Cable size selection is based on three main factors: Current carrying capacity. Voltage regulation. Short circuit rating.
The guiding principle in choosing the right cable size is how well your cable can carry the required current load in your installation environment, without causing excessive voltage drop from your supply voltage.
For cables both in air and in soil; conductor size, conductor material, circuit grouping, duct size and duct material affect current rating.
Electrical Considerations
The dominant factor in determining a safe and efficient wire size is ampacity, the amount of current the wire can carry. The second most common consideration is voltage drop, or the amount of voltage reduced due to losses.
Insulation type, ambient temperature, and conductor bundling are three primary factors in determining how big a conductor has to be for it to safely carry the current imposed on it.
Reduction in Cable Sizing
By improving the power factor, less current is required, which means smaller conductors. A factor of 0.5 could require cross-sectional cable sizing twice that of a power factor of 1.0.
In this paper the effects on ampacity of conductor size, ambient temperature, bonding arrangement, duct size, soil thermal resistivity, resistivity and size of backfill (or duct bank) and depth of installation for underground installations are presented.
Cable Factors
Single core PVC insulated cables in straight runs of conduit exceeding 3m in length OR in runs of any length incorporating bends or sets.
Over size cables can in some instances cause problems. In so far as they can reduce the resistance/impedance to such an extent, that the Prospective Fault Current can be higher than the breaking capacity of equipment connected to the circuit. The chances of such being an issue in a domestic installation, are slim.
The larger the cross-section of a wire, the less its resistance. Also, the larger the cross-section, the greater the amount of current (amperage) the wire can safely carry before overheating. A wire with a smaller gauge (larger diameter) can carry more power than one with a larger gauge.
A common way for referencing a cable size is its “gauge.” The American Wire Gauge (AWG) is used as a standard method of denoting wire diameter, measuring the diameter of the conductor - measured as only the bare wire with the insulation removed. AWG is sometimes also known as Brown and Sharpe (B&S) Wire Gauge.
The grouping factors are based on the assumption that all cables in a group are carrying rated current. If a cable is expected to carry no more than 30% of its grouped rated current, it can be ignored when calculating the group rating factor.
The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed. The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
The k-factor is a multiplier representing the DC resistance for a given size conductor 1,000 feet long and operating at 75°C. From this information, the code user can find either the minimum size conductor needed to carry the load (measured in circular mils or kcmils), and/or the percentage of voltage drop.
The three factors that Lectromec recommends are the current carrying capacity, signal degradation, and voltage rating. For current carrying capacity, there is already a wealth of information on recommended maximum current ratings for different gauge wires; Lectromec has a couple of articles already on this.
The factors on which the strength of the electric current flowing in a given conductor depends are its resistance and the potential difference across its ends.
As a rule of thumb, when the wire length is increased, the wire gauge must be increased. Likewise, the lower your voltage, the larger your wire gauge must be. In any case, calculate the voltage loss and make your wire selection based on your findings.
So, the cable's diameter is to be sized as per the requirements. It must be ensured to provide a load with a suitable voltage i.e. with minimum voltage drop. Cable with a small diameter will have a higher resistance. Also, it will cause more voltage drop across the cable.
The smaller the wire diameter, the higher the resistance there will be to the flow of energy. When you have high resistance, you generate heat that can turn into a fire hazard. Proper wire size is important for any electrical wire installation.
The easiest way to determine cable size is by finding the American Wire Gauge (AWG) on your cable. AWG is the standard for measuring cable size in North America. 40 AWG has the tiniest diameter, 0.0031 inches, while 0000 AWG has the biggest possible diameter, 0.46 inches.
The temperature rating of the cable is a determining factor in the current carrying capacity of the cable. The maximum temperature rating for the cable is essentially determined by the insulation material.
Measuring. In commerce, the sizes of wire are estimated by devices, also called gauges, which consist of plates of circular or oblong form having notches of different widths around their edges to receive wire and sheet metals of different thicknesses.