Conductors, Cables, and Raceways

Conductors, Cables, and Raceways

Introduction

This chapter covers one of the most heavily tested areas on the Red Seal exam for construction electrician(s): conductors, cables, and raceways. You must master conductor types, their ampacities, service temperatures, correction factors, as well as the installation rules for raceways (conduits, wireways, cable trays). Questions cover both theory and practical calculations. An ampacity calculation error or a wrong cable type choice can cost you valuable points.

Fundamental Definitions

Conductor

A conductor is a wire or group of wires intended to carry electric current. In Canada, conductors are manufactured in copper or aluminum, in accordance with CSA C22.2 standards (series of standards for conductors). Copper is the most common material due to its high conductivity and corrosion resistance. Aluminum is used for larger sizes to reduce weight and cost, but it requires special precautions at connections (antioxidant paste, specific torque values).

Cable

A cable is an assembly of insulated conductors, with or without a protective sheath. Cables are classified according to their type (NMD90, NMWU, TECK90, ACWU, etc.) and their intended use. The outer sheath protects the conductors against mechanical damage, moisture, and chemical agents.

Raceway

A raceway is a closed or semi-closed system that contains and protects conductors. Raceways include conduits (EMT, PVC, rigid steel, etc.), wireways, cable trays, and cellular floors. The choice of raceway depends on the installation environment (indoor, outdoor, wet, corrosive, risk of mechanical damage).

Types of Conductors and Cables

Bare and Insulated Conductors

Bare conductors are used for grounding and bonding. Insulated conductors are covered with one or more layers of thermoplastic or thermosetting insulation. Common insulations include:

Single-Conductor and Multi-Conductor Cables

Single-conductor cables (one insulated conductor) are used in conduits. Multi-conductor cables (several conductors under a common sheath) are used for residential, commercial, and industrial installations.

Armoured and Unarmoured Cables

Armoured cables (TECK90, ACWU) have a metallic armour that protects the conductors against mechanical damage. Unarmoured cables (NMD90, NMWU) are used in locations where the risk of damage is low.

Comparison Table of Common Cables

Ampacity of Conductors

Definition

Ampacity is the maximum current a conductor can carry continuously without exceeding its maximum service temperature. Ampacity depends on:

Service Temperatures

The maximum service temperatures of insulations are standardized:

Important: The ampacity of a conductor is determined by the service temperature of its insulation, but connections to equipment terminals are often limited to 75 °C. If the terminal is marked 75 °C, you must use the 75 °C ampacity, even if the insulation is rated 90 °C.

Ampacity Table (excerpt from the Canadian Electrical Code, Part I, Chapter V, Table 2)

Table 2 of the Code gives ampacities for copper and aluminum conductors, at 75 °C, for different installation methods. Here is an excerpt for copper conductors:

Note: These values are for conductors in a conduit, with an ambient temperature of 30 °C. For other temperatures, apply the correction factors from Table 5A.

Correction Factors

Ambient Temperature

If the ambient temperature exceeds 30 °C, the ampacity must be reduced. Use Table 5A of the Code. For example, for an ambient temperature of 40 °C, the correction factor is 0.88 for 75 °C insulation, and 0.91 for 90 °C insulation.

Calculation: Corrected ampacity = Base ampacity × Temperature factor.

Grouping of Conductors

When more than three current-carrying conductors are in the same conduit, the ampacity must be reduced. Use Table 5C of the Code. For example:

Important: The neutral conductor is counted as a current-carrying conductor if it carries unbalanced current (for example, in a three-phase circuit with a shared neutral). In a single-phase 120/240 V circuit, the neutral is considered current-carrying.

Ampacity Calculation: Complete Example

Problem: You must install 6 copper conductors, size 4 AWG, THHN insulated (90 °C), in an EMT conduit, in a location where the ambient temperature is 40 °C. What is the maximum allowable current?

Solution:

Answer: The maximum ampacity is 69 A (rounded down).

Trap: Don't forget that if the equipment terminals are rated 75 °C, you must use the 75 °C ampacity as the base. In our example, the 75 °C ampacity for 4 AWG is 85 A. Corrected: 85 × 0.88 × 0.80 = 59.84 A. The actual ampacity would therefore be 59 A.

Voltage Drop

Principle

Voltage drop is the decrease in voltage between the source and the load, caused by the resistance of the conductor. Excessive voltage drop can cause poor equipment operation. The Canadian Electrical Code, Part I recommends a maximum voltage drop of 3% for lighting and heating circuits, and 5% for motor circuits (Rule 8-200, note).

Calculation Formula

For a single-phase circuit:

Voltage drop (V) = 2 × L × I × R / 1000

Where:

For a three-phase circuit:

Voltage drop (V) = √3 × L × I × R / 1000

Typical Resistances (copper, at 75 °C)

Voltage Drop Calculation Example

Problem: A single-phase 120 V circuit supplies a 15 A load at a distance of 50 m. You use 10 AWG copper conductors. What is the voltage drop in volts and as a percentage?

Solution:

Answer: The voltage drop is 5%, which exceeds the 3% recommendation for lighting. You would need to increase the size to 8 AWG or reduce the distance.

Raceways: Conduits, Wireways, and Cable Trays

Types of Conduits

Conduit Fill

The maximum number of conductors in a conduit is limited to prevent overheating and facilitate pulling. The Code (Rule 12-1014) specifies fill percentages:

Calculation: The total cross-sectional area of the conductors (including insulation) must not exceed the allowable percentage of the interior cross-section of the conduit.

Conduit Dimensions Table (excerpt)

Cable Trays

Cable trays (or cable ladders) are open supports used to support cables in industrial and commercial installations. They allow more efficient heat dissipation than closed conduits. Installation rules are given in Rule 12-2200 and following.

Advantages:

Disadvantages:

Conductor Installation Rules

Rule 12-100: Scope

Rule 12-100 establishes the general requirements for the installation of conductors and raceways. Conductors must be installed in raceways or cables, except for specific exceptions (grounding, bare conductors, etc.).

Rule 12-108: Continuity of Raceways

Raceways must be continuous between boxes, devices, or panels. Joints must be mechanically strong and electrically continuous.

Rule 12-110: Supports for Raceways

Raceways must be supported at regular intervals to avoid mechanical stress. Maximum distances between supports are specified in Table 12-110:

Rule 12-140: Identification of Conductors

Conductors must be identified by their colour or by markings. The grounding conductor must be green or green with a yellow stripe. The neutral conductor must be white or grey. Phase conductors can be black, red, blue, etc., depending on the system.

Rule 12-3000: Cables

Cables must be installed in accordance with the specific rules for their type. For example, NMD90 cables must be supported at maximum intervals of 1.4 m (Rule 12-510). TECK90 cables can be installed in wet locations or exposed to mechanical damage (Rule 12-604).

Minimum Conductor Size Calculations

General Method

The minimum size of a conductor is determined by:

Complete Example

Problem: A three-phase 10 HP, 208 V motor has a full-load current of 30 A. The circuit is 40 m long. The conductors are copper, THHN insulated, in an EMT conduit, with an ambient temperature of 35 °C. Determine the minimum conductor size.

Solution:

Answer: Use 8 AWG copper, THHN conductors.

Traps to Avoid

Summary

Traps to Avoid (Reminder)

Exam Tips

This chapter gives you a solid foundation to tackle questions on conductors, cables, and raceways on the Red Seal exam. Review the Code tables, practice the calculations, and stay methodical in your answers.