A guide for selecting the right electrical cables.

Introduction

It is essential to follow certain standards and methods for selecting the right cable size for switchgear, process control and automation applications within the electrical switchgear and control panel industry in UAE. It is quintessential to be aware of cable ratings, types, materials, characteristics, and basic cable construction to understand problems related to cable systems.  This not only helps in solving problems related to electrical cable systems but also provides a safe and reliable electrical power system, safeguarding the devices and their operators.

Go Switchgear will guide you through this process with some quick tips and methods to be able to select the right cable size and brief you through the cable types as well.

Note that Dubai electrical switchgear industry follows DIN and BS standards for most of the electrical installations. Other Emirates within UAE follow similar standards, though it is always essential to check with local building companies or authorities such DEWA, ADDC (Abu Dhabi Distribution Company) or AADC (Al-Ain Distribution company). The focus of this article is primarily for low voltage copper conductor cables and wires (Such cables are provided by Go switchgear Dubai- offered here!)

General Principles for cable selection

Cable installation

Cable installation incorporates the type and location of installation of the cables. The cables could be placed outdoors or indoors. Outdoors cables can further be classified ‑ if they are underground or above, and also, on the location of the electrical cabinet. Cable management or protection systems such as conduit, trunking, or cable trays, can also affect the selection of cables.

Cable construction

Cable construction is one of the primary criteria for sorting or classifying cables into different types.

Factors that determine cable construction are:

Insulation

The layer that touches the conductor directly is known as insulation. It plays a vital role in the selection of cables, as it determines the maximum temperature of the cable during operation. There are various insulation materials such as PVC, XLPE, and EPR. PVC has a max operating temperature of 70°C, EPR and XLPE have 90°C. Other factors such as flexibility also play a vital role. EPR is a type of rubber and is more flexible than XLPE. Along with these factors, the most primordial one is cost. For the same length, size, current rating (both short time and continuous current rating) and voltage rating, the cost of XLPE would be much higher than PVC insulated cable.

Note that Go switchgear Dubai, UAE provides PVC flexible cables by Havells, which are heat resistant and flame retardant (HFFR). They have high insulation resistance, high heat resistance and are RoHS compliant.

Conductor and Cores

Conductor materials such as copper and aluminium are given precedence due to their chemical and physical properties. Aluminium conductors have more critical consideration factors than copper conductors.

There are single-core and multi-core conductors available in the Dubai switchgear industry. Single-core cables can have a higher current carrying capacity due to the fact that it can dissipate heat better than multi-core cables. But multicore cables provide the ease of installing multiple conductors at one go, saving time. Moreover, multi-core cables are less susceptible to damage caused by mechanical stresses due to high inrush currents and short circuit currents.

Our webshop provides single core unsheathed cable by Havells in Dubai UAE and SM cables, Malta. The number of cores and conductors finally depends on factors such as type of installation, and current and voltage rating.

Technical Parameters affecting cable Sizing.

After determining installation conditions, environmental conditions, load requirements, cable length and circuit conditions, the next step is to determine the current carrying capacity, followed by voltage drop.

Current-Carrying Capacity Method.

Table 1 shows the current-carrying capacities for PVC single-core unsheathed and unarmoured copper cables at a conductor working temperature of 70°C and ambient temperature of 30°C.

Voltage Drop Method

The voltage drop can be calculated only once the length of the cable is known. The drop is measured in mV/A/m and conveys the potential drop across the running cable in the system, or desired part of the system.

Table 2 shows the max voltage drop for single-core copper wires. r,x, and z are resistance, reactance and impedance respectively.

In the UAE electrical industry, standard practice is to have a maximum voltage drop of 2.5% -8% across the cable run. Please note that a range of values is mentioned since it could be vary based on the Emirate’s local electrical regulations set by organizations such as DEWA, or AADC. For instance, if the supply voltage is 415V, then the voltage drop cannot exceed 10.375V (2.5%).

The following formulas in Box 1 will help in calculating the voltage drop. Here, V­­_max is the max permissible voltage drop in the circuit (V), L is the length of the cable (m), I is the current (A), V_d is the max voltage drop in the circuit (mV/A/m), V_ds is the voltage drop of the selected cable (mV/A/m) and V_t is the total voltage drop in the circuit (V).

Selecting the right values

Table 1 and Table 2 will help you select the right cable size and Buy PVC single-core cables by Havells from goswitchgear.com. After calculating the size using both methods, the one with a higher conductor cross-sectional area or size has to be selected.

Remember that the smaller the cable size, the longer the cable and the higher the voltage drop. If the voltage drop exceeds the max permissible voltage drop (2.5% in this case), then the cable size has to be increased.

Other factors such as, ‘ambient temperature-correction factor’ for PVC cables, and the ‘number of circuits-correction factor’ has to be selected for further optimizing the cable size. The tables provided consider conductor operating temperature at 70°C and the cables are a part of just one operating circuit.  These factors will be discussed in the coming articles of our blog/knowledge centre.

Keep following the Go switchgear blog to stay updated and get more valuable insights to build an efficient low voltage control panel in the UAE.

References.

1. Cable Sizing Guide by Keystone Cable
2. Electrical engineering portal – Five key factors to the correct cable selection and application
3. Selection Of Electrical Power Cables