Whichever cable is selected, it must be capable of continuously carrying the currents required by the loads. This is known as the full thermal current rating of a cable. Incorrect cable specification can result in prolonged operation beyond its rated design temperature, which can lead to premature failure or, in the worst case, a potential fire hazard.
The current rating capacity of a cable is determined by calculating the maximum current (Iz) required and the AC supply rating. This calculation is based on several factors including the design current (Ib) of the circuit and the maximum input rating of the UPS to be installed, the rating of each of the protection devices (In), and the rating of the cable (It). The cable rating is the value of the current for a particular conductor located within a specific environment.
The current-carrying capacity of unshielded multicore cables (thermoplastic or thermosetting PVC insulation) can be affected by location. Current carrying capacity is reduced under load due to less heat dissipation. For example, a three or four core 10mm cable enclosed within an insulated wall, the maximum current carrying capacity is reduced to 39A. In free air, the current carrying capacity would increase to 60A because the heat from the cable dissipates more easily.
It is also important to calculate the voltage drop based on the length of the cable and the amount of current passing through it at full operating load. The greater the load or the distance, the greater the voltage drop. Wiring regulation standards (in the UK BS EN 7671 IEE Wiring Regulations 17th Edition) around the world specify the requirements that must be met for acceptable levels of voltage drop etc.
Other factors to consider when sizing and installing cables in uninterruptible power supply installations are ground fault loop impedance; operating current (Ia); room temperature; thermal insulation (Ci) and grouping (Cg).
Ground Fault Loop Impedance
As part of the electrical installation of an uninterruptible power supply system, it is important to calculate this value since it must comply with regulations. An acceptable value will also ensure that the necessary fault current is generated in the event of a fault condition or short circuit. Once the fault current level has been calculated, it can be compared to the time curve of the protection device to ensure it will trip properly if required.
Operating current (Ia)
The operate current is the current required to trip the chosen protection device within the required operate time during a fault condition.
Ambient Temperature (Ca)
As already highlighted, the ambient temperature around the installed cables must be taken into account to ensure, during operation, that they do not overheat and can dissipate heat effectively. This is especially important where there are high ambient temperatures and/or direct heat sources such as radiators or heaters.
Thermal Insulation (Ci)
When cables are installed in an insulated location, that insulation will prevent the cable from dissipating heat effectively and that must be considered in any electrical installation.
Grouping (Cg)
Heat dissipation will also be compromised when multiple cables are bundled together in one location. It is inevitable, particularly in new or specially built installations, that there will be a need for this to happen. Provided all of the above are taken into account and accurate calculations are made, pooling can be carried out in many installations.
This article was compiled from information available in The Power Protection Guide: The Design, Installation, and Operation of Uninterruptible Power Supplies (ISBN: 9 780955 442803). By Robin Koffler and Jason Yates of Riello UPS.