Electrical System Architecture
How a commercial building power system fits together — from the utility service and main switchboard through feeders, transformers, and panelboards to branch-circuit loads, plus the emergency/standby power path and the grounding & bonding system. Hover, tap, or focus any component or circuit for its description and NEC reference.
Component Reference
Every component in the diagram above, with its function and the NEC reference that governs it.
Service & Main Distribution
Utility Service
The utility service — a transformer and service drop/lateral — delivers medium- or low-voltage power to the building service point. Everything downstream of the service point is the owner’s premises wiring.
📕 NEC Art. 230Utility Meter / CT Cabinet
The metering equipment (self-contained meter or a CT cabinet with instrument transformers on larger services) measures energy use for utility billing ahead of the service disconnect.
📕 NEC Art. 230 / utilityMain Switchboard (MSB)
The main switchboard contains the service disconnecting means and main overcurrent device and distributes power to feeders. It is the origin of the building one-line diagram and sets the available fault current downstream.
📕 NEC Art. 230 & 408Surge Protective Device (SPD)
A Type 1/Type 2 surge protective device limits transient overvoltages from lightning and switching. NEC now requires SPDs at services supplying dwelling units and many other occupancies.
📕 NEC 230.67 / Art. 242Grounding Electrode System
The grounding electrode system (ground rods, building steel, concrete-encased electrode, water pipe) bonds the service neutral to earth through the grounding electrode conductor, establishing the system ground reference.
📕 NEC 250 Part IIIPower Distribution
Distribution Panelboard
A distribution panelboard (often 480Y/277V) takes a feeder from the switchboard and distributes it to transformers, sub-panels, and large equipment via feeder breakers.
📕 NEC Art. 408Step-Down Transformer
A dry-type distribution transformer steps 480V down to 208Y/120V for receptacle and lighting panels. Primary and secondary conductor and overcurrent protection follow NEC Article 450.
📕 NEC Art. 450Branch Panelboard (Sub-Panel)
A branch-circuit panelboard distributes a feeder into the individual branch circuits serving a floor or area, with isolated neutral and ground bars when fed as a sub-panel.
📕 NEC Art. 408Busway / Bus Duct
Busway (bus duct) is a rigid, high-ampacity distribution backbone used in risers and industrial buildings, with plug-in units tapping power along its length. Installed per NEC Article 368.
📕 NEC Art. 368Branch Circuits & Loads
Lighting
Lighting branch circuits and controls serve luminaires per NEC Articles 210 and 410, with load calculated per NEC 220 and energy-code control requirements.
📕 NEC Art. 210 & 410Receptacles
Receptacle branch circuits supply general-purpose and dedicated outlets, with GFCI/AFCI protection required by location per NEC 210.8 and 210.12.
📕 NEC 210.8 / 210.52HVAC Equipment
HVAC equipment circuits supply air conditioners, heat pumps, and air handlers. Conductor and disconnect sizing for air-conditioning and refrigeration equipment follows NEC Article 440.
📕 NEC Art. 440Motors / VFD
Motor loads — pumps, fans, and machinery — fed directly or through variable frequency drives. Branch-circuit conductors, overload, and short-circuit protection follow NEC Article 430.
📕 NEC Art. 430EV Charging (EVSE)
EV supply equipment provides Level 2/3 charging. EVSE circuits, load management, and disconnects follow NEC Article 625, and are an increasingly large share of building load.
📕 NEC Art. 625Emergency & Standby Power
Standby Generator
An engine-generator provides an alternate power source during a utility outage. Generator output conductors, overcurrent protection, and ratings follow NEC Article 445 and the emergency/standby articles.
📕 NEC Art. 445 / 700Automatic 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 is restored, with required timing for life-safety systems.
📕 NEC Art. 700UPS
An uninterruptible power supply provides instantaneous battery-backed power for critical loads (IT, life-safety controls) during the gap before the generator picks up, per NEC Articles 645/700 as applicable.
📕 NEC Art. 645 / 700Emergency / Life-Safety Panel
A dedicated emergency panel supplies legally-required life-safety loads — egress lighting, exit signs, fire alarm, fire pump controls — fed from the ATS and kept independent of normal distribution per NEC Article 700.
📕 NEC Art. 700Grounding & Bonding
Grounding Electrode (Ground Rod)
Grounding electrodes — ground rods, concrete-encased (Ufer) electrodes, building steel, and metal water pipe — connect the system to earth. Electrode types and requirements are in NEC 250.52.
📕 NEC 250.52Grounding Electrode Conductor
The grounding electrode conductor connects the grounded (neutral) service conductor and equipment to the grounding electrode system. It is sized from NEC Table 250.66 by service conductor size.
📕 NEC 250.66Main Bonding Jumper
The main bonding jumper bonds the grounded (neutral) bus to the equipment grounding/enclosure at the service disconnect — the single point where neutral and ground are tied together (NEC 250.28).
📕 NEC 250.28Equipment Grounding Conductor
Equipment grounding conductors bond all non-current-carrying metal parts and provide the low-impedance fault-return path so overcurrent devices clear ground faults. Sized from NEC Table 250.122.
📕 NEC 250.122Protection & Power Monitoring
Overcurrent Devices (Breakers)
Circuit breakers protect conductors and equipment from overload and short circuit, and must have an interrupting rating equal to or greater than the available fault current. Governed by NEC Article 240.
📕 NEC Art. 240Fused Disconnect
Fused disconnect switches provide a local disconnecting means and fuse-based overcurrent/short-circuit protection at equipment such as motors, HVAC units, and feeders.
📕 NEC Art. 240 & 404Power Monitoring / Sub-Metering
Power monitoring and sub-metering devices track energy, demand, and power quality at panels and feeders for tenant billing, energy management, and troubleshooting.
📕 NEC 230.82 (metering)Power Factor Correction
Power-factor-correction capacitors supply reactive power locally to raise system power factor, reducing demand charges and freeing capacity. Capacitor installations follow NEC Article 460.
📕 NEC Art. 460Circuits & Connections
The conductor and circuit types that wire the system together — each shown as a colored line in the diagram above.
Service Conductors
The service-entrance conductors that bring utility power to the building’s service disconnecting means (main switchboard). Their size, routing, overcurrent protection, and the number/grouping of disconnects are governed by NEC Article 230.
📕 NEC Art. 230Feeders
Conductors between the service equipment and downstream distribution panels, transformers, and sub-panels. Feeders are sized for the calculated load with feeder overcurrent protection per NEC Articles 215 and 240.
📕 NEC Art. 215Branch Circuits
The final conductors from a panelboard’s overcurrent device to outlets and utilization equipment — lighting, receptacles, and dedicated equipment. Branch-circuit ratings and load limits follow NEC Articles 210 and 220.
📕 NEC Art. 210Emergency / Standby Power
Feeders from the generator and automatic transfer switch to legally-required and optional standby loads. These systems are kept independent and prioritized for life safety per NEC Articles 700–702.
📕 NEC Art. 700–702Grounding & Bonding
The grounding electrode conductor, equipment grounding conductors, and bonding jumpers that establish a low-impedance fault-clearing path and reference the system to earth. Governed by NEC Article 250.
📕 NEC Art. 250Protection & Monitoring
Overcurrent protective devices (breakers and fuses), surge protection, and metering/monitoring that protect conductors and equipment and track power usage. Overcurrent protection is governed by NEC Article 240.
📕 NEC Art. 240