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I know that typically electronics draw only as much current as they need. Is this the case with UPSs? I have a 120 volt, 30 amp circuit, and am replacing the UPS plugged into that circuit with a lower wattage UPS. This UPS has a 15 amp plug. Is it safe to use an adapter to do this?

Also, the current plug is NEMA L5-30, whereas the newer UPS is 5-15. If this is safe, are there adapters that are L5-30P to 5-15R?

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Have a look at the adapter you propose to use, and see what current it is rated for. If it's 15A or more, you should be OK. Disclaimer: we're not electricians here. –  MadHatter Jun 13 '14 at 15:59
I think you meant "L5-30R to 5-15P". L5-30R is the receptacle (wall) side, Locking 120V 30A Receptacle. –  MikeyB Jun 13 '14 at 18:10
No, I didn't mean that. The adapter would have to have an L5-30P, because it's plugging into an L5-30R. –  bsamek Jun 14 '14 at 16:20
It's safe. Just beware of cheap adapters that don't provide good coverage of the pins when plugging in/out and have a very small area to grip. –  Matt Jun 15 '14 at 20:17

6 Answers 6

up vote 22 down vote accepted

You should be safe.

It is always OK to put a lower load on a higher rated receptacle. (At the proper voltage that is. Don't mix 230V and 115V).

Just think of it this way: If it wasn't OK nobody could plug a phone charger (about 2 Amp max) in a standard wall-outlet (10 Amps or more).

And for the record: I am a qualified electrician.
Even though it's been over 20 years since I worked in that field, the laws of electrical physics haven't changed.

I still recommend you have a electrician sort out the converter plug or cable. The cheap stuff you can buy online is often of shoddy quality. If you buy it yourself get it from a reputable source.
An UPS for, presumably, important equipment is not the place to skimp on quality.

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If the laws of physics no longer apply God help you - Cave Johnson –  Canadian Luke Jun 13 '14 at 19:56
Wait... 2 A 120 V phone charger? Are you saying phones need 200+ W to charge in the States? No wonder people complain about the price of electricity. It's probably on the order of 1/10th to 1/20th of that here (10-20 W). Valid point, but check your numbers. –  Michael Kjörling Jun 14 '14 at 18:33
@MichaelKjörling He's probably talking about 2A out, at 5 volts, which is 10 watts, which is more like 0.08A @ 120V. –  Bob Jun 14 '14 at 20:37
@MichaelKjörling 2 A OUTPUT ON THE CHARGER. That's a standard iPad charger. –  Tonny Jun 14 '14 at 21:00
@Tonny It was not obvious at least to me that that was what you meant, when you mixed wall outlet 10 A and phone charger output 2 A in the same sentence. Even so, +1 for a good answer. –  Michael Kjörling Jun 16 '14 at 7:33

Why not just plug into a NEMA 5-15 or 5-20 receptacle? You don't need to use the L5-30 circuit.

But yes, you can do this. It's possible to just buy an L5-30P-NEMA 5-15R pigtail adapter online or manufacture your own. However, I'm assuming you have plentiful 5-15 receptacles available to you. Is there a reason you can't use one of those instead?

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Sometimes you don't have a smaller circuit available. –  Andrew Medico Jun 13 '14 at 19:37
Not in this case. It's pretty rare to see a lone L5-30 receptacle without any NEMA 5-15/5-20 outlets nearby. –  ewwhite Jun 14 '14 at 0:53
I'm actually dealing with multiple UPSs with this configuration, so I don't have enough 5-15 receptacles to handle all of them, but I do have enough L5-30s. Point taken though, thank you. –  bsamek Jun 14 '14 at 16:22
Just use the adapter. –  ewwhite Jun 14 '14 at 16:50

You are safe.

All you're taking about doing is plugging a 120V device into a 120V circuit, using a plug adapter you can purchase off the shelf.

For anything beyond this, consult a local licensed electrician.

If this wasn't safe, you couldn't just buy the adapters off the shelf. But you can buy these kinds of adapters:

Your UPS isn't going to draw any more current through a 30A breaker than it does through a 15A or 20A breaker.

All of your circuit breakers are clamped directly onto the fat aluminum buses in your service panel and subpanel(s), which are in turn directly connected to the fat cables that connect directly to the windings on the step-down transformer coil on the pole outside your home or office.

All the devices you plug into the wall, from a tiny 5 watt light bulb to your phone charger to your dishwasher, are directly connected to that transformer outside (which is directly connected to the high voltage power distribution grid).

Essentially all your circuit breakers (or fuses) do is protect the wires downstream from them, by cutting the power when more current passes through them than the wires can carry without getting too hot. Have you ever noticed how thin the high voltage wires on power poles seem to be? They are air-cooled, not closed up inside walls in buildings. Circuit breakers are there to prevent fires. When current gets too high, electrical resistance in the wire increases, the wire gets hot, potentially extremely hot, and buildings burn down.

But again, the breaker doesn't affect how much current a device draws. It just cuts the circuit if the cumulative load passing through the breaker is more than the wire is rated to handle. You have a 30A breaker, in this case, protecting #10 wires, so barring some kind of equipment failure you're not going to have a fire.

