Cable Sizes

MaintenanceCircleTeam

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Maintenance

December 14, 2008

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: CABLE SIZES

One of the most important component in transmitting the power, signal or data is cable. Even though there are thousands of varieties of cables available for various purposes, in this edition let us try to understand how a cable size is calculated & determined for electrical power distribution. We will share a series of articles in further editions on more specific details of cables & its properties. Readers are encouraged to contribute. In any manufacturing setup, the only method of supplying power to a machine is thru an insulated electrical cable. Aluminum and copper are the most widely used conducting materials for electrical power transmission & distribution. The choice of conductor depends on cost, design requirement, machine manufacturer’s specification & environmental conditions prevailing in any site. For example, copper is widely used in Europe & America whereas aluminum is widely used in India & many other Asian countries, mostly due to their availability volumes & cost involved. Cable size is one of the most important parameters to be properly selected for any given application. As we are all aware from basics of electrical engineering, the current (which directly means electrical load, generally rated in Kilowatts) carrying capacity of any conductor is directly proportional – higher the cross-sectional area, higher the current carrying capacity – to its cross-sectional area. Of course, there are many other parameters like harmonics, voltage drop, distance, insulation resistance, working conditions that also decide the cable size. But in this article, let us discuss very briefly about cable size as an independent parameter. SWG (Standard Wire Gauge) / AWG (American Wire Gauge) are the most common used terminologies when cable size needs to be mentioned. Even though each standard identifies cable size with different number system, the basic dimension used for cable size is the circular cross-sectional area of the conductor material. Two important points need to be understood when selecting proper size of power cables. i. Multi & Single strand (or wire) cables – This terminology refers to number of conductors that are “combined” together to form one “core.” A single strand cable will have only one conductor forming a core. Multi strand cable will have more than one conductor forming “core.” Almost all power conductor & heavier cables are multi stranded where as panel and control wiring cables can be single stranded type.

ii. Single & Multi core cables – This terminology refers to number of independently insulated conductors that are carried “within” another external insulator. For example, the most commonly used electrical cable will have four cores for R, Y, B (three phases) & N (neutral) system. Cables for communication & telephone lines can have upto one thousand cores within a single system. Number of cores depends on the purpose for which cable is manufactured. All domestic wirings – lighting, fan – are generally done using single core cables. So, it is possible to have cables of following combinations a) Single strand Single core b) Single strand Multi core c) Multi strand Single core and d) Multi strand Multi core. We will discuss more details about this in further articles. Now, let us take an example to understand how cable size is calculated. Example-1: 3 ½ Core 7-strand 11,000V insulation tested armored PVC Aluminum 200 Sq.mm. power conductor cable (meant for carrying 415V AC voltage in a typical factory) Here the cable size is 200 square mili meters. 3 ½ means R, Y & B phases will have conductor with 200 Sq.mm. cross-sectional area and neutral (N) will have HALF this value, i.e.,100 Sq.mm. Each of these refers to one “core” and each “core” has seven strands within it. Refer to figure-1 for illustration.

This document contains information for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam

Page 2

December 14, 2008

Each of the phases – R Y B – has seven conductors forming one core. The above mentioned cross-sectional area of 200 Sq.mm. is TOTAL sum of seven conductors and NOT the overall area of conductor which also includes insulator. So, the area of each conductor is therefore equal to 200 / 7 = 28.58 Sq.mm. From the basics of geometry, we know that the area of a circle, Overall PVC A = π x d2 / 4, where d is the circular diameter

insulation

Armors – to give additional strength to the cable

7-strand multi core / multi strand power cable

Replacing the values, 28.58 = π x d2 / 4 which when re-converted will give the value of diameter of individual phase conductor, d = 6 millimeter or 0.6 centimeters. Therefore, the diameter of neutral conductor will be half this value, 3 millimeter or 0.3 centimeters. Example-2: Same as above, except that it has a single core conductor. So we need to calculate the diameter of only ONE conductor. 200 = π x d2 / 4, when reconverted will give the value of diameter of one conductor as 16 millimeter or 1.6 centimeters. Subsequently, the diameter of neutral conductor will be 8 millimeter or 0.8 centimeters.

From above calculations, you can note that the diameter values are different for single and multi core cables. An ordinary vernier caliper or inside micrometer can be used to measure the diameter of individual conductor. Due to mass production process involved in conductors, the cross section may not be perfectly circular. It is an usual practice in maintenance department to “approximately” judge the size of cable from past experience or some abstract method, which should be avoided as much as possible. For any given load, the cross-section area of a copper conductor will be smaller – approximately by 30 to 40% - than an equivalent aluminum conductor due to its superior properties including higher conductivity. The insulation composition and overall cable diameter may vary based on the application & hence should not be taken as a bench mark for arriving at “actual” cable area. Single & multi strand cables have their own advantages & disadvantages. This and many other interesting details will be elaborated in future articles. PVC or Poly Vinyl Chloride is the most commonly used insulating material for covering the conductor core. XLPE or Cross-Linked Poly Ethylene is also becoming equally popular. The insulation value – 11000 volts – mentioned in the example signifies cables resistance to high voltage and shows its ability to withstand any surge voltages. Armored (or unarmored) cable has small strips of aluminum / steel / other metal running around the external periphery of all the cores to give it an additional mechanical strength for withstanding external forces viz., digging & heavy load. Revisiting the widely used standards, AWG or SWG for cable size denomination, G or gauge indirectly indicates the diameter of each conductor in a cable. Higher the gauge value, lesser will be the conductor diameter and vice-versa.

This document contains information for reference only. We assume no responsibility for its implication.