I have a 20 amp UPS I want to use with a 15 amp electrical circuit, safely.
What I need is an adapter that goes from 5-20R to 5-15P and has a built-in
fuse for 15 amps, so as to never overload the installed circuit.
Does this exist off the shelf, and if yes I would appreciate any references
to where I might purchase it.
Just how are you going to persuade the UPS not to draw over 15A, eg
when first switched on? What type of UPS is it?
I'm not that familiar with US mains wiring, but won't this circuit
already be individually fused for 15A, at the local distribution board?
So, what exactly, is the built-in fuse going to protect, in addition?
Thus I can't see why anyone would manufacture such a thing.
You can, presumably, wire your own "adapter lead". Or just take the plug
off and fit a 15A one.
You can also write to the UPS manufacturer and ask their advice
regarding running it from a 15A service, on light load. I would suspect
that they will recommend that you don't. BICBW
This is a SmartUPS SU3000NET 5U rackmount. Apparently it does not draw
more than 15 amp when switched on because we have one in a test lab that
powers on with a 15 amp circuit and never trips the breaker on power on.
Yes, there is a fuse on the circuit at the panel. But in this case I'm
just trying to add an extra level of protection, because we are knowingly
plugging in a device that could in theory (but probably never in practice)
overload the circuit.
In any case, sounds like it would be a custom adapter.
You can waste your money on a power strip with a
15 amp breaker. That's the closest you'll get to
the adapter you describe. It won't make things any
No adapter is needed between a 5-20R and a 5-15P.
The 5-15P mates perfectly with the 5-20R; the extra
slot in the 5-20R is for identification purposes
only. You don't have a 5-20R if you have a properly
wired 15 amp branch circuit.
There is no way to plug a 20 amp device into a 15 amp
I rather think that the OP intends that the device wont ever draw 15A
(let alone 20A) in normal use. Mapping protective devices to the SOAR
of the circuit should allow it to be used "safely". One suitable device
may easily be enough.
BTW, it was the adapter lead that was to have the 5-20R...
OK, a line-interactive unit.
Anything you plug in could, under fault conditions, present an overload.
The only thing special about what you plan is that the UPS could easily
present a *small* overload over a prolonged period. Which is a pretty
rare "fault". You need to ensure that your protective device can
discriminate against this small overload and clear the circuit. Fuses
are good with large overloads but pretty ineffective with very small ones.
But it is certainly something that could be done *safely* - with the
corect protective devices present.
Wouldn't matter if he put in a 15 amp fuse. The requirement
here (US - National Electrical Code) is that no single device
plugged into the circuit may draw more than 80% of the branch
circuit rating. If the device is rated at 20 amps, the NEC
doesn't care about whatever Rube Goldberg he rigs up. The
device rating itself can't be more than 12 amps, regardless
of what is in some external adapter.
But set that aside for a moment and consider that the branch
circuit is certainly not dedicated to a single receptacle.
Adding a 20 amp device that doesn't draw over 15 amps (because
the op says so) is still screaming for an overload. The
other outlets on the 15 amp branch easily allow an overload
to occur, even if his device never draws over 15. He's
got a 20 amp rated device which is supposed to be installed
on a *30* amp branch (to meet the 80% rule) and he's going
to install it on a *15* amp circuit??! No way that can be
Yes, but it doesn't matter. The 5-20R is no different
functionally than a 5-15R. There is absolutely no need
for an adapter with regard to 5-15P and 5-20R. His device
has to have a plug that mates with a 5-20R. If it mates
with a 5-20R, it will mate with a 5-15R. But this suggests
that the device has already been, or is planned to be,
illegally modified. If it is truly a 20 amp device, then
its plug should be a 30 amp twist lock, not a straight blade
I can't agree that it can't be made safe. For example, if the branch
protection fuse was replaced by a 1A, where, exactly is the risk? It may
not be code, but it is certainly possible to make it safe.
There you go, as I said, I am not into US electrics. I had thought that
a 20R would take either a 20P or a 15P - but that a 15R wouldn't take a
20P. And that this was rather the point of the OP. IIUC, he wanted a
lead with a 20R on one end, to take the plug from his UPS and a 15P on
the other, to fit his wall socket. With a protective device in the
middle, to "make things safe". If that protective device was a 1A fuse,
where's the danger? You seem to be saying thatit could ahve a 12A fuse
and still be safe, although not "legal".
I believe the device was manufactured with the 5-20P, and is not really
a "20A" device, but something that draws more than 12A (thus ineligible
for the 5-15P) but no more than 16A, and because it draws no more than
16A, is eligible for the 5-20P.
Umm, no. The 5-20P has one vertical and one horizontal blade. It will
not fit into a 5-15R, the "normal" US outlet, which has two vertical slots.
