Feeders and services

In residential work (ELEC-130), the electrician typically installs one service, one panel and a set of branch circuits. Commercial and industrial buildings are larger and more varied: a utility transformer or customer-owned substation feeds switchboards, which feed distribution panels, motor control centres and transformers through feeders, which in turn feed branch circuit panels throughout the building. This course applies the code to commercial and industrial installations. This first lesson covers calculating loads for non-dwelling occupancies, sizing services and feeders, continuous loads, voltage drop, neutral sizing, feeder taps, parallel conductors and service equipment requirements, including ground fault protection. The section at the end of this lesson shows how the twelve lessons fit together.

References in this course are to the Canadian Electrical Code, Part I (CSA C22.1:24, the 26th edition), as adopted with amendments by each province and territory. Always check the edition and local amendments in force where you work.

The distribution system

A typical commercial or industrial distribution system includes:

• The service: the conductors and equipment that deliver energy from the utility to the building's wiring, ending at the service disconnecting means.
• Main switchboard or switchgear: the service disconnect, metering and overcurrent devices for the feeders.
• Feeders: circuit conductors between the service equipment (or a separately derived system) and the final branch circuit overcurrent devices.
• Distribution and branch circuit panelboards, motor control centres, and dry-type transformers that step down voltage for lighting and receptacles.
• Branch circuits: the conductors between the final overcurrent device and the outlets.

Common Canadian commercial systems are 600Y/347 V three-phase four-wire, with 347 V lighting and 600 V motors, plus 120/208 V from step-down transformers for receptacles; smaller buildings use 120/208 V services directly. Industrial plants may take service at 4.16 kV, 13.8 kV or 25 kV.

Calculated loads for non-dwelling occupancies

For occupancies other than dwellings, Rule 8-210 sets the minimum calculated load using Table 14, which gives a basic load in watts per square metre of floor area for each type of occupancy (for example, 50 W/m² for office buildings), with demand factors for larger areas, plus the actual loads of heating, air conditioning and other equipment with the demand factors permitted in Section 8. Where the actual connected load is known to be larger, it must be used.

Continuous loads

A continuous load is one where the maximum current is expected to continue for three hours or more, such as lighting in a store, office HVAC or a data centre load. Because overcurrent devices and their terminations heat up under sustained current, the code limits their continuous loading:

Rule 8-104 limits the calculated continuous load on a consumer's service, feeder or branch circuit to 80% of the rating of the overcurrent device and of the conductor ampacity, unless the equipment is marked for continuous operation at 100%, in which case 100% is permitted (with conductors sized to the 75 °C column, or 90 °C where permitted). Equivalently, a continuous load requires a rating of at least 125% of the load.

Voltage drop

Conductors that are adequate for heating may still cause excessive voltage drop on long feeders, starving motors and lighting of voltage. For a three-phase circuit:

where \(R_L\) is the resistance of the conductor length (ohms per unit length times one-way length).

Rule 8-102 limits voltage drop to 3% in a feeder or branch circuit and 5% overall from the supply side of the consumer's service to the point of utilization, measured at the calculated demand load (or 80% of the overcurrent device rating where the load is not known).

Neutral sizing and harmonics

In a three-phase four-wire system with balanced linear loads, the neutral carries little current, and the code allows the neutral to be sized for the maximum unbalanced load, with a demand reduction for the portion above a threshold. However, nonlinear loads such as LED drivers, computer power supplies and variable-frequency drives produce triplen harmonic currents (3rd, 9th, ...) that add in the neutral rather than cancelling (ELEC-210), so the neutral may carry more current than the phases.

Rule 4-024 permits the neutral to be sized for the maximum unbalanced load, with a reduction permitted on the portion above 200 A, but not for neutrals carrying nonlinear loads (electric-discharge lighting, data processing equipment and similar), which must be sized for the full load. A neutral carrying harmonic currents must also be counted as a current-carrying conductor when applying the ampacity correction factors of Section 4.

Feeder taps

A tap is a conductor with an ampacity smaller than the rating of the overcurrent device protecting the feeder it connects to. The code permits short taps without overcurrent protection at the point of connection, provided they are short, protected from damage and end at an overcurrent device:

Rule 14-100 allows tap conductors from a feeder without overcurrent protection at the tap under conditions including: a length not exceeding 3 m with ampacity not less than the calculated load and terminating in a single overcurrent device or set of fuses not exceeding the tap ampacity; and a length not exceeding 7.5 m with ampacity at least one-third of the feeder overcurrent device rating, suitably protected from mechanical damage and terminating in a single overcurrent device. Longer taps are permitted in high-bay industrial buildings under supervision conditions.

Parallel conductors

Large feeders and services carry currents too large for a single conductor per phase, so several conductors are run in parallel. To share current equally they must be the same length, conductor material, size, insulation type and termination, and each parallel set in separate raceways must contain all phases and the neutral, to avoid induced heating in metal raceways.

Rule 12-108 permits conductors of size 1/0 AWG and larger to be run in parallel under these conditions.

Service equipment and ground fault protection

Large solidly grounded wye services can have arcing ground faults that draw too little current to trip the service overcurrent device quickly but release enough energy to destroy switchgear and start fires. Ground fault protection (GFP) senses the vector sum of the phase and neutral currents and trips the service disconnect when ground fault current exceeds its setting.

Rule 14-102 requires ground fault protection for each consumer's service disconnecting means rated 1000 A or more on a solidly grounded system with more than 150 V to ground and not more than 750 V phase-to-phase (such as 600Y/347 V), with a maximum setting of 1200 A and an operating time of not more than 1 s for currents of 3000 A or more. It is also required for disconnects rated 2000 A or more at lower voltages in some cases. Fire pumps and some continuous processes are exempt, because an unplanned shutdown would be more dangerous.

Service equipment must also be rated for the available fault current at its line terminals, marked with that value, and installed with the required working space in front of it (ELEC-130). In large services, the utility's available fault current can exceed 50 kA, requiring equipment and series ratings chosen accordingly.

How this course is organized

The twelve lessons cover the code requirements that set commercial and industrial work apart:

• Lesson 1 (this lesson): loads, feeders, services, voltage drop, taps, parallel conductors and ground fault protection.
• Lesson 2: wiring methods: raceways, cables, conduit and box fill, cable trays, busways and firestopping.
• Lesson 3: motor circuits, protection, disconnects, controls and drives.
• Lesson 4: overcurrent protection, available fault current, equipment ratings, transformers and arc flash.
• Lesson 5: hazardous locations.
• Lesson 6: grounding and bonding in commercial and industrial systems.
• Lesson 7: special occupancies and equipment: health care, pools, EV charging, solar and storage, fire pumps and IT rooms.
• Lesson 8: industrial control panels, machine safety and lockout.
• Lesson 9: emergency and standby power systems.
• Lesson 10: fire alarm, communication and low-energy systems.
• Lesson 11: commercial lighting, controls and energy codes.
• Lesson 12: permits, inspection, testing and a commercial design project.