Building Electrical Systems and Standards

Fundamental Definitions and Sources of Electricity

Electricity is defined as a form of energy generated by friction, induction, or chemical change. It possesses magnetic, chemical, and radiant effects and is characterized by the motion of free electrons through a solid conductor.

The primary sources of electricity include:

Storage Batteries: These are utilized to supply emergency lighting circuits for hallways, stairways, and exits. They also energize police and fire alarm systems along with various signaling systems.
Generators: These machines generate electric current. They are categorized into:
- Alternating Current Generators (Alternators): These produce the bulk of electrical energy used today for both power and lighting.
- Direct Current Generators: These provide electrical energy specifically for elevators, escalators, intercommunicating telephone systems, clock systems, and the control of signal systems.

Additional dynamo-electric machines include:

Motors: Devices that convert electrical energy into mechanical energy.
Transformers: Equipment used to convert voltage from lower to higher or from higher to lower levels.
Rotary Converters: Machines that change alternating current (AC) to direct current (DC) and vice versa.

Types of Current and Units of Electricity

There are two primary types of electric current:

Alternating Current (AC): A current that periodically varies in time rate and direction. It rises from zero to a maximum value, falls back to zero, reverses direction, and returns to zero again.
Direct Current (DC): A current that flows at a constant time rate and maintains a singular direction.

Standard units of electricity include:

Coulomb (Unit of Quantity): Comprises approximately $6.25 \times 10^{18}$ electrons.
Ampere (Unit of Current): Represents a flow rate of one coulomb or $6.25 \times 10^{18}$ electrons per second through a given cross-section.
Volt (Unit of Potential): The electromotive force or potential difference between two points in an electric field that will move a charge of one coulomb between those points.
Ohm (Unit of Resistance): The resistance that allows one ampere of current to flow when one volt is impressed upon it.
Watt (Unit of Power): The rate of doing electrical work.
Watt-Hours (Unit of Energy): The capacity for doing work.

Ohm’s Law defines the relationship between these units in a DC circuit:

$I = \frac{V}{R}$

In this equation, current ( $I$ , in amperes) is directly proportional to voltage ( $V$ , in volts) and inversely proportional to the resistance ( $R$ , in ohms).

Electric Load Control and Measurement

Electric load control involves the effective utilization of available energy by reducing peak loads and lowering demand charges. Systems used for this include:

Load Scheduling and Duty-Cycle Control: Analyzing and scheduling loads to restrict demand by shifting large loads to off-peak hours and preventing coincident operation.
Demand Metering Alarm: Continuously meters demand and triggers an alarm when a predetermined level is exceeded.
Automatic Instantaneous Demand Control (Rate Control): An automated version of the alarm system that automatically disconnects or reconnects loads as required.
Ideal Curve Control: Operates by comparing the actual rate of energy usage to an ideal rate, controlling total energy used within a metering interval.
Forecasting Systems: Computerized systems that forecast the energy remaining in a demand interval and prioritize connected loads to decide the proper course of action.

Measuring electric consumption is performed using:

Wattmeter: Measures active power.
Kilowatt-Hour (KWH) Meters: Measures energy by introducing the factor of time ( $\text{energy} = \text{power} \times \text{time}$ ). These AC meters are essentially small motors with a speed proportional to the power being used, with dials calibrated in kilowatt-hours.

Building Electrical Systems and Principles

A branch circuit is a complete conducting path carrying current from a source to an electrical device (load) and back to the source. A two-wire circuit consists of a live wire (carrying current to devices) and a neutral or grounded wire (return wire).

Circuits are organized in two primary configurations:

Series Circuit: Components are connected in tandem. All loads carry the same current, and total resistance ( $R$ ) is the sum of individual resistances: $R = R_1 + R_2 + R_3 + ...$
Parallel (Multiple) Circuit: Components are arranged so that current divides between them. Each outlet has a live wire and a neutral wire. The total current is the sum of the currents in each outlet. Total resistance is calculated as: $\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...$

Classifications of Branch Circuits

General Purpose Branch Circuit: Supplies outlets for lighting and appliances, including convenience receptacles.
Appliance Branch Circuit: Supplies outlets for feeding appliances; does not supply fixed lighting.
Individual Branch Circuit: Designed to supply a single specific item, such as a motor or a unit air-conditioner.