The 5-20R has a T shaped slot, and will accept either a 5-15P or a 5-20P.
To the original poster: Check whether the outlet you want to use is
on a 20A circuit, and is wired with #12 wiring. If so, you can simply
replace the 5-15R outlet with a 5-20R, if you can replace an outlet
Also, FWIW, a possible reason for the 5-20P is the UPS was designed to
power devices drawing up to 16A total, but won't draw that much current
without such a load.
That's how I understand it too, and he wants an adapter with a 5-15P
and a 5-20R. According to APC's website, the SU3000NET has a
3000 V-A rating and a L5-30P plug. That makes sense for a unit that
can draw 25 amps. The 5-20P is the wrong plug in the first place, and
the OP might as well change it to 5-15P if he can get the #10 AWG
conductors into a smaller plug. I've seen in done before (my input was
Not Needed). None of this makes it any more code compliant. What
the OP really needs is to reinstall the right plug and connect it to a
30 amp circuit with a single L5-30R receptacle.
By the way, per NEC 210.21(B)(2) and 210.23(A) and (B), the 80%
limit for a cord-and-plug connected load only applies to 15, 20 and
30 amp branch circuits with two or more outlets or receptacles. I read
that and the first paragraph of 210.23 to allow connecting the full load
on a branch circuit with a single receptacle.
That's stretching things all to hell and back, and still wouldn't
make it safe. It would just blow fuses. That would lead to extension
cords draped all over the place, as the 15 amp branch wouldn't
support much at all. The same effect as your 1 amp fuse can be
achieved by unplugging it.
You seem to be saying that you can violate the safety requirements
and still be safe, while I'm saying that violating the safety
requirements automatically means it cannot be considered safe.
That is correct. But he's got a 20R and a 15P - at least that's
what he said. From a 20R to a 15P does NOT require an adapter.
Others think that he is referring to the connectors on the adapter,
not the receptacle and plug he already has, and they may have
the correct idea, while my idea of what he has may be incorrect.
And it still doesn't matter. What he is talking about is *unsafe*.
His premise that it doesn't draw more than 15 amps is: "Apparently
it does not draw more than 15 amp when switched on because we have
one in a test lab that powers on with a 15 amp circuit and never
trips the breaker on power on." His statement is that this is a 20
amp device: "I have a 20 amp UPS I want to use with a 15 amp
electrical circuit, safely. " When you look up the device, it
requires a *30 amp circuit*. The manufacturer's rating trumps
whatever the op, you or I think is safe.
Fuses or breakers do not make a circuit safe. What makes a
circuit safe is proper design and implementation. The fuse
or breaker is there to protect against some defect that would
make an otherwise safe circuit unsafe, and when it blows/trips
it is telling you there is an *unsafe* condition. In the case
the op requested, that unsafe condition is designed in to the
That's bending things all to hell. Might as well leave it
I don't mean to make that impression, it's too "soft".
No outboard device can make it safe.
If the device itself contains a 12 amp fuse as part
of the original design, then it meets safety requirements
for installation on a 15 amp circuit. (But in that case
it would not be rated for 20 amps.) If it does not meet
safety requirements, then it is not safe.
Yes, you are correct. I was sloppy with my wording.
I was speaking with regard to a 15P, which has to
mate with a 20R.
The problem is actually very simple, and a lot of respondents
were sucked in. That model UPS is designed to be installed on
a 30 amp circuit. Unmodified, it has a 30 amp twist lock plug.
The op's original post mentions a 20 amp rating, a 5-15P plug,
and a 5-20R receptacle, which had the effect of diverting
attention from the original design of the device.
No qualified engineer in his/her right mind would authorize the
proposed installation with all the facts in view.
Exactly right. I've never seen a 15 amp dedicated circuit,
thus my comment that the branch circuit is certainly not
dedicated to a single receptacle. 'Course that doesn't
mean there are no 15 amp dedicated circuits anywhere in
the world, and it is possible the op has one. But that
doesn't change my thoughts on the matter.
Nope, what I am saying is that you can violate safety regulations and
still be safe but you cannot violate safety principles and be safe. The
regulations were drawn up based on the principles, but of necessity had
to present a simple set of rules to be followed. A technician has to
work to the set of rules. An engineer has the depth of understanding of
the underlying physics needed to both draw up the rules and determine
when one or more of them can be waived.
As I wrote previously, the safety requirements were drawn up by
engineers, for the blind obedience of technicians. Engineers wrote the
book and can re-write the book. And often do - safety requirements are
not immutable but are subject to constant change. What never changes is
the underlying principles - the physics the links action to effect.
It isn't inevitably unsafe to breach safety regulations - even if it is
inevitably unlawful to do so.