How Size Cable for UPS

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How size cable for UPS For many prospective power engineers especially involved in the business of alternative power generation primarily involving inverters, UPS and related technologies, solutions design is usually one of the major challenges. Often times they have an inverter that is adequately rated to take care of a particular load (in terms of rating) however, due to wrong cable section, the entire installation is a mess within the shortest possible period you can think of. A simple and safe approximation method of obtaining the right cable size is illustrated below: Whatever cable is selected it must be capable of continually carrying the currents required by the loads. This is referred to as a cable's full thermal current rating. Incorrect specification of cables can result in prolonged operation beyond its designed temperature rating, which can cause premature failure, or at worst, a potential fire risk.

he current calculating capability of a cable is determined by calculating the maximum current (Iz) required and the ac supply rating. This calculation is based upon several factors including the design current (Ib) of the circuit and the maximum input rating of the UPS that is to be installed, the nominal rating of each of the protective devices (In) and the cable rating (It). Cable rating is the value of current for a particular conductor located within a specific environment. The current carrying capability of non-armoured, multi-core cables (with thermoplastic PVC or thermosetting insulation) can be affected by location. Current carrying capability reduces under load due to reduced heat dissipation. For example, a three or four-core 10mm cable enclosed within an insulated wall, the maximum current carrying capability is reduced to 39A. In free air, current carrying capability would increase to 60A because heat from the cable is more easily dissipated. It is also important to calculate voltage drop based on the length of the cable and quantity of current passing through it at full operating load. The greater the load or distance, the greater the voltage drop. Wiring regulation standards (in the UK BS EN 7671 IEE Wiring Regulations 17th Edition) throughout the world specify requirements that must be met for acceptable levels of voltage drop and so forth. Other factors to consider when sizing and installing cables in uninterruptible power supply installations are Earth fault loop impedance; operating current (Ia); ambient temperature; thermal insulation (Ci) and grouping (Cg).

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Earth Fault Loop Impedance As part of the electrical installation of an uninterruptible power supply it is important to calculate this value as it must satisfy regulations. An acceptable value will also ensure the necessary fault current will be generated should a fault condition or short-circuit occur. Once the level of fault current has been calculated, it can be checked against the time curve of the protective device to ensure it will trip appropriately if necessary.

Operating Current (Ia) Operating current is the current required to trip the chosen protective device within the required operating time during a fault condition. Ambient Temperature (Ca) As already highlighted, ambient temperature around the installed cables must be considered 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 restrict the cable from effectively dissipating heat and that should be borne in mind in any electrical installation. Grouping (Cg) Heat dissipation will also be compromised when several cables are grouped together within one location. It is inevitable, particularly in new or purpose-built installations, that there is a necessity for this to happen. As long as all the above are taken into consideration and accurate calculations made, grouping can be put into effect in many installations. Case Study: Consider an installation that involves a Powerware 40kVA 9355(3-phase) UPS rated at 432VDC. Calculate the cable sizes required to install the UPS assuming the UPS will be operating at full load for a backup time of 12 hours. Solution: UPS rating; 40,000VA UPS DC rating; 432V - 36 batteries

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Battery cable Selection: Maximum DC current(current drawn from battery) will be as calculated below: Recall, Power = Current x Voltage' Then, Current = Power / Voltage Therefore, Battery Current = UPS Power Rating / UPS DC Voltage rating Battery Current = 40000VA / 432V = 92.59A Battery cable size selector guide(safe assumption) 1mm sq => 4A From the above assumption, the cable size that can accomodate 92.59A will be 92.59 / 4 = 23.15mm. The minimum cable size that can be used for such an application( from battery to UPS should be 25mm sq. However, to be on a safer side, 35mm sq. can be used. UPS Output Cable selection: Since the output of the UPS is rated at 40kVA, and the output will be available as three phase, then the power per phase will be Power per phase = 40000VA / 3 = 13333.333VA Phase voltage = 230V Therefore, phase current = 13333.333VA / 230V = 57.97A At 1mm sq. => 4A, Minimum cable size to accommodate 57.97 A will be 57.97 / 4 = 14.49mm sq. 16mm sq. will be the recommended minimum size UPS input cable selection: Recall, UPS output rating is 40kVA. However, the input power will usually be greater Let’s join



than the output power. This is because charging power and overloads have to be considered. A safe assumption is 30% extra on the output power. To this end, the input power will be about Input power = 130% of 40kVA = 52kVA Therefore, input phase power will be 52kVA / 3 = 17333.333VA Input phase current will be 17333.333VA / 230V = 75.36A At 4A per mm sq., cable size will be 75.36 / 4 = 18.84mm sq. That is, minimum cable size required will be 18.84mm sq(25mm sq. will be ideal) Hence, cables required are: Input cable - 25mm sq. Output cable - 16mm sq. Battery cable - 25 - 35mm sq. For single phase UPS systems, the only difference is that you don't need to divide the power by 3.

References: 1. by: Jason 2. by Phlow!

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How Size Cable for UPS

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