What is a cable?

A cable can be a rope, chain or wire that functions according to its application. It can either connect or tie two points together. In electricity, it is used for conveyance or data transmission. It can also be a loadbearing or non-loadbearing tension to support.

Selecting a cable

Low voltage cable sizing of copper conductor

Proper Sizing of Cables is essential so that the cable can function without halting, under full load with no damage and supply the load with an appropriate voltage and prevent extreme voltage drops.

Firstly, collection of data is necessary which comprises of the material, length, temperature of the cable, and so on.

Step 1: Current-carrying capacity method

• Installation method

It is a suitable reference method for obtaining the current-carrying capacity from the standard reference tables based on the installation method.

• Cable construction

In the sizing of cables, the insulation material of the cable plays a crucial role since it affects the cable’s maximum operating temperature directly. Some extensively used insulation materials include Polyvinyl chloride (PVC), Cross-linked polyethylene (XLPE), ethylene propylene rubber-insulated (EPR), and so on. After receiving the installation method, the current-carrying capacity comes next wherein, to find the correct cable size reference of current carrying capacity, the table is important for the desired current load. According to the standard cables, PVC material has a maximum operating temperature of around 70-degree Celsius than XLPE (cross-linked polyethylene cable) and EPR (ethylene propylene rubber-insulated). The material of the cable assists to predict how well it can function inside the installation environment. If the surrounding is an enclosed area with very little space, then EPR (rubber) is advisable since it is more ductile than other insulation materials. Based on the installation requirement, one can pick between single-core cables or multi-core cables. A single-core cable is capable of spreading heat than a multi-core cable and thus, it has a higher current-carrying capacity but a multi-core cable is faster and less tedious to install.

On the evaluation of the conductor operating temperature, ambient temperature, and ground temperature along with the material of the cable, one can determine the cable size by the above corresponding values to the sizing of cables.

Step 2: Voltage drop

 The length of the cable is necessary to determine the voltage drop. Voltage drop is the loss of electrical potential energy along the cable. It is usually for cables with a voltage rating up to 600 V or 1,000 V. Care must be taken that the voltage drop of the cable run must be under 4% of the supplied voltage. Also, the voltage drop is affected by the cable size. Length of the cable and cable size are inversely proportional whereas the cable length and voltage loss are directly proportional to each other, that is, the smaller the cable size, the longer the length of the cable, the larger the loss in voltage.

Calculation of voltage drop

1. V_max = 4% x supply voltage

Where I = current (A)

L = Length of the cable to be installed (m)

V_max = Maximum permissible voltage drops in the circuit (V)

2. $V_d=\frac{V_{max} x 1,000}{I\times L}$

V_d = Maximum voltage drops in the electrical circuit (mV/A/m)

3. V_ds ≤ V_d

V_ds = Voltage drops of the chosen cable (mV/A/m)

4. $V_t=\frac{V_{ds}\times I\times L}{1,000}$

V_t = Total volt drop in the electrical circuit (V)

Now, comparing the values from step 1, by the current-carrying capacity method and step 2, by the voltage drop method, the highest of the two values is considered for the cable size.

Other key factors

If the desired standard temperature conditions do not prevail, then it is advisable to apply the correction factor for modifying the current load required for overheating control.

Generally, the two correction factors are,

The ambient temperature correction factor

Typically, according to the reference tables, the assumption of a standard ambient temperature of 30 degrees Celsius (free air) or 15 degrees Celsius of ground temperature with 0.5 meters of depth is taken into consideration. The ambient temperature is directly in relation to the routing of the cable and ventilation. Hence, it is vital to take account of the condition of the installation along the full cable’s length. If the deviation from the expected temperature persists, one should apply the correction factor.

Circuits correction factor

The conventional current carrying capacity tables consider only single-phase or three-phase conductors but if circuit grouping in the installation process persists, one should apply the correction factor of the number of circuits for suitable selection of a bigger cable size to avoid overheating problems.

For the application, the current load/correction factor will give the altered current load to utilize instead.

