Is it safe to plug 20A device into 30A receptacle?

Thank you for the compliment, but you got my sex wrong. I am a man. My name is a French phrase and the title of a song in that language.

..name means black eyes/your black eyes.

I had a reply already drafted yesterday, saying "Dale get a grip! How can a 20 amp load provoke a 30 amp outlet into increasing its voltage?" Then I noticed the OP was asking about a UPS so I deleted the reply before anybody saw it. Here is the revised version. My original point still stands; the inverter of a UPS usually supplies a pretty stable output as long as the load is within limits, and if the voltage were to vary from rated, it would be downward.

I would not personally think of running a significant motor load from a UPS unless I knew the UPS was rated for that kind of service. There is a lot we don't know. We are told the receptacle is a "30 A outlet". Does that mean the UPS is rated for 30 amps RMS continuous into a motor load? Or does it just mean it is a 30 amp type outlet wired up to God knows what?

Also a motor driven appliance rated at 20 A will draw a much bigger current as the motor starts up, and also a stalled or laboring router would drive up the current. I would not be surprised to see the UPS objecting strongly to this type of treatment. If you were lucky it would just trip an overload relay and (maybe) connect the load to the external AC supply, if you were less lucky you could cook the inverter. A big question I would have is "why would you do this?".

Another point is that many data center type UPSs supply an AC waveform which is not a true sine wave like the AC from the power utility. This might be OK for IT equipment where it is going to be turned into DC anyway, but the motor in the router might well object to this. We have not even thought about power factor, which might de rate the UPS.

A couple of points. 1. A "router" which draws 20 amps is not a computer network router. It is a motor-driven woodworking machine, and if it is a home type it is towards the powerful end of the range. 2. A UPS or Uninterruptible Power Supply is a device where incoming AC is transformed down and rectified into DC. This is used to charge a lead acid battery or batteries which feed the DC to an inverter which outputs supply-voltage AC to the final load. If the AC supply fails or dips the inverter still runs, being supplied from the battery which runs down if the AC supply does not come back. The idea is that the users of IT equipment can shut down servers, etc in an orderly fashion during the time the battery allows. Sometimes a huge UPS system might run for hours or days. A UPS designed to run motor loads such as air conditioners for server rooms, etc, would be an expensive and specialized piece of equipment. They do exist and have a rating much bigger than just the motor load shown on the rating plate might suggest.

Viens-là, viens avec moi, ne pars pas sans moi
Oh ! Oh ! viens reste-là ne pars pas sans moi
Oh ! Oh ! tu prends tes vêtements tu les mets sur toi
Oh ! Oh ! cette nuit dans ce lit tu es si jolie...

Mais qu'est-ce qui brille sur nos regards ?
ce sont tes yeux noirs
un corps musclé des cheveux courts
tu ressembles à un garçon
et je sens ta peau même sans lumière
les serpents sont légendaires

Où vas-tu les yeux noirs ?
tu t'en vas vers nulle part

Allez ! Viens-là, viens avec moi ne pars pas sans moi
Eh ! viens-là reste-là ne pars plus sans moi
Oh ! Oh ! tu prends tes vêtements tu les mets sur toi
Eh ! cette nuit dans ce lit tu es si jolie

On se reverra tous les jours dès notre retour
Prends tes vêtements tu as froid
et mets-les sur toi
mais tu cries dans l'eau même en hiver
et brillent tes yeux noirs

Où vas-tu quand tu pars ?
dans la rue vers nulle part

Allez ! Viens-là, viens avec moi ne pars pas sans moi
Allez viens reste-là ne pars plus sans moi
Oh ! Oh ! tu prends tes vêtements tu les mets sur toi
Eh ! cette nuit dans ce lit tu es si jolie...

A quote from an electrical installation forum:

The VA terminology reminds us about power factor...

bravissimo. Good thorough job.

Pushing, yea Tes; but don't know about that "pulling" 'cause the implication that the appliance is doing work doesn't apply to its power source

Guy, getting back to your q, in spite of all claims to the contrary, given a reduced load the UPS voltage is likely be ever-so-slightly higher, depending on such factors as various resistances associated with source

A UPS of simple-minded design might be more vulnerable, but one more sophisticated with built-in regulation, would probably deliver the same voltage over a wide range of load

Guys who use this kind of stuff might elaborate

Higher than what? A double conversion UPS typically will regulate the output within 3% of nominal, ie a nominally 110 V device will provide between 106.7 volts and 113.3 volts, which is hardly going to matter if you are trying to run a 3 HP motor which you shouldn't be doing anyway. (Did you even read my post above?). It is not going to matter anyway.

Higher than with 30A load

See the post I just made.

Clearly Tes you're misunderstanding me. With increasing load the output voltage of an unregulated supply can be expected to drop, however slightly

Partly my fault 'cause I'm not familiar with the term "double conversion"

You are not familiar with anything technical, Dale.

You have been told that countless times.

You should desist giving bad advice in matters unknown to you.

At your age, why don't you rest and have fun?

Well Tim, this sort of activity is a sort of fun, that is until I get the occasionally raggedy posting.....

Admitting that this old fella does harken to a time when loading down the typical PS slightly reduced its output voltage, however slightly depending on factors such as its "internal resistance". They didn't even have semiconductors back then....

So my most sincere apologies to all...../

Aw, come on, everyone. . . . Let's play nice.

Wm, thought was being nice