Power reaches a building at high voltage from the utility, is stepped down by a transformer, enters through the service entrance and main breaker, and is then divided again and again: by the main switchboard into feeders, by transformers into lower voltages and by panelboards into the branch circuits that feed outlets, lights and equipment. Each step has its own protection, so a fault in one place can be cleared without shutting down everything upstream.
Knowing the path helps you understand your building's drawings, talk to electricians and engineers, and make sense of why adding a new load is sometimes easy and sometimes a major project. The details vary between a house, a strip mall and a hospital, but the sequence is the same.
The path at a glance
Think of the system as a tree. The trunk is the utility service, which carries all of the building's power. The main switchboard is where the trunk splits into large branches, called feeders. Each feeder serves a panel, a transformer or a large piece of equipment. Panels split feeders again into many small branch circuits, and those end at the outlets, light fixtures and machines people actually use.
Step by step
Step 1: the utility supply
Utilities move power around a city at medium voltage, typically several thousand volts or more, because higher voltage means less current and smaller, cheaper conductors for the same amount of power. Before it can be used in a building, that voltage is reduced by a utility transformer. For a house it is usually a pole-mounted or small pad-mounted transformer shared with neighbours. A large building may have its own pad-mounted transformer outside or a utility vault inside, and very large sites sometimes take power at medium voltage and own their transformers.
The voltage the building receives depends on its size. Houses get single-phase 120/240 V. Smaller commercial buildings commonly get three-phase 120/208 V. Larger commercial, institutional and industrial buildings in Canada commonly get three-phase 347/600 V, which carries more power with less copper and runs 347 V lighting directly.
Step 2: the service entrance
The service entrance is where utility power becomes the building's power. It includes the service conductors, the revenue meter and the main disconnect, which is a breaker or fused switch that can shut off the whole building. This is also where the electrical system is connected to the grounding electrode, a connection to earth that helps limit voltages during lightning and utility faults, and where the neutral and ground are bonded together. Exactly where the utility's responsibility ends and the owner's begins varies by utility and by type of service, so check your utility's service requirements.
The size of the service, expressed in amperes at a given voltage, is the hard limit on how much power the building can draw at once. When people talk about a building "running out of capacity", this is usually what they mean.
Step 3: the main switchboard and feeders
In anything larger than a house, the service feeds a main switchboard or switchgear: a line-up of steel sections with a main breaker and a set of feeder breakers. Each feeder breaker protects a set of large conductors running to a panel, a motor control centre, a transformer or a large piece of equipment such as a chiller or elevator. Feeders are sized for the load they carry and checked for voltage drop on long runs.
In a 347/600 V building, mechanical equipment such as fans, pumps and chillers usually runs directly at 600 V from a motor control centre, and lighting runs at 347 V from lighting panels. That leaves only the smaller loads that need lower voltage.
Step 4: step-down transformers
Receptacles, computers, small appliances and most office equipment need 120 V. In a 347/600 V building, dry-type transformers in electrical rooms on each floor or zone step 600 V down to 120/208 V. Putting transformers close to the loads keeps the long feeders at the higher voltage, which keeps them smaller. In a building that is served at 120/208 V from the street, this step does not exist.
Step 5: panelboards and branch circuits
A panelboard is the familiar grey box full of breakers. It takes a feeder and divides it into branch circuits, each protected by its own breaker sized to the wire it feeds. A branch circuit might serve a group of receptacles, a run of lights, a single appliance or a piece of equipment. The panel schedule on the door, when it is kept up to date, tells you which breaker feeds what.
Branch circuits are where most of the code's detailed rules apply: how many receptacles per circuit, which circuits need ground fault or arc fault protection, and which loads need dedicated circuits. They are also where most everyday problems show up, such as tripped breakers from overloaded circuits.
Protection at every level
Every step down the tree has its own overcurrent protection: the main breaker, the feeder breakers, the panel main and the branch breakers. Each one protects the conductors and equipment downstream of it. In a well-designed system, these devices are coordinated, so that a fault on a branch circuit trips only the branch breaker and not the feeder or the main. Equipment at each level must also be rated for the fault current that could flow there, which is highest near the service and drops further downstream.
A walk through a typical building
Picture a four-storey office building served at 347/600 V. The utility's pad-mounted transformer sits beside the parking lot. Underground service conductors run to an electrical room on the ground floor, where the meter and main switchboard are. From the switchboard, one feeder goes to a motor control centre for the rooftop units and pumps, another to an emergency transfer switch, and one rises through a vertical shaft to an electrical room on each floor.
On each floor, the riser feeds a 347 V lighting panel and a dry-type transformer, which feeds a 120/208 V panelboard. From that panel, branch circuits run through the ceiling to receptacles in the offices. If a tenant plugs in a space heater and overloads a circuit, the branch breaker trips and the rest of the floor stays on. If they want to add a server room, the question becomes whether the panel, the transformer, the floor feeder and finally the service each have room for it.
What owners and managers should know
- Where the main disconnect is, and who is allowed to operate it.
- Where each electrical room is, and that clear working space in front of panels is kept free of storage.
- Whether you have a current single-line diagram that matches the building.
- Whether panel schedules are accurate after the last renovation.
- How much spare capacity remains at the service and in the main switchboard.
Never open energized equipment yourself. Work on electrical systems is done by licensed electrical contractors under permit, and opening live panels exposes people to shock and arc flash hazards.
How ehZee can help
We design new and upgraded distribution systems and document existing ones, so owners know what they have and how much room is left. See our power distribution design service, or read more about transformers in commercial buildings, panelboards, switchboards and MCCs and 120/208 V vs 347/600 V systems.
Common questions
What is the difference between a feeder and a branch circuit?
A feeder carries power from the switchboard or a distribution panel to another panel or large piece of equipment. A branch circuit is the final circuit from a panel to the outlets, lights or devices it serves.
Why do large buildings use 600 V?
For the same power, a higher voltage needs less current, so conductors and equipment can be smaller. That saves cost on long feeders and large motors, and 347 V lighting can run directly from the same system.
Who owns the transformer outside our building?
Usually the utility, but not always. Some large customers own their transformers. Your utility's service agreement and the single-line diagram will show where ownership changes.