Electrical power distribution and standby power

Almost everything in a modern building depends on electricity: lighting, HVAC fans and pumps, elevators, computers, security, fire alarm and, increasingly, heating and vehicle charging as buildings electrify. The electrical distribution system takes power from the utility, transforms it to usable voltages, divides it safely into thousands of circuits and protects people and equipment from faults. Standby systems keep critical loads running when the utility fails.

This lesson explains how the system is arranged, how voltage, current and power relate, what drives the size and cost of electrical equipment, and what the constructor must coordinate. Electrical hazards on site, approach boundaries and arc flash protection are covered in HSE-214; this lesson focuses on the permanent building system.

From the utility to the outlet

Power flows through a series of components, shown on the single-line diagram, the electrical engineer's map of the system.

• Utility service: the utility's supply, either at a usable low voltage from a pad-mounted or vault transformer, or at medium voltage (for example around 13.8 kV or 27.6 kV) that the building transforms itself.
• Service entrance and metering: the point where the utility's responsibility ends, with revenue metering.
• Main switchgear or switchboard: the main disconnect and overcurrent protection, and feeder breakers to the rest of the building.
• Distribution transformers: step down voltage for lighting and receptacle loads.
• Panelboards: smaller enclosures with breakers that feed branch circuits.
• Feeders and branch circuits: conductors that carry power to panels and from panels to devices and equipment.

Common Canadian commercial voltages are 347/600 V three-phase, with 347 V used for lighting and 600 V for motors and large equipment, and 120/208 V three-phase for receptacles and small loads, obtained through step-down transformers. Houses and small buildings use 120/240 V single-phase.

Voltage, current and power

Voltage (volts) is electrical pressure, current (amperes) is flow, and power (watts) is the rate of energy use. For resistive loads, power is voltage times current. For motors and electronic loads, current and voltage are not perfectly in step, so real power is reduced by the power factor (pf), and equipment such as transformers and generators is rated in kilovolt-amperes (kVA). Three-phase systems deliver power more efficiently and are used for all but small buildings.

The higher distribution voltage means less current for the same power, so conductors and equipment are smaller. A 600 V system carries 25 per cent more power than a 480 V system at the same current, which is one reason Canadian equipment and conductor sizes differ from US practice.

Transformers

Dry-type step-down transformers inside the building convert the main distribution voltage to 120/208 V for receptacles and small equipment. They are rated in kVA and come in standard sizes such as 30, 45, 75, 112.5, 150, 225 and 300 kVA. Energy efficiency standards apply to them in both countries. Transformers are heavy, produce heat that the HVAC system must remove and hum, so they need ventilated rooms, structural support and attention to noise near quiet spaces.

Overcurrent protection and conductors

Circuit breakers and fuses protect conductors from overheating due to overloads and short circuits. Each conductor's ampacity, the current it can carry continuously without overheating, depends on its size, material (copper or aluminium), insulation temperature rating and installation conditions such as how many conductors share a raceway and the ambient temperature.

In Canada, conductor ampacities come from the tables in Section 4 of the Canadian Electrical Code (such as Table 2 for copper), with correction factors. Loads that run for three hours or more are treated as continuous, and in most cases a breaker may be loaded to only 80 per cent of its rating (Rule 8-104).

Wiring methods include conductors in metallic or PVC conduit, armoured cable and busway. Canadian projects often use Teck90 armoured cable for industrial feeders and AC90 (BX) cable for branch circuits; US projects commonly use THHN conductors in EMT conduit and MC cable. Each method has code limits on where it may be used, especially in fire-rated assemblies, plenums and hazardous locations.

Selective coordination means arranging protective devices so that only the device nearest a fault trips, leaving the rest of the building powered. Codes require it for emergency systems, elevators and fire pumps in many cases, and the coordination study affects which breakers are bought.

Voltage drop

Long conductors lose voltage through resistance, and equipment at the end may run poorly. Designers keep voltage drop within recommended limits.

The Canadian Electrical Code (Rule 8-102) limits voltage drop to 5 per cent from the supply side of the consumer's service to the point of use, and to 3 per cent in a feeder or branch circuit.

Grounding and bonding

Grounding connects the electrical system to the earth to stabilize voltage and dissipate surges such as lightning. Bonding connects all metal parts that could become energized, such as conduit, enclosures, piping and structural steel, so that a fault produces enough current to trip the breaker quickly instead of leaving exposed metal at a dangerous voltage. Ground fault protection devices protect people in wet locations, and large services need ground fault protection of equipment. Grounding electrodes, such as ground rods, building steel and concrete-encased electrodes (Ufer grounds), must often be installed during foundation work, which is easy to miss.

Emergency and standby power

When utility power fails, some loads must keep running. Codes distinguish levels of priority.

In Canada, the National Building Code requires emergency power for life safety loads such as exit and emergency lighting, fire alarm and, in high buildings, fire pumps, elevators and smoke control. Emergency generators are designed to CSA C282 Emergency electrical power supply for buildings, and Section 46 of the Canadian Electrical Code covers emergency systems and unit equipment (battery packs). Optional standby power for business continuity follows the owner's requirements.

Automatic transfer switches (ATS) sense the loss of utility power, signal the generator to start, and transfer the load once the generator is stable. Emergency loads are kept on separate panels and wiring, often with fire-rated protection for feeders. Uninterruptible power supplies (UPS) use batteries to carry critical electronic loads without any break, bridging the seconds before a generator picks up.

Electrification and new loads

Electrical services are growing as buildings switch from gas to heat pumps, add electric vehicle charging and install battery storage and rooftop solar. Many codes and municipalities now require EV-ready parking spaces, and utilities may need months or years to upgrade a service. The constructor should confirm utility capacity, application timelines and any service upgrade costs early, because they can control the schedule.