And you won't damage your UPS, either, because, the voltage potential on that circuit is 120V, which is exactly what the UPS is designed for. It's only going to pull as much current (Amps) as it's designed for, and also has its own internal overload protection (it's UL listed, right?). You could attach it directly to the bus bars in your service panel and it would function without burning up (not advised for all kinds of other reasons, of course).

The breakers (or fuses) do not modify the power in any way, except possibly through a tiny bit of voltage loss to thermal inefficiency (like how an old incandescent light bulb uses more energy producing heat than it does producing light). The step-down transformer on the pole outside is where the power is modified. That's why you get 120 Volts in your outlets instead of the 17,000 Volts that is energizing the local distribution lines.

GFCI and arc-interrupt breakers don't modify the power that passes through them, either. They just detect and trip the circuit under additional conditions:

  • GFCI trips the circuit when a sudden voltage imbalance appears between the hot and neutral legs of the circuit, indicating that the juice found an alternative path to ground, which may well be straight through a human being. So GFCI protects against electrocution rather than protecting against fire.

  • Arc interruption breakers detect when an electrical arc occurs somewhere on the circuit and cut power. Electrical arcs start fires. Arc interrupt breakers are also a pain because they'll cut circuits when you do things like unplug your vacuum cleaner and a little arc occurs inside the wall receptacle as the prongs on the vacuum cleaner's plug break contact with the prongs inside the receptacle. But the electrical code requires them in many instances now.


  • DO NOT do something like jerry-rig a 120V receptable somewhere downstream on a 240V circuit (the 240V circuit will have 2 120V hot conductors, a ground and usually a neutral unless it is a very old circuit). That neutral wire (or the ground wire if it's an old hot/hot/ground circuit) carries the imbalance of the load between the hot conductors and you don't want to go around intentionally creating imbalanced circuits, and I'm pretty sure the N.E.C. says this is illegal. (EDIT: Actually, a 230/240V circuit will be a little different depending on whether the incoming service is single phase ("Edison") or 3-phase, but this explanation is probably close enough.)

  • Also, if you have a noisy load like a big motor on that same 30A circuit, it could potentially impact your sensitive electronics (although your UPS might mitigate that, but you could also potentially shorten the life of your UPS).

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+1 - Thank you for the comprehensive explanation. In this case, I feel the OP purchased poorly, as he has multiple UPS devices that require this. It's clearly a case of consolidation or buying the right equipment in the first place. –  ewwhite Jun 15 '14 at 18:37
Just a follow-up note to reiterate and confirm the issues with plugging your UPS into the same circuit with something like an appliance (anything with a high-draw electric motor). Somebody plugged a UPS in our server room into the same outlet (same 20A/120V circuit) as the wall-mounted AC unit. The UPS, I discovered, kept cycling between about 15% and close to 100% charged, and the NAS unit with a USB connection to that UPS kept shutting itself off, while the other one kept running, but would have lasted only a minute or so in a power outage. I labeled that outlet "NO COMPUTERS." –  Craig Aug 5 '14 at 16:17

Is it safe to use an adapter to do this?

If the voltage is the same (120v in your case) and the Amp rating is less (15 < 30 in your case), then Yes.

Are there adapters that are L5-30P to 5-15R?

Yep. An eletrical supply house would have this. You should be able to find one on the Googles too.

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Like MadHatter mentioined, we are not electricians here.

But my take on that is that the Amps are a like a maximum load you can bruden the circuit with. What's more important in this case (with my non-electrician knowledge) is that your new UPS can deal with 120 volts. It's most likely the case quite honestly.

I don't think you will find adapters for this, I believe the point of twist type plugs is to secure the connection physically. Having an adapter would pretty much defeat the purpose.

I would get an electrician over just to change the receptacle.

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If the electrician changes the receptacle, he'll be required to change the fuse/breaker. It's easier (and future-compatible) to just buy an adapter. –  MikeyB Jun 13 '14 at 18:12
He does not need to change the breaker. The UPS will never send anything above 15 amps. If it was the other way around (getting a 30 amp UPS and plug it in 15 amp circuit) then I would agree with you. –  Alex Jun 13 '14 at 18:20
Please reread. IF the electrician changes the receptacle, he'll be required to change the fuse/breaker. It's against code in many countries to have a fuse/breaker with higher amperage than the outlets on that circuit. –  MikeyB Jun 13 '14 at 19:07

This UPS has a 15 amp plug. Is it safe to use an adapter to do this?

It LIKELY is safe, but the UPS fuse LIKELY is OUTPUT - and the UPS can suck a lot more power because it msut deliver it's maximum load AND LOAD AT THE SAME TIME. The documentation (of the UPS) will have requirements for the input circuit.

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"UPS fuse LIKELY is OUTPUT"? Sense you no make. –  MikeyB Jun 13 '14 at 18:08
Umm... really? The UPS isn't going to draw more current through a 30A breaker than it does through a 15A or 20A breaker. –  Craig Jun 15 '14 at 17:34
We don't rely on circuit breakers to enforce the maximum load on hardware; they're to prevent fires due to overloaded wiring. To fill an entire 15A circuit at 110V, the UPS (and attached equipment) would need to draw 1650W, which is quite a lot for the kind of UPS I suspect this is. –  Falcon Momot Jun 15 '14 at 19:16
@FalconMomot, Thank you -- exactly my point. :-) –  Craig Jun 15 '14 at 19:57

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