I'm trying to improve power reliability for a network closet at my business. Two UPS units originally provided about seven minutes of runtime, so I brought two older units back into service and optimized the load. The equipment now runs for roughly an hour, but I have another UPS available and was considering adding it.
Since UPS units should not be daisy-chained, I looked at automatic transfer switch (ATS) devices and transfer-switch PDUs that can switch between separate power sources. I purchased an inexpensive 120 V, 15 A ATS with NEMA 5-15 connections for testing, and it appears to work. However, I discovered that it is not UL- or ETL-listed, which makes me concerned about electrical safety and the possibility of overheating or a fire, even though the connected equipment is below the rated amperage.
Would using this ATS with several UPS units be a reasonable way to increase runtime, or would an expandable-battery UPS, a larger enterprise UPS, or a generator be a better approach?
5 Answers
For a more resilient rack, use two properly installed power circuits and two standards-compliant PDUs, with dual-power equipment connected to both sides. Each side can have its own UPS if needed. Single-cord devices can use a certified ATS, but it should be a listed commercial unit designed for the application—not the cheapest unlisted switch available.
The cleaner solution is usually one properly sized enterprise UPS with external battery modules. Multiple old UPS units add self-consumption, battery-maintenance issues, and more points of failure. A UPS that has been sitting unused may also have degraded batteries, and an ATS-fed backup unit can fail silently because it carries no load until the primary source is unavailable.
First measure the real load with a power meter, then size a single UPS and battery expansion for the runtime you actually need. Many outages that last longer than a short UPS window will last for hours, so it may be more practical to configure an orderly shutdown after a defined period instead of trying to keep everything running indefinitely.
I would avoid the inexpensive, unlisted ATS for critical power. The fact that the load is below 15 amps only confirms one rating; it does not establish that the device is safely constructed, properly protected, or suitable for continuous operation. Equipment intended to protect your network should not introduce an unknown electrical hazard.
If the business truly needs long runtime, consider a generator or a professionally designed standby-power system. The UPS can then bridge the short gap while the generator starts, rather than trying to support the entire closet for hours with a collection of aging units and an ATS.
A battery runtime test and network management card are useful here. You want the UPS to report its actual runtime and alert you when batteries need replacement, rather than discovering a problem during an outage.