Electrical Systems & Distribution

The electrical system is the nervous system of a building — it powers everything from lights and outlets to elevators, fire alarms, data networks, and HVAC equipment. Electrical work is the most regulated building trade because the consequences of error are severe: electrical faults cause an estimated 46,000 home fires per year in the United States, resulting in hundreds of deaths and billions of dollars in property damage. Understanding the electrical system from the utility transformer to the receptacle is essential for construction professionals, even those who are not electricians, because electrical work interacts with every other trade and governs spatial planning, fire ratings, and life-safety systems.

Voltage, Current, and Power:

• Voltage (V): Electrical pressure — the force that pushes electrons through a conductor. Measured in volts.
• Current (I): The flow rate of electrons through a conductor. Measured in amperes (amps).
• Resistance (R): Opposition to current flow. Measured in ohms. Ohm's Law: V = I × R
• Power (P): The rate at which electrical energy is consumed. P = V × I (for resistive loads). Measured in watts (W) or kilowatts (kW). 1 kW = 1,000 watts.
• Energy: Power consumed over time. Measured in kilowatt-hours (kWh). A 100-watt light running for 10 hours consumes 1 kWh.

AC Power and Phases:

Building electrical systems use alternating current (AC) at 60 Hz (in North America). The utility generates power as three-phase AC — three sinusoidal voltages offset by 120° — and distributes it through the grid.

• Single-phase: Two hot conductors and a neutral, derived from one phase of the three-phase system. 120/240V single-phase is the standard residential service — 120V between each hot and neutral (for lighting and receptacles), 240V between the two hots (for ranges, dryers, and HVAC equipment). Residential service sizes: 100A, 150A, or 200A typical.
• Three-phase: Three hot conductors (and sometimes a neutral), providing more efficient power delivery for large motors and equipment. Commercial and industrial buildings use three-phase power.120/208V three-phase, 4-wire (wye): 120V phase-to-neutral (lighting and receptacles); 208V phase-to-phase (small equipment). Common for small commercial buildings.277/480V three-phase, 4-wire (wye): 277V phase-to-neutral (commercial lighting); 480V phase-to-phase (large motors, HVAC equipment). Standard for medium and large commercial buildings. Step-down transformers within the building provide 120/208V for receptacles and small equipment.

The service entrance is where utility power enters the building. Components include:

1. Utility transformer: Steps down distribution voltage (4,160V–34,500V) to utilization voltage. Pad-mounted (ground-level, green box) for underground services; pole-mounted for overhead services. Residential transformers provide 120/240V single-phase; commercial transformers provide 120/208V or 277/480V three-phase.
2. Service conductors: Wires from the transformer to the building. Overhead (service drop) or underground (service lateral).
3. Meter: Utility-owned device that measures energy consumption (kWh) and, in commercial installations, demand (kW). Residential meters are typically socket-type mounted on the exterior wall. Commercial meters may be in a CT (current transformer) cabinet.
4. Main disconnect: A switch or breaker that can disconnect all building power. Required to be accessible and at a code-compliant location. In residential, typically the main breaker in the panelboard. In commercial, typically a fused switch or circuit breaker in a switchboard or switchgear.
5. Switchboard or switchgear: (Commercial) A large assembly containing the main disconnect, metering, and distribution breakers or fusible switches that feed branch panelboards throughout the building. Switchgear is higher-rated (above 600A) with draw-out breakers for easier maintenance; switchboards are lower-rated with fixed or bolt-on breakers.

Panelboards (Panels):

A panelboard is a wall-mounted enclosure containing circuit breakers (or fuses) that protect and control branch circuits. Think of it as the traffic control center for electrical circuits.

• Residential panel: Main breaker panel (100–200A) with 20–42 spaces for branch circuit breakers. Located in garage, basement, or utility room. Single-phase 120/240V.
• Lighting and appliance panelboard: Commercial panel serving 120V and 277V lighting and receptacle circuits. Fed from a transformer if the building service is 277/480V.
• Power panelboard: Commercial panel serving 208V or 480V equipment circuits (HVAC units, kitchen equipment, elevators).