Cable installation

The components essential for a smooth cable installation experience are as follows:

Installation temperature

When we install a cable, lower temperatures affect the cable by reducing its capacity and durability. Thus, one should never install cables with temperatures below the cold bend rating temperature and 15°C in addition to it.

Equipment checklist

The appropriate usage of the right equipment is important for a successful cable installation. The equipment advised for various installations must be chosen for a specific installation.

Open wiring or cable system

NEC (National Electric Code) in U.S. exposed open wiring systems or cable systems are on or fixed to the surface or behind panels designed to permit access. The wiring methods to install cables include metal-clad cable, medium-voltage cable, and messenger-supported wiring.

Cable tray

A cable tray system is a unit or a gathering of units or sections that are linked to fittings creating a structural system used to fasten securely and support cables and raceways.

Underground installations

Underground installations include cables allowed to be directly concealed by the cables or conductors and they need approval to be installed in raceways for direct burial.

Raceways

For the power cables, heat is produced as current flows through the conductor. A raceway is a channel made for catching wires, cables, or busbars and needs to be qualified in the CEC rules. Conductor derating reduces the quantity of current that wires are permitted to carry. The logic behind it is to preserve the insulation of the wire from degradation by extreme heat since current flows through a wire and heat is generated.

$\%Fill=\left(\frac{Total cable cross-sectional area}{Raceway inside area}\right)\times 100$

Cable measurements

The dimensions of the cable come in a variety of diameters suitable for the allowable expansion and area to install the cables.

Physical limitations

Supporting conductors in vertical raceways

Supports must maintain the raceway’s continuity and not break the conductors or their coverings.

Allowable Sidewall Bearing Pressure

Sidewall bearing pressure (SWBP) is the radial force deployed on a cable being pulled around a conduit bend or sheave. Extensive SWBP can smash a cable and thus is the most limited component to install containing bends and higher pulling tension.

Major references for installation specifications

• NEC (National Electrical Code) issued by NFPA (National Fire Protection Association)
• CEC (Canadian Electrical Code) issued by CSA (Canadian Standards Association)
• NEMA (National Electrical Manufacturers Association) and ICEA (Insulated Cable Engineers Association)

Context and Applications

• Bachelors of Technology (Electrical Engineering)
• Masters in Science (Electronics and Telecommunication)
• Masters of Technology (Electrical Engineering)
• Masters in Science (Robotics)

Practice Problems

Q1. What are the steps involved in cable selection and sizing?

1. Current carrying capacity method
2. Voltage Drop
3. Both a and b
4. None of these

Correct Option: c

Explanation: The steps involved in Cable Selection and Sizing are the current carrying capacity method and the voltage drop.

Q2. Which one of the following has a maximum operating temperature of 70-degree Celsius?

1. PVC insulation material
2. XLPE insulation material
3. EPR insulation material
4. AWG

Correct Option: a

Explanation: PVC has a maximum operating temperature of 70-degree Celsius.

Q3. Which of the following statements is correct?

1. The voltage drop of the cable run must be above 4% of the supplied voltage
2. The voltage drop of the cable run must be below 4% of the supplied voltage.
3. The voltage drop of the cable run must be at par 4% of the supplied voltage.
4. The voltage drop of the cable run must be nearly 5% of the supplied voltage.

Correct Option: b

Explanation: The voltage drop of the cable run must be below 4% of the supplied voltage.

Q4. What is the full form of CEC?

1. Canadian Electronics Code
2. Canadian Energy Code
3. Canadian Efficiency Code
4. Canadian Electrical Code

Correct Option: d

Explanation: The full form of CEC is the Canadian Electrical Code.

Q5. What is the SI unit of current?

1. Joule (J)
2. Kelvin (K)
3. Ampere (A)
4. Watt (W)

Correct Option: c

Explanation: The SI unit of current is ampere (A).

Related Concepts

• IEC (International Electrotechnical Commission)
• Wiring substation and resistivity
• AWG (American Wire Gauge)
• Blended Learnings