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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?

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    The circuit breaker that turns the circuit off, does it say 15 or 20 on the handle? Do you know if the wires are 12 AWG or 14 AWG (yellow sheath, will not easily fit in modern backstab hole on cheapie receptacle) Commented Nov 14, 2021 at 1:10

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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.

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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.

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  • "overheating the wire and tripping the breaker." - I think tripping the breaker is not the failure mode to be concerned about in that situation Commented Nov 13, 2021 at 23:33
  • @whatsisname, I am curious why you don’t think overloading a circuit until a breaker trips is not something to worry about? , in the U.S. code is to only load a circuit to 80% of the breaker listing. Although breakers are safe there are failures I have rewired a few homes that failures caused massive failures, several ended up toasting every wire in the service panel but the “fire” was contained in the metal boxes in several cases one was not so lucky and the wall in the home burned into the attic, a hand full of small fires in the last 10 years from renters jury rigging grow lights same bkr
    – Ed Beal
    Commented Nov 14, 2021 at 0:03
  • I'm referring to the fact that the breaker tripping is a million times preferable to the wires heating up and starting on fire. As you said, breakers do sometimes fail, so the proposed adapter can overload the circuit, and we've seen plenty of times where people just replace a 15 amp with a 20 amp breaker on 14ga wire and call it good. Commented Nov 14, 2021 at 0:35

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