Circuiting Guidelines:

Buildings should be supplied with a load of $30\,\text{watts/m}^2$ ( $3\,\text{watts/ft}^2$ ), excluding porches, garages, and basements.
Lighting, convenience receptacles, and appliances should generally be on separate circuits.
A minimum of two $20\,\text{ampere}$ appliance branch circuits is required for kitchen, pantry, dining, and family room outlets.
Convenience receptacles in a single area should be wired to at least two different circuits to prevent total power loss in that area.
General purpose circuits are typically rated at $20\,\text{amperes}$ and use minimum No. $12\,\text{AWG}$ wire. Switch legs may use No. $14\,\text{AWG}$ if lighting loads permit.

System Components and Distribution terms

Service Drop: The overhead portion of service conductors extending from a utility pole to a building.
Service Lateral: The underground portion of service conductors extending from a main power line or transformer to a building.
Service Entrance Conductor: The portion of the service conductor extending from the drop or lateral to the service equipment.
Service Equipment: Necessary for controlling, metering, and protecting the power supply, typically located near the entrance in a switchgear room.
Main Switchboard: A panel containing switches, overcurrent devices, metering instruments, and busbars.
Feeder: Conductors extending from service equipment to various distribution points.
Sub-feeder: Line extensions of a feeder that distribute power to local sub-panel boards.
Panelboard: A board mounted with switches, fuses, and circuit breakers for controlling and protecting similar branch circuits. It is also called a cut-out.
Busbar: A heavy conductor, usually a solid copper bar, used for distributing large currents.
Transformers: Used to step down high supply voltage to service voltage. Oil-filled transformers inside buildings require fire-rated, ventilated vaults.

Electrical Protection Devices

Fuse: A device containing a fusible metal strip that melts under excess heat to interrupt current.
Circuit Breaker: An automatic switch that interrupts a circuit to prevent damage from excess current. It can be reset and reused.
Ground Fault Circuit Interrupter (GFCI/GFI): An overcurrent device providing ground fault protection in addition to standard circuit breaker functions.
Lightning Rod (Air Terminal): A conducting rod installed at the top of a building and grounded to divert lightning.
Lightning Arrester: Protects equipment from high-voltage currents using spark gaps to carry current to the ground.

Single-Phase and Three-Phase Systems

Single-Phase System: Used for homes and small commercial buildings. Can be 2-wire or 3-wire ( $220\,\text{V}$ / $110\,\text{V}$ ). It involves two hot legs and a neutral wire.
Three-Phase System: Used for industries and large buildings. It is essentially a triple circuit. Lighting and outlets are connected between a phase leg and neutral, while heavy machinery is connected across phase legs ( $220\,\text{V}$ ).

Electrical Materials and Wiring Methods

Conductors:

Wires: Single insulated conductors No. $8\,\text{AWG}$ or smaller. The smallest permitted size is No. $14\,\text{AWG}$ .
Cables: Single insulated conductors No. $6\,\text{AWG}$ or larger, or multiple assembled conductors.

Conductor Sizes:

Sizes from No. $16$ to No. $4/0$ (0000) are expressed in AWG (American Wire Gauge).
Sizes larger than $4/0$ are expressed in MCM (Thousand Circular Mil).
One circular mil is the square of the cable diameter when expressed in mils ( $1000^{\text{th}}$ of an inch). Calculation: $\frac{\text{CM}}{1000} = (\text{diameter in mils})^2 / 1000$ .

Cable Types:

Type AC (Armored Cable): Flexible metal sheath.
Type MC (Metal Clad Cable): Individually insulated conductors in a metallic sheath.
Type NM/NMC (Non-Metallic Sheathed Cable): Trade name Romex. Integrated non-metallic outer sheath.
Type UF (Underground Feeder): Moisture resistant for direct burial.
Type SE/USE: Service Entrance cable.
Type FCC (Flat Conductor Cable): Used inside floor surfaces for individual branch circuits.
Type MV (Medium Voltage): Rated at $2,000$ to $35,000\,\text{volts}$ .

Conduit Types:

Rigid Steel Conduit (RSC): Heavy-wall steel ( $0.117\,\text{mm}$ thick).
Intermediate Metal Conduit (IMC): Mid-weight steel ( $0.071\,\text{mm}$ thick).
Electrical Metal Tubing (EMT): Thin-wall steel; not threaded; uses set screws. Not for hazardous areas or embedding in concrete.

