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When sizing a load for power protection and an appropriate power solution it is important to include any critical and essential loads that could be required to keep a system running. This includes small modems, routers and hubs that are sometimes simply plugged into the nearest wall or available extension lead socket, which are required to keep a network running during a mains power supply failure or can damaged by spikes, lightning transients and electrical noise.
When sizing a load we are also looking for ‘Right-sizing’ in terms of calculating or measuring both the peak (start-up / in-rush) and normal running loads, and then building in a factor to cater for future expansion. Rear rating plate labels and manuals can provide an indication of the total power drawn by a load. Website-based load advisers can also provide further information but often the power ratings given can be larger than the true normal running values. Power ratings may also be given in either Amps or Watts which can further complicate the calculations.
If there is any doubt we recommend a Site Power Survey by one of our Power Projects team. Using their experience and measurement tools including power analysers, RMS volt-meters and current clamps they can easily identify the right information to size a power protection system with, including supply voltage, frequency and site stability with respect to power problems.
Whilst the majority of power solutions installed by EcoPowerSupplies are uninterruptible power supplies and standby power solutions including DC power and generators, the following calculations can be used for any power protection system. Care should also be taken when assessing the load type. The majority of installations are to protect PCs, file servers, server racks and telecoms applications. Like most modern electronic-based devices, their built-in power supply will be a Switch Mode Power Supply (SMPS) with capacitive load characteristics. Other types of load which can affect the sizing of a power protection solution include inductive (transformers and motors) and resistive (heaters and load banks).
Most power protection systems are sized in VA (kVA or even MVA) including uninterruptible power supplies, inverters (including solar PV and wind installations), power conditioners, automatic voltage stabilisers, filters and filter strips and power distribution units. Total Amps or Watts (or kW, MW) may be used for DC standby power solutions. kVA or kW may be used for generators.
Summary calculations table – please see the explanations below.
|Apparent Power||Capacitive||VA, kVA, MVA||Volts (V) x Amps (A)|
|Real Power||Capacitive||W, kW, MW||Apparent Power (VA) x Power Factor (pF)|
|Real Power||Inductive||W, kW, MW||Apparent Power (VA) x Power Factor (pF)|
|Real Power||Resistive||W, kW, MW||Amps (A) x Volts (V)|
VA or kVA measures the Apparent Power drawn by an electrical load. VA is calculated by multiplying the RMS supply Voltage (V) by the load current draw Amps (A).
Apparent Power (VA) = Volts (V) x Amps (A)
Example: if the load is connected to a 230Vac single-phase mains power supply and the current draw is 2A, the VA value is 460VA. An appropriate UPS would be a 500VA or a 600VA or larger to build in future expansion.
For a three-phase load, the calculation has to take into account the balancing of the load across three phases. First the VA per phase has to be calculated and assuming an even load sharing, these can then be added up to give the total kVA required from the UPS. Unbalanced loads can lead to phase conductor overheating, inefficiencies and an improperly sized UPS.
Example: if the load per phase is 10kVA on phase 1, 9kVA on phase 2 and 8kVA on phase 3, the UPS could be sized at 3 × 10kVA = 30kVA as the loads are fairly balanced at approximately 10kVA per phase. As the UPS would 100% loaded on phase 1, a 40kVA UPS would be recommended to allow for expansion and the running of the UPS at less than 100% load (80% often given as being an optimal loading for any electronic device). Three-phase UPS tend to be quoted in kVA or even MVA when operated in parallel configurations at or above 1000kVA.
An alternative rating calculation is to use Real Power and the Watts drawn.
Real Power (also known as Apparent Power) is measured in Watts. It can be important for larger UPS systems to calculate both the UPS kVA and kW ratings to ensure that excess demands in terms of supplying Apparent and Real Power are not presented to the installed load. Watt ratings are also used to calculate battery set sizes.
Watts is a typical measurement for linear resistive loads and can be calculated using the formula:
Real Power (W) = Amps (A) x Volts (V).
However, for capacitive (Switch Mode Power Supplies) and inductive loads, calculations need to include the load Power Factor; the degree to which the current and voltage waveforms are out of phase with one another using the following formula:
Real Power (W) = Apparent Power (VA) x Power Factor (pF)
When selecting a power solution and sizing power protection systems here is a summary of some of the factors to consider:
Written by Robin Koffler.