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How to correctly size a UPS

Published by Lee Kelly on 17 September 2019

While this may seem like an easy task, choosing an Uninterruptible Power Supply with the right capacity for your application proves difficult for many people. In order to make the correct choice, a person must understand their site's power load. A site survey can be done to check the nature of the critical load and how it varies at your data centre. Site surveys can either be free or paid for if the project is complex and requires designs to be created. This article reviews the uninterruptible power supply (UPSs) and its crucial element, helping you to understand what is needed to make the right choices for your company.

What is a UPS?

A UPS (uninterruptible power supply) is a device that provides emergency power to a load when the power fails. These devices perform two primary functions. They allow you to shut down your system during a power outage, and cover sags in power during short-term power outages.

In the past, UPS systems were often associated with data centre; however, with so many other applications relying on electronics, they are becoming more mainstream. Different applications, such as retail environments, hospitals, and industrial manufacturing, create different loads on their UPSs. In order to find the most effective solution, these loads must be understood. UPS can also be used on IT equipment and motors, which are either Variable Speed Drives (VSD) or Direct on Line (DOL).

What factors do I consider when looking for a UPS?

There are several things to consider when taking into account the load for your UPS. Obviously, your load will be uniquely yours, but consider the cycles of your environment. There will be daily variations, such as office equipment that isn't used overnight or during weekends, and seasonal variations, especially in the summer when air-conditioning is in high demand. There will also be fluctuations from specific occurrences, such as needing to add new employees and equipment.

When looking to determine the UPS, we need to look at things like the required voltage, load current, frequency, number of phases, and power consumption. The supply voltage for your equipment is often stated on a label attached to the equipment. In the UK, and elsewhere, 240 V is common, but because of the EU harmonised tolerance, your equipment could be rated 220 V, or 230 V also. Be sure to check.

The nature of the load should also be looked at, as different pieces of equipment can draw higher currents. Equipment like motors and compressors, laser printers, air conditioning, and blad servers come to mind. These types of equipment can also have an inrush of current during startup that can overload the UPS. While they are resilient, these loads can present challenges for the UPS.

If harmonic currents are present, they can also affect the load by generating high peak currents. This can vary from 1.42 times the steady-state value for linear loads, to 2.5 times for computer systems.

Measuring these loads can take place during a site survey that includes installing measuring and monitoring equipment. This equipment will record information about the load over a selected period of time, usually, when the office is at it's busiest. It wouldn't make sense to monitor an office network at night or a manufacturing plant on a Sunday afternoon. If you monitor during peak working hours, you'll find your highest loads.

Which UPS should I select?

UPSs use a volt-amperes rating system, ranging from 300 VA to 5,000 kVA. After reviewing all the information you collect, you should know your required UPS capacity and where it fits in this range. The given rating is the maximum load that the UPS supports, but best practice says not to match this exactly. You need to allow room for company growth, new equipment, and new employees.

By going with a UPS that has 1.2 times the load capacity of what you need, you will be able to allow for this growth. Companies expecting rapid growth and companies with new servers may need to use a higher multiplier to account for their systems.

Your equipment and the industry you are in will determine whether you need a single-phase UPS or a three-phase UPS. Most UPS suppliers, such as Eaton and Riello Multi Sentry will have both readily available.

These phases, single-phase and three-phase, account for the electrical phases of receiving and transmitting. Utility companies generate three-phase power to send through the power lines because it is the most efficient way to send power. Large power consumers, like hospitals and manufacturing plants, keep the three-phase electricity. Smaller power consumers, like schools, homes, and office buildings, use less electricity, so their power is converted to a single-phase power. You will need to know which type of power you receive.

If you are using a three-phase UPS for single-phase components, you will need to be sure to evenly distribute the single-phase components to help balance the load. This also shows a balanced load to the mains if the UPS is bypassed for some reason.

For non-IT applications, it is important to consider that VSD drives can be programmed for a soft start to suit the UPS; however, a DOL should be rated at 10 times the nominal load unless the load can be documented as requiring a smaller UPS solution.

Miscellaneous Considerations

Also influencing your final decision on UPS size and configuration is the load's criticality. Almost all applications today demand a redundancy in their UPS. Medical facilities, manufacturing plants, office building, and schools are looking for UPSs that are resilient to the failure of its components. That means that if the power goes off, they will be able to shut down systems as needed or have the backup power to finish their work.

UPS batteries don't have a long lifetime. Most are intended to support loads while shutting down the system, for example, the office worker that needs to save a document. If you need additional time during a power outage, many companies offer extended battery UPSs, allowing you several hours of runtime.

This can all be a bit overwhelming and confusing when you are first looking into UPSs. Suppliers like Riello and Eaton will be able to provide further information and help with site surveys. They can also walk you through load criticality, and advise you as to the right UPS configurations for your organisation.

Taking the time to understand how uninterrupted power supplies work and how they can help your business will assist you in making the right choices for your company. Losing that critical data because of a power failure is never an option. A UPS just may save the day.