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Sizing Conductors

Within the walls of your office, are miles of copper wires that safely transmit electricity to every light bulb, power socket and piece of electrical equipment. It's important when sizing conductors to remember that these wires and cables will be used for decades to come. The right-sized conductors will ensure a safe and reliable operation of your building's electrical distribution system.

As electrical engineering consultants, the sizing of conductors is explicitly designed for:

Fuses will blow before the conductors pose a fire hazard.

The fuses are designed to blow before the conductors can pose a fire hazard to the building.

  • The continuous load current of 80% of conductor ampacity for 3 or more conductors (CEC Rule 8-104),
  • The normal short-term overcurrents that can be safely handled,
  • The voltage drops to be within specified limits to ensure proper equipment operation (CEC Rule 8-102),
  • The over-current protection devices to operate before the conductor poses a fire hazard.

This is what we do as electrical engineers.

No two buildings are the same - We work with business owners to calculate your immediate electrical needs and your needs well into the future. After understanding your overall electrical needs, we determine the size of the conductors to fit your requirements.

We can save substantial costs when planning your the electrical system for future growth. In some cases, experience has shown that right-sizing the conductors will work just fine.

For example, a dining room in a restaurant does not require many electrical receptacles now or in the future. By right-sizing the branches to this location, we can allocate money to other areas that may expand, like the mechanical HVAC system in the kitchen.

How to Right Size Conductors

Step 1 - Power Demand

What is the overall electrical load requirement? This depends on your specific business needs and the building you're going to occupy. In general, the Canadian Electrical Code makes allowances for basic levels of occupancy (CEC Table 14) and then we customize the commercial space according to your needs and plans.

Determining the overall electrical load is done with a load calculation. The electrical distribution system is then built with the right connections, with the right conductors, in the right locations to match your business operations.

Step 2 - Installation Considerations

The electrical engineers and electricians take care of these details. The type of conductor we specify depends on:

Underground conductors need to be armoured to protect against water and vehicle damage.

Underground cables must either be armoured (shown above) or placed in conduits for protection from water, rodents and vehicle weight.

  • The installation environment,
  • The method of installation,
  • Various electrical correction factors for multiple cables,
  • Required ampacity,
  • Number and size of motors on the circuit.
  • Outdoor installations require sunlight resistant conductors.

This is the engineering that takes place behind the scenes. The conductors are specified on the drawings and installed by the hired electricians.

Step 3 - Voltage Drop

Paralynx Engineering gets called to investigate problems on electrical systems and a lot of times, it turns out to be voltage drop.

Feeder cables can be copper or aluminum.

Feeder cables going to electrical panels can be copper (more expensive) or aluminum (thicker in diameter).

Older buildings were designed before the invention of computers. Modern electronics can have electrical requirements that draw too much current, over too long a distance for the size of conductor. The demand makes the voltage at the far end too low for the equipment to operate. And the worst part is, this is always an intermittent electrical problem.

The Canadian Electrical Code specifies the maximum voltage drop to be 5%. When it comes to motors, the design should consider the number of motors and how they are started. Across-the-line starting is the hardest method and can consume 5-7x the amount of full rated amperage (FLA).

Specifying the size of the conductors will ensure that the motor will not stall and suffer premature failure.

Step 4 - Short-Circuit Analysis

This calculation is a lot of fun for electrical engineers. It's the application of Ohm's law and determines the maximum short-circuit current that can be found at various points in the electrical distribution system.

It starts by assuming the maximum amount of current that can be delivered by the utility company's transformer. Depending on the size and length of the conductors, the equipment needs to be rated to meet these fault loads.

Again, a reliable electrical system works silently and reliabily for decades. And the first priority is safety.

Step 5 - Protection Devices

Conductors are the pathways for the electricity. They are often buried deep within walls, underground or high overhead. When a fault occurs, a circuit breaker will trip or a fuse will blow before the conductor can poses a safety hazard.

Circuit breakers and fuses are known as overcurrent protection devices (OCPD) and are specified to operate as per code. Some are rated for slower trip time. Some are rated to handle more fault current.

It all depends on the main supply, the building and the business layout. For industrial facilities, a coordination study is required to ensure a costly failure can be isolated to the lowest possible level in the electrical distribution system and not affect other areas of the plant.

For commercial buildings like restaurants and stores, experience builds this level of safety and coordination into every design.

Putting It All Together for You

Sizing conductors is an engineering art. It follows a set of rules but also a gut-feel for what may happen in the future. A good design will be reliable and money-saving for years to come.