preassembled disconnect and circuit breakers, and are commonly used in residences.

• A commercial building's electrical distribution system has feeders extending from the switchboard to lighting/appliance and power distribution panelboards.

• Circuits then extend from the panelboards to outlets.

Building Transformers

• Transformers are used to transmit and distribute power; measured in kVA. Common building sizes range from 3 to 1500 kVA.

• Transformer nameplates typically include:

  • Rated KVA
  • Primary and secondary voltage
  • Impedance (if 25 kVA or larger)
  • Required clearances (if there are ventilating openings)

• Step-down transformers have lower secondary voltage

• Step-up transformers have higher secondary voltage, often used to reduce building system voltages.

• Single-phase transformers have single primary and secondary windings, used in residential and small commercial applications.

• Three-phase transformers have three primary and secondary windings.

• Two main types of three-phase transformers.

  • Delta: windings are connected in series in a triangle (Δ).
  • Wye: three independent transformer windings connected at a common neutral or star point (Y).

Overcurrent Protection

• Overcurrent Protection (OCP) safeguards building services or circuits from excessive current flows.
• Fuses and circuit breakers act as automatic overcurrent protection devices.

Circuit Breakers

• Acts as a switch that can be manually opened and closed
• Automatically "trips off" to open the circuit when the current exceeds the rating.

Fuses

• Consist of a metal strip with a low melting temperature
• When the current rating is exceeded, the strip melts and opens the circuit.
• Three basic types of fuses:
  • Plug fuses: Rated 5-30 A.
  • Cartridge fuses: cylindrical, with ferrule-contact (up to 60A) or knife-blade types.
  • Time delay fuses: Handle temporary overloads without blowing, used for electric motors.
• Circuit breakers can deteriorate with age and thus must be periodically replaced.
• Overcurrent protection is typically installed in enclosures such as switchboards, panelboards, motor control centers, or individual enclosures.

Overcurrent Protection Device Ratings

• Overcurrent rating is the highest amperage an OCP device can continuously carry without overheating.
• Interrupting rating is the maximum fault-current than an OCP device can handle.
• Standard AIC rating for circuit breakers and fuses: 10,000.

Utilization Equipment and Devices

• Utilization equipment includes electrical or electronic machines designed for mechanical, chemical, heating, or lighting functions such as electric motors, air conditioning, units, and elevators.

• Appliances are end-use pieces of utilization equipment such as refrigerators, ranges, clothes washers and dryers, freezers, and blenders.

• Electrical devices carry but do not use electricity, and include components such as switches, receptacles, and relays.

• Outlets are locations in a branch circuit where electricity is used.

• Receptacles are female connecting devices with slotted contacts, installed at outlets for plug insertion.

• Plugs are male connecting devices with prongs, connected to flexible cords of appliances, lights, or equipment.

Electrical Conductors

• Materials that conduct electrical current; wires are common electrical conductors.

• Wire gauge is classified via a standard based on cross-sectional area in circular mils (cmil).

• A mil is equal to 1/1000 inch.

• Solid conductors consist of a single solid wire.

• Stranded conductors consist of smaller wire strands, and are more efficient because electrons flow more easily near the surface.

• Copper and aluminum are common conductor materials that provides a balance of conductivity and economy.

Conductor Insulation

• Provides electrical isolation and physical protection to prevent power loss and short circuits.

• Cable contains multiple bundled conductors with an outer sheathing and may be made of copper (better conductor and safer) or aluminum.

• Conductivity describes electricity flowing from one point to another through an electrical wire.

Common Cable Types

• Nonmetallic-sheathed (NM) cable: for dry, indoor use

• Underground Feeder Cable: flame retardant, moisture, fungus, and corrosion resistant.

• Service entrance cable: suitable for exposed uses

• Armored cable: contains 2 to 4 copper conductors, for protection in harsh conditions.

• Metal-clad Cables (MC): Similar to AC except it is not limited to the number sizes of conductors it can carry.

• Flat conductor cable (FCC): Under carpet squares

• Thermostat cable: Used in application with voltages less than 30V, such as wiring to doorbells, chimes and thermostats.

• Cords are flexible stranded conductors inside flexible insulation, used for tools, equipment, or computer power cords.

• Conductor Ampacity is the maximum current a conductor can carry continuously without exceeding temperature limits; affected by wire thickness, conductor material, insulation, bundled conductors, temperature, and exposure.

• Voltage Drops in Conductors results from power losses and leads to inefficient appliance operation i.e voltage across conductors is lower at the usage end than at the power supply end of the circuit.

Enclosures and Raceways

• Enclosures are electrical boxes and cabinets made of either metal / non-metal to that protects electrical components and wiring from damage
• Electrical boxes are metal / non-metal that houses switches / outlets for wiring connections. Available in square, rectangular, octagonal, or round shapes.
• Junction boxes (J-boxes) are electrical boxes used to enclose wiring joints
• Pull box allows access to a raceway for snaking conductors through the raceway.
• Knockouts in most boxes are removed to allow wiring to enter the box.
• Cover plates, made from plastic / stainless steel / ceramic, is required for switches, outlets, and dimmers

Raceway

• Enclosed channel designed for holding wires, cables, or busbars.

  • Rigid metal conduit: Heavy galvanized steel or aluminum tube.
  • Intermediate metal conduit (IMC): Thinner-walled galvanized steel or aluminum tube with threaded connections.
  • Electrical metallic tubing (EMT): Thin-walled galvanized steel or aluminum tube joined with threadless compression couplings.
  • Electrical nonmetallic tubing (ENT): Flame-retardant, semiflexible corrugated plastic tube for use behind fire-rated materials.
  • Rigid nonmetallic conduit: Thin-walled PVC pipe joined with solvent welding.
  • Flexible metal conduit: Installed without cables; wiring must be pulled through.
  • Liquid tight flexible metal conduit: Flexible metal conduit covered in watertight plastic.
  • Liquid tight flexible nonmetallic conduit: Flexible plastic conduit, cannot exceed 6 ft in length.
  • Cellular concrete floor raceways: Hollow voids in precast concrete floor slabs.

• Busway factory-assembled enclosure consists of outer duct-like housing for delivering large amount of current

• Wireways is used to describes sheet metal or plastic troughs used to enclose and protect conductors (accessed through a door/removable cover)

Occupant Protection

• Tamper-resistant receptacles: Incorporate shutter mechanisms for enhanced safety, preventing accidental electrocution particularly for kids.

• Ground fault interruption: ground fault happens when electrical current accidentally flows between an ungrounded (hot) wire and grounded wire.

• Ground fault circuit interrupter (GFCI): detects even extremely low electrical current (6 mA) and acts quickly by shutting off appliance power.

• Arc Fault Protection: Arc faults happen when an unintentional electric discharge of low, erratic current occurs.

• Arc fault circuit interrupter (AFCI): Detects hazardous arching in building circuits due to damaged wires by using electronics that automatically recognize and interrupt electrical discharges.

• Surge Protection is a power surge from an increase in electrical current or voltage that very short in duration cause by lightning.

• Inrush Current Protection is an inrush current or input surge that refers the peak instantaneous current of appliance.