Electrical Systems Overview
Electrical systems questions on the ARE cover power distribution basics, lighting design, and - critically - the life-safety electrical systems that protect building occupants. These topics span PPD (systems integration) and PDD (detailed design and code compliance).
Power Distribution
Electrical power typically enters a building at a utility transformer and service entrance, passing through a main switchboard/distribution panel before being stepped down and routed to branch panels serving individual areas. Key concepts include:
- Voltage systems: Common building voltages include 120/208V (typical for receptacles and small equipment in commercial buildings) and 277/480V (typical for lighting and larger mechanical equipment).
- Demand load and diversity: Actual electrical demand is typically less than the sum of all connected loads because not everything operates simultaneously; demand factors account for this in sizing.
- Panel schedules: Document circuit assignments, breaker sizes, and loads for each panel, coordinated between architect and electrical engineer.
Lighting Design Fundamentals
- Illuminance is measured in foot-candles (fc) or lux and varies by task - office work typically requires 30–50 fc, while circulation spaces require less.
- Lighting power density (LPD), measured in watts per square foot, is regulated by energy codes (e.g., ASHRAE 90.1) and varies by space type.
- Color temperature and CRI (Color Rendering Index) affect the visual quality and mood of a space; higher CRI values render colors more accurately.
- Lighting controls - occupancy sensors, daylight harvesting, and time-based scheduling - are frequently required by energy codes to reduce unnecessary lighting energy use.
Emergency and Egress Lighting
Building codes (IBC) and life safety codes (NFPA 101) require illumination of the means of egress during normal operation and continued illumination during a power outage:
- Egress illumination: A minimum average illumination level (typically 1 fc at the walking surface) must be maintained along the entire egress path.
- Emergency power: Provided by battery units, generators, or unit equipment, required to maintain egress lighting for a minimum duration (typically 90 minutes) after normal power loss.
- Exit signage: Illuminated exit signs must remain visible and legible during both normal and emergency power conditions.
Fire Alarm Systems
Fire alarm systems detect fire conditions and notify occupants, and often interface with other building systems:
- Initiating devices: Smoke detectors, heat detectors, and manual pull stations that trigger the alarm.
- Notification appliances: Horns, strobes, and combination devices that alert occupants, with strobe placement and intensity regulated for accessibility (ADA) compliance.
- System interfaces: Fire alarm systems commonly interface with HVAC (shutting down air handlers or activating smoke control), elevators (recall to a designated floor), and door hardware (releasing magnetic locks).
- Mass notification and voice evacuation: Larger or high-rise buildings often require voice-based systems capable of providing specific evacuation instructions rather than a single alarm tone.
Electrical and life-safety systems must be coordinated with Fire Protection, HVAC Systems, and Plumbing Systems to resolve routing conflicts and shared code requirements.
Study further in the PPD division guide and PDD division guide. Sign up free for building-systems practice questions and flashcards.