A person would assume that due to the importance of the uninterruptible power supply (UPS) system, more attention would be applied to these devices in the National Electrical Code. Unfortunately, the answer is no. UPS systems are commonly used in residential, commercial and industrial installations. These systems provide emergency and critical power, standby power for computers and backup power for critical applications in medical facilities, among many other applications. Despite their extensive use, many people have heard of these systems but few know how they function or where they are located.

What’s a UPS system?

The 2023 NEC defines a UPS in Article 100 as a “device or system that provides quality and continuity of AC power through the use of a stored-energy device as the backup power source for a period of time when the normal power supply is incapable of performing acceptably.” 

In other words, a UPS system is a device that provides backup power when the main power source fails. In doing research for this article, I discovered that Wikipedia provides the following additional definition: “An uninterruptible power supply or uninterruptible power source is a type of continual power system that provides automated backup electric power to a load when the input power source or main power source fails. A UPS differs from a traditional auxiliary/emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions by switching to energy stored in battery packs, supercapacitors or flywheels.”

None of the following information is provided in the NEC, but in my opinion, it should be. This device is cost-effective and will provide backup power, protect against power surges and supplies automatic voltage regulation to correct abnormal voltages, such as spikes, sustained undervoltage, momentary reduction in voltage, sags, high or low frequency, noise and harmonic distortion. 

UPS types

There are three general categories of UPS systems: (1) online, (2) line-interactive and (3) standby. In an online UPS, the batteries are always connected to the inverter, so power transfer switches are not necessary. When power loss occurs, the rectifier drops out of the circuit and the batteries keep the power steady and unchanged. 

When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from damaging the batteries (similar to a voltage regulator by regulating battery power charging). The main advantage of an online UPS is its ability to provide an “electrical firewall” between the incoming utility power and sensitive electronic equipment.

A line-interactive UPS system maintains the charge to the batteries by converting AC to DC and then supplies current from the DC source (usually batteries) back to AC only when power is lost. The line-interactive UPS is similar in operation to a standby UPS but with the addition of a multitap variable-­voltage autotransformer. This is a special type of transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the transformer’s output voltage. 

The offline/standby UPS offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below or rises above a predetermined level, the UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS then mechanically switches the connected equipment onto its DC-AC inverter output. The switchover time can be as long as 25 milliseconds, depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS is designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Section 700.12(E)(1) in the 2023 NEC covers stored-energy power supply systems, which include UPS systems. The enhanced content in NFPA Link states the following: “UPS systems generally include a rectifier, a storage battery, and an inverter to convert DC to AC. Uninterruptible power supplies can be very complex systems with redundant components and high-speed solid-state switching. A common practice is to include an automatic bypass for UPS malfunction to permit maintenance. UPS systems often are used to provide a ‘ride-through’ power source between when the normal source is lost and when the engine generator is started and brought online.” 

This immediate source of power ensures there isn’t even a 10-second loss of power. 

During the total rewrite of the NEC for the 2029 cycle, a separate article in Chapter 6 for special equipment must be established specifically addressing UPS systems.

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