Switch Types and Classifications

Switches are categorized by:

Duty: Normal Duty (ND), Heavy Duty (HD), or Light Duty (LD).
Service: Service switches (main disconnect), Power switches (disconnecting/isolating), and Wiring switches.
Wiring Configurations: Single-pole, Three-way (control from 2 points), Four-way (control from 3+ points), and Electrolier (multi-circuit for fixtures).
Mechanism: Rotary, Push-button, Toggle/Tumbler.
Poles and Throws: Poles are the insulated contact parts; Single Throw (one position closes circuit) vs Double Throw (closes in two positions).

Specialized switches include:

Automatic Transfer Switch (ATS): Automatically supplies emergency power upon failure of normal service.
Remote Control (RC) Switch: A relay that latches after being operated wireless from a distance.
Mercury Switch: Quiet operation using a sealed glass tube of mercury.
Time Controlled Switch: Includes a miniature drive motor (timer).

Electrical Regulations and Clearances (PD 1096)

Locational Requirements:

Poles and transformer supports must be located no more than $500\,\text{mm}$ inside from the road right-of-way or property line.
Clearance from fire hydrants: minimum $5\,\text{m}$ .
Clearance from curbs: minimum $150\,\text{mm}$ .

Vertical Clearances for Power Lines:

Primary Lines: $10\,\text{m}$ from the crown of the pavement when crossing highways; $7.5\,\text{m}$ when installed along the side of the road in urbanized areas.
Secondary/Neutral/Service Lines: $7.5\,\text{m}$ from the crown of the road when crossing; same for sides in urbanized areas.

Clearances for Buildings:

Buildings over $15\,\text{m}$ high: A clear zone at least $2\,\text{m}$ wide must be left beginning no more than $2.5\,\text{m}$ from the building for fire ladders.
Wire horizontal clearance from building surface ( $300$ to $8,700\,\text{V}$ ): $1.0\,\text{m}$ .
Wire vertical clearance from building surface ( $300$ to $8,700\,\text{V}$ ): $2.5\,\text{m}$ .
Service drop horizontal clearance from windows/doors/fire escapes: minimum $1\,\text{m}$ .
Service drop vertical clearance above windows: minimum $500\,\text{mm}$ .

Service Drop Over Roofs:

Standard clearance: $2.5\,\text{m}$ from the highest point.
Exception: If voltage is $\leq 300\,\text{V}$ and roof slope is $\geq 1:3$ , clearance may be $1\,\text{m}$ .
Exception: If $\leq 300\,\text{V}$ and only passing over $\leq 1.2\,\text{m}$ of roof overhang for termination, clearance may be $500\,\text{mm}$ .

Transformer and Vault Specifications

Transformer Vault Construction:

Required for expected load demands of $200\,\text{kVA}$ or above.
Walls and roofs: Reinforced concrete (min $150\,\text{mm}$ ), masonry/brick (min $200\,\text{mm}$ ), or load-bearing CHB (min $300\,\text{mm}$ ).
CHB walls must have a coating of cement or gypsum plaster at least $20\,\text{mm}$ thick.
Floor: Concrete (min $100\,\text{mm}$ thick).
Fire Rating: Walls and floor must have a minimum fire resistance of $2.5\,\text{hours}$ .
Doorways: Must have a sill at least $100\,\text{mm}$ high to confine oil.

Transformer Types and Locations:

Dry-Type Indoors ( $\leq 112.5\,\text{kVA}$ ): Minimum separation of $300\,\text{mm}$ from combustible materials.
Dry-Type Indoors (> 112.5\,\text{kVA}): Must be in a fire-resistant room unless separated from combustibles by $1.85\,\text{m}$ horizontally and $3.7\,\text{m}$ vertically.
Askarel-Insulated (> 25\,\text{kVA}): Requires a pressure relief vent.
Oil-Insulated (> 35,000\,\text{volts}): Must always be installed in a vault.

Emergency Systems and Maintenance

Emergency systems supply power for exit lighting, fire pumps, fire alarms, and life-support ventilation. Standby systems supply non-safety loads like water treatment or industrial processes.

Performance Standards:

Transition time for hospitals from normal to emergency power must not exceed $10\,\text{seconds}$ .
Storage batteries must be able to maintain at least $90\,\%$ of system voltage for the total emergency load for at least $0.5\,\text{hours}$ .
Emergency wiring must be kept entirely independent of all other wiring and raceways, except within transfer switches or emergency fixtures supplied from two sources.

Maintenance:

Systems must be tested periodically under the schedule of authority having jurisdiction.
A written record of all tests and maintenance must be kept.
Branch circuit overcurrent devices in emergency circuits must be accessible to authorized persons only.