Branch circuit breakers:

• Standard: 15A and 20A for receptacles and lighting; 30A–50A for appliances (ranges, dryers)
• GFCI (Ground Fault Circuit Interrupter): Senses imbalance between hot and neutral current (indicating current leaking to ground, possibly through a person) and trips in less than 1/25 of a second. Required for receptacles in bathrooms, kitchens (within 6 feet of sink), garages, outdoors, basements, crawl spaces, and laundry areas. 5 milliamp trip threshold.
• AFCI (Arc Fault Circuit Interrupter): Detects dangerous arcing conditions (frayed wires, loose connections) and trips before the arc can ignite a fire. Required for virtually all 15A and 20A branch circuits in dwelling units (bedrooms, living rooms, kitchens, etc.) per NEC 210.12.
• Dual-function (AFCI/GFCI): Combines both protections in one breaker; increasingly common in residential installations.

Conductors (Wires):

Conductors carry electrical current. Residential wiring typically uses copper conductors; large feeders may use aluminum (less expensive but requires larger sizes and special termination techniques to prevent oxidation and loose connections).

• Wire sizing: American Wire Gauge (AWG); smaller numbers = larger wire. Common sizes: #14 AWG (15A circuits), #12 AWG (20A circuits), #10 AWG (30A circuits), #8 AWG (40A), #6 AWG (60A). Feeders and services use larger sizes: 1/0, 2/0, 3/0, 4/0 AWG, and beyond that, MCM (thousands of circular mils).
• Insulation types: THHN/THWN-2 (most common for commercial, used in conduit), NM-B (non-metallic sheathed cable — "Romex" — residential standard), UF-B (underground feeder, direct-burial rated), MC (metal-clad cable — flexible metal armor around THHN conductors).

Raceways (Conduit):

Raceways protect and route conductors. Commercial electrical systems use conduit almost exclusively; residential uses NM cable (Romex) with conduit only where exposed.

• EMT (Electrical Metallic Tubing): Thin-wall steel conduit with set-screw or compression fittings. The standard for commercial indoor branch circuits. Lightweight, easy to bend. Not suitable for direct burial.
• IMC (Intermediate Metal Conduit): Thicker-wall steel conduit, threaded. Suitable for outdoor and wet locations. Heavier than EMT.
• RMC (Rigid Metal Conduit): Heaviest steel conduit, threaded. Maximum protection. Required in hazardous locations and where subject to physical damage.
• PVC (Rigid Nonmetallic Conduit): Plastic conduit for underground and corrosive environments. Joined by solvent cement. Requires a separate grounding conductor inside.
• Flexible metal conduit (FMC or "Greenfield"): Flexible steel spiral conduit for connections to vibrating equipment (motors, transformers) and where routing through tight spaces is needed. Liquidtight flexible metal conduit (LFMC) adds a plastic jacket for wet locations.

Wire Pulling: In commercial construction, conduit is installed empty during rough-in, then conductors are pulled through using a fish tape or pulling machine during wire-pulling phase. Wire fill (the number and size of conductors in a conduit) is limited by NEC Chapter 9 tables to prevent overheating — typically 40% fill for three or more conductors.

Grounding and bonding are the most important safety aspects of the electrical system.

• Grounding: Connecting the electrical system to the earth through a grounding electrode (ground rod, concrete-encased electrode (Ufer ground), metal water pipe, or ground ring). The neutral conductor is bonded to the grounding electrode at the service entrance. This limits voltage to earth and provides a path for fault current to operate overcurrent protection.
• Equipment grounding conductor (EGC): A conductor (green or bare wire, or the metallic conduit or cable armor itself) that connects every metal enclosure, box, panel, and equipment housing back to the service ground. If a hot wire contacts a metal enclosure, fault current flows through the EGC back to the panel, tripping the breaker. Without proper grounding, the enclosure remains energized — a lethal shock hazard.
• Bonding: Connecting all metallic systems (water pipes, gas pipes, structural steel, communications systems) to the grounding electrode system to ensure they are at the same electrical potential. This prevents dangerous voltage differences between systems if a fault occurs.