Overview: Electrical Power Systems

The service is where utility ownership ends and the building's system begins, usually at the meter or the transformer's secondary terminals.

Large buildings and campuses may take service at medium voltage and own their own transformers. They then pay a lower utility rate but take on maintenance of the medium-voltage equipment.

Main distribution splits the service into large feeders, and intermediate equipment carries them to each floor, wing, or major load.

Overcurrent protection at every split is coordinated so a fault trips only the device nearest to it. NEC requires this selective coordination for emergency and legally required standby systems.

The branch panel is the last distribution point; each breaker in it feeds one branch circuit to the endpoints.

Endpoints are where power is used: devices people plug into, fixtures, and equipment wired directly to a circuit.

These systems run alongside the main chain at every level.

• Grounding and bonding. Grounding electrodes, equipment grounding conductors, and bonding of metal piping and structure. They give fault current a safe path so breakers trip.
• Emergency and standby power. A generator feeds loads through automatic transfer switches (ATS). NEC splits these into three systems:
• Article 700, emergency: egress lighting, exit signs, fire alarm
• Article 701, legally required standby: smoke control, some fire pumps and ventilation
• Article 702, optional standby: business-critical loads the owner chooses
• Uninterruptible power supply (UPS). Battery ride-through for IT and controls during the seconds before a generator starts.
• On-site generation and storage. Solar PV, battery energy storage, and inverters. Some sites combine them with a generator into a microgrid that can run without the utility.
• Metering and monitoring. Submeters per tenant or system, power quality meters, and links to the building management system (BMS).
• Surge protection. SPDs at the service and at panels serving sensitive electronics.

A house has the flattest hierarchy: the meter, main breaker, and load center are often one combined unit, and every branch circuit runs straight from that panel to its endpoints.

• Larger homes add a subpanel for a garage, addition, or ADU, fed from a breaker in the main panel.
• Electrification (heat pumps, induction cooking, EV charging) is pushing new services from 200 A toward 320–400 A, or toward load-management devices that avoid the upgrade.

Apartments and condos add a layer between the service and the units: a meter stack gives each unit its own utility meter and feeder, while a separately metered house panel serves everything the building shares.

• Units receive two legs of the three-phase system, so they run at 120/208 V rather than 120/240 V. Appliances must be rated for 208 V.
• High-rise buildings take 480Y/277 V service, run busway risers, and place meter banks and transformers on each floor or every few floors.
• Fire pumps are connected ahead of the main disconnect so a building shutdown does not cut them off.

Offices have the deepest hierarchy of the four types. Power travels at 480 V as far as possible, up a riser to each floor, then a local transformer steps it down to 208Y/120 V for receptacles.

• Lighting runs at 277 V directly from the floor panel, so no transformer is needed for it.
• Electrical rooms stack vertically in the building core, which keeps riser runs short and straight.
• Core-and-shell buildings provide the riser and floor panels; tenant fit-outs add branch circuits, furniture feeds, and any tenant UPS or supplemental cooling.
• Receptacle density, flexible floor power (floor boxes, raised floors, poke-throughs), and data-room backup drive the design.

Retail design is driven by tenancy. A shopping or strip center usually has one service, a meter section with a meter and disconnect for each tenant space, and a house panel for shared site loads.

• Load profiles vary more than in any other type: restaurants and grocery stores draw several times the power per square foot of a clothing store.
• Each tenant space is sized for a range of future uses, since tenants change more often than the building.
• Life-safety lighting is often handled by battery units in each fixture rather than a central generator; grocery stores commonly add a generator to protect refrigeration.
• Freestanding big-box stores have their own service and look more like the office pattern, with 277 V lighting and 480 V mechanical loads.

Schools look like offices at the top of the hierarchy but branch by space type rather than by floor. Specialty rooms (labs, shops, kitchens, gyms, auditoriums) each get dedicated panels with their own safety features.

One 480Y/277 V service feeds distribution panels by wing, plus dedicated panels for the kitchen, gym and stage, and mechanical plant.

• Labs and shop classrooms need emergency shutoffs that a teacher can reach to kill power to the student stations.
• Assembly spaces (gyms, auditoriums, cafeterias) drive larger emergency lighting and fire alarm systems.
• Many districts add solar, batteries, and generators so schools can serve as community shelters during outages.
• Portable classrooms are fed from exterior panels or pedestals tied back to the main building.

Large campuses take medium-voltage service and run their own distribution loop to a unit substation at each building, often alongside a central heating and cooling plant.

• The loop lets campus staff isolate a damaged cable section without cutting power to any building.
• Owning the medium-voltage system lowers utility rates but requires trained staff and maintenance contracts.
• Research buildings and data centers on campus add their own standby generators and UPS systems.