Commercial Electrical System Architecture
How a complete commercial electrical system fits together — from the medium-voltage utility service, switchgear, and service transformer through the 480Y/277V main distribution, emergency generator and transfer switch, panelboards and motor control center, down to the branch-circuit load systems, low-voltage/communication systems, building automation, and grounding. Hover, tap, or focus any component or feeder for its description and NEC reference.
Hover, tap, or focus any component on the drawing (or a circuit below it) for details. Click to pin; click the pinned component again to clear. Unpinned details follow your pointer.
Component Reference
Every component in the drawing above, with its function and the NEC reference that governs it.
Service & Generation
Utility Service
The incoming medium-voltage utility service (13.2 kV / 34.5 kV) is delivered to the building service point, where the utility’s responsibility ends.
📘 NEC Art. 230Utility Transformer (Pad-Mount)
A pad-mounted utility transformer provides the connection between the medium-voltage primary and the building’s service, ahead of the metering point.
📘 NEC Art. 450 / utilityMain Switchgear (Metal-Clad)
Medium-voltage metal-clad switchgear provides the main service disconnect, metering, and protective relaying at the primary voltage.
📘 NEC Art. 230 / IEEE C37Metering
Utility revenue metering — typically CT-metered at this service size — measures the building’s energy and peak demand for billing.
📘 NEC 230.66 / utilitySurge Protective Device (SPD)
A service-entrance Type 1/2 SPD limits transient overvoltages from lightning and switching, protecting downstream distribution and equipment.
📘 NEC 230.67 / Art. 242Service Transformer (Step-Down)
The service transformer steps the primary voltage down to the building secondary distribution voltage — 480Y/277V, 3-phase, 4-wire.
📘 NEC Art. 450Main Distribution Switchboard
The 480Y/277V main switchboard distributes secondary power through feeders to panelboards, step-down transformers, and the motor control center.
📘 NEC Art. 408Emergency Generator
An engine-generator provides legally-required emergency/standby power, feeding the emergency distribution through the automatic transfer switch.
📘 NEC Art. 700 / 445Automatic Transfer Switch (ATS)
The ATS monitors the normal source and automatically transfers emergency/standby loads to the generator on power loss, then retransfers when utility power returns.
📘 NEC Art. 700Emergency Distribution Switchboard
The emergency distribution switchboard distributes generator-backed power to life-safety and essential loads, kept independent of normal distribution.
📘 NEC Art. 700Distribution (Panels & MCC)
Lighting Panel “LP”
A 120/208V panelboard serving the general lighting branch circuits throughout the building.
📘 NEC Art. 408 / 410Power Panel “PP”
A 120/208V panelboard serving general-purpose receptacle and plug-load circuits.
📘 NEC Art. 408HVAC Panel “HP”
A 480/277V panelboard serving HVAC equipment — rooftop units, chillers, pumps, and exhaust fans.
📘 NEC Art. 440 / 408Elevator Panel “EP”
A 480/277V panel feeding elevator and escalator equipment, with the required shunt-trip and selective coordination.
📘 NEC Art. 620Kitchen Equipment Panel “KP”
A panel serving commercial kitchen equipment — ovens, hoods and exhaust, and dishwashers.
📘 NEC Art. 210 / 422Motor Control Center (MCC)
The MCC groups motor starters, VFDs, and overload protection for pumps, fans, and conveyors in a single coordinated assembly.
📘 NEC Art. 430 / UL 845Capacitor Bank
A capacitor bank supplies reactive power to correct system power factor, lowering demand charges and freeing distribution capacity.
📘 NEC Art. 460Essential / Emergency Panel
A panel on the emergency/essential branch serving life-safety loads such as egress lighting, exit signs, and fire pump/alarm power.
📘 NEC Art. 700Branch Circuits & Load Systems
Lighting System
Interior and exterior lighting branch circuits with occupancy and daylight controls, designed to the energy-code lighting power allowances.
📘 NEC Art. 410 / ASHRAE 90.1Receptacle System
General receptacle, plug-load, and dedicated equipment circuits, including controlled receptacles required by the energy code.
📘 NEC Art. 210Special Systems (IT / Data)
Dedicated and isolated-ground circuits plus UPS-backed power for IT/data rooms and server equipment.
📘 NEC Art. 645Building Systems (MEP)
Power to mechanical, electrical, and plumbing building equipment — HVAC, pumps, elevators, and other MEP loads.
📘 NEC Art. 440 / 430Power Systems (UPS / PFC)
Power-quality and backup systems — UPS, battery systems, power-factor correction, and surge protection for critical loads.
📘 NEC Art. 645 / 705Low Voltage, Controls & Grounding
Low Voltage & Communication Systems
Low-voltage and communication systems — fire alarm, public address / voice alarm, mass notification, security / CCTV, access control, and structured cabling.
📘 NEC Ch. 8 / Art. 760Building Automation System (BAS)
The BAS provides monitoring, scheduling, alarms, energy management, and trend logging over BACnet/IP, integrating the building’s systems.
📘 ASHRAE 135 (BACnet)Grounding System
The grounding electrode system, main grounding bus (MGB), and equipment bonding establish the low-impedance fault-clearing path and reference the system to earth.
📘 NEC Art. 250Feeders & Distribution
The power and control distribution types that wire the system together — each shown as a colored line in the diagram above.
Utility / Primary Power
The medium-voltage utility service feeding the building at primary voltage (e.g., 13.2 kV / 34.5 kV).
📘 NEC Art. 230Primary Distribution
Medium-voltage distribution between the service switchgear and the service step-down transformer(s).
📘 NEC Art. 230 / 450Secondary Power (480Y/277V)
The 480Y/277V secondary feeders from the main switchboard to panelboards, transformers, and the motor control center.
📘 NEC Art. 215Branch Circuits (120/208V)
The 120/208V branch circuits from panelboards to lighting, receptacles, and equipment.
📘 NEC Art. 210Life Safety / Emergency
Generator-backed feeders to life-safety and emergency loads, kept independent of normal distribution for survivability.
📘 NEC Art. 700–701Control / Communication
Low-voltage control and communication wiring — building automation, signaling, and data.
📘 NEC Art. 725 / Ch. 8Grounding / Bonding
Grounding electrode and equipment bonding conductors establishing the fault-clearing path and earth reference.
📘 NEC Art. 250