Rating current-carrying conductors

Properties and ratings of current-carrying conductors 30/919 Table 30.3 Multiplying factors for copper sections... 

These are the basic maximum ratings that a current-carrying conductor can carry under ideal operating conditions. They are influenced by the service conditions and other design considerations, a.s discussed in Section 28.5. Apply suitable derating factors to arrive at the actual current ratings of these conductors under actual operating conditions. 

The ampacity or rated current carrying capacity of the electrical service entrance conductors that connect the utility company s lines to the plant s service entrance equipment must be a minimum of 125% of the calculated maximum demand for continuous loads plus 100% of the maximum calculated demand for non-continuous loads. Service entrance conductors and equipment with higher ratings or provisions to increase the rating of the service entrance conductors are recommended. 

For higher rating systems, say 2500 A and above, sleeving is normally not used. Instead, a non-metallic, semi-glossy black paint may be provided to make the bus conductors act like a black body and dissipate more heat. This will also add to the current-carrying... 

This minimum conductor size will take account of the healing effects only during the fault, irrespective of the current rating of the conductor. The required conductor size may be more than this, depending upon the continuous eurrent it has to carry, as discussed later. 

We can derive the same inference from Tables 30.2, 30.4 and 30.5, specifying current ratings for different cross-sections. The current-carrying capacity varies with the cross-section not in a linear but in an inconsistent way depending upon the cross-section and the number of conductors used in parallel. It is not possible to define accurately the current rating of a conductor through a mathematical expression. This can be established only by laboratory tests. 

As discussed later, the enclosure of an IPB may carry induced currents up to 95% of the current through the main conductors. Accordingly, the enclosure is designed to carry longitudinal parasitic currents up to 90-95% of the rated current of the main busbars. The cross-sectional area of the enclosure is therefore maintained almost equal to and even more than the main conductors to account for the dissipation of heat of the main conductors through the enclosure only, unless an additional forced cooling system is also adopted. The outdoors part of the enclosure exposed to atmospheric conditions is also subjected to solar radiation. Provision must be made to dissipate this additional heat, from the enclosure. 

Properties and current ratings for aluminium and copper conductors Current-carrying capacity of copper and aluminium conductors... 

A similar reduction in rated current occnrs when several conductors are combined in one cable. Single-core cables can carry more current than three or fonr core cables. Vertically run cables carry less cnrrent than those run horizontally by a factor of approximately 5%, dne to the convection of heat given ont by the lower part of the cable. 

HWI Hot-wire ignition Measures resistance to ignition from high temperatures from a device or conductor failure carrying more than its rated current Mean number of seconds to ignite a sample wrapped with a nonchrome resistance wire dissipating a specified level of energy... 

The above examples show that the resistance of an aluminium cable is some 60% greater than a copper conductor of the same length and cross-section. Therefore, if an aluminium cable is to replace a copper cable, the conductor size must be increased to carry the rated current as given by the tables in Appendix 4 of the lET Regulations and Appendix 6 of the On Site Guide. 

Overheating of cables and equipment will occur if they become overloaded. Electrical equipment and circuits are normally rated to carry a given safe current which will keep the temperature rise of the conductors in the circuit or appliance within permissible limits and avoid the possibility of fire. These safe currents define the maximum size of the fuse (the fuse rating) required for the appliance. 

In LT systems, where the neutral is groiintled, the neutral as well as the ground conductor tnay have to carry unbalanced currents up to hall the rating of the line currents due to single-phase loads. A ground conductor should also be rated for the same size as the neutral, irrespective of the setting of the relay. 

Example Determine the current rating factor for a 300-kcmil copper conductor required to carry a nonlinear load with the following harmonic frequency content ... 

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