I bought a large UPS for my server and didn't realize it comes with a NEMA 5-20 plug. We're in a residential setting and don't have those outlets. I see Nema 5-20 Female to Nema 5-15 Male Power Adapters but it seems not safe to me, if the device is expecting a dedicated 20 amp circuit. This is the UPS: https://www.cdw.com/product/cyberpower-smart-app-online-ups-series-ol2200rtxl2u-ups-1.8-kw-2200-v/3059881?pfm=srh ; is it safe to use an adapter and plug it into a residential circuit?
If your circuit is a 20 amp / 12awg wire it would be safe to replace the receptacle. Using those 20-15 amp devices is not considered safe and you probably can only find them on line because they would not be code compliant in the US anyway. If the circuit is a 15 amp with 14 awg wire it may work but you take a chance of overheating the wire and tripping the breaker.
You have two possible circuits, and the only way to tell is to see what type of breaker you have.
- 20 Amp
If you have a 20 Amp circuits with NEMA 5-15 15 Amp receptacles, then you should replace the receptacles. Note that the existing NEMA 5-15 are 100% fine. Code allows for 15 Amp receptacles on 20 Amp circuits, as long as there are at least two (which a duplex receptacle has, even if it is the only thing on the circuit). Builders will often use NEMA 5-15 everywhere to keep costs down - one part is cheaper to stock than two, and most homeowners never need a 5-20.
That will cost you between $2 and $5, depending on the brand, quality, style (standard or Decora) at a typical big box or hardware store. If you find 99 cent specials, spend a little more for "commercial" or "spec" grade. But basically any UL-listed NEMA 5-20 will do just fine.
- 15 Amp
TL;DR Use an adapter to cheat and you are playing with fire
This is where you have the real problem. A 15 Amp circuit is likely to have 14 AWG wire in at least part of the circuit. That wire is rated for 15 Amp. A load with a 5-20 plug can draw up to 16 Amps continuously, and a UPS with a 5-20 plug can be expected to use more than 12 Amps, at least when charging after power is restored, because if it didn't need more than 12 Amps then the manufacturer would have used a 5-15 plug.
The problem is that if a load draws 15 Amps for a long time, it might never trip a 15 Amp breaker, since the breaker is designed to only trip when it senses more than 15 Amps. Meanwhile, the wires will be overheating as they are really only supposed to have 12 Amps continuously. At 16 Amps it is possibly even worse, as the breaker should trip eventually, but 16 Amps is a level where it will not get an instant trip (that's in the 2x or more range) but instead taking several minutes, or more, to trip, while the wire is really getting hot.
It is, of course possible to have multiple 5-15 receptacles on a circuit and end up with a similar situation - e.g., a 12 Amp heater on one receptacle, a 3 Amp computer on another. But in typical usage, most people end up with:
- Lots of small loads - chargers, lamps, TV, computer, etc. - not a problem
- Big loads that result in a quick trip and then "I won't try that again" - e.g., 2 1500W heaters on one circuit
- Occasional usage of big loads together with small loads. And if those small loads are for a short time, the delayed trip mode of the breakers at low overloads (1500W hair dryer + a few other things, but not a 2nd hair dryer) combined with the built-in safety margin for the wiring allows for this to all work safely.
But a single large load that requires more continuous power than a 15A circuit can safely supply must be installed properly - which is why it has that NEMA 5-20 plug.