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I was browsing amazon and found this strange extension cord that takes two 110v circuits and combines them into one 220v circuit. I vaguely understand how two outlets can be on a different circuit/phase, but this still intuitively seems very 'fire hazard' to me. Which of the below sentences would best describe this adaptor?

  • Industry known accepted pattern
  • Works sometimes, fails silently other times
  • Works sometimes, fire other times
  • Fire all the time

enter image description here

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7 Answers 7

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There is one more hazard to this kludge that hasn’t been mentioned yet. That is the risk of electrocution! If you plug in one of the 120 volt plugs and have a 240 volt load connected to the other end, then the other plug has lethal (120 volt) voltage exposed on the prongs! If you touch the exposed hot prong, you could be completing the circuit from the plugged-in hot, through the 240-volt load, to the exposed hot prong.

Please avoid this piece of junk!

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DANGER! DO NOT USE THIS!

There are a few different problems:

  • Legs (a.k.a., Phases)

If you connect this to two hots that are on the same leg then the hot-hot on the 14-50 connector will be 0V and nothing 240V will work.

If you connect this to two hots that are on different legs then you are essentially reversing a Multi-Wire Branch Circuit. Except for GFCI, in theory it should work OK, but only up to 15A.

In all (or mostly) 240V mode, there will be little neutral current and the neutral will be relatively safe. If it was actually split again to separate 120V lines and the legs were incorrect (same leg for both hots) then if one of the neutrals in this contraption (or in house wiring) breaks then all the current will go on one neutral. That means even if used at the safe level of 15A @ 240V == 15A input 120V x 2 and then output 15A @ 120V x 2, there will be 30A on a neutral wire. Which will overheat the wire and possibly burn down your house. Neutral wires are not overcurrent protected in standard US residential wiring. This is an admittedly unlikely scenario with this particular contraption (why split to 2 x 120V when you used this to join 2 x 120V to a non-working 240V) but stranger things have happened.

  • Total Current

A 14-50 is normally used for 40A or 50A. (There is no 14-40 receptacle.) The web page references 30A. But all of those are at 240V. What this contraption can actually supply is 15A @ 120V x 2, which is 15A @ 240V. If you run this at 15A you are OK. If you run it at 20A then you will be pushing the house wiring to the limit of 20A circuits and over the limit if the circuits are actually 15A circuits. (Because of the type of plugs, this could be plugged into 15A or 20A circuits.) But if you push it to 30A (or more!) then either you will trip breakers ("nuisance" trips, but actually saving your house from burning down) or, if there are other problems in your electrical system, possibly causing serious problems.

Aside from the 30A vs. 40A vs. 50A issue, the basic problem is that "X"A @ 120V x 2 does not result in "2X"A @ 240V, it results in "X"A @ 240V (the doubling is on the voltage and not on the current). So you never get to where you want to be.

WHY?

The nominal rationale for this is, based on pictures, to connect to two 120V receptacles on a generator to provide 240V power to an RV. However:

  • A small generator that doesn't have 240V receptacles (14-30 or 14-50) is very, very likely to actually be a 120V generator. If it is a 120V generator then combining two receptacles like this will have the "same leg" problem - you won't actually get any 240V power.
  • A large generator will typically have at least one 14-30 or 14-50 receptacle and you use that with an appropriate extension cord to power an RV (or a house through an interlock) directly.
  • Using this with two separate small generators leads to a whole bunch of problems because the AC power won't be synchronized between them.

So the use case is extremely limited - a 240V generator with no 14-30 or 14-50 receptacle. Which is unlikely to produce nearly enough power for a full 240V RV anyway.

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    You have this all backwards. If the two plugs are connected to two circuits on the same phase, it's not the case that "nothing will work". 120V loads (plenty of those in an RV) would function. However, neutral could be overloaded by a factor of two (depending on whether it really wires the socket neutral to both plugs). If connected to two circuits on different phases, 240V loads will work and neutral definitely will not be overloaded (assuming circuits of equal size). That's the whole beauty of multi-phase AC power.
    – nobody
    Commented Nov 9, 2023 at 1:49
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    @nobody I don't think I have it all backwards. But I think you're right (and I'm wrong) about part of this. Need to rethink and edit. Commented Nov 9, 2023 at 2:43
  • RV supplies can be expected to have highly variable loads on the other end - so it's perfectly possible to use the cable without overloading its supply. Of course in that case you might be able to connect only one phase (if it's to power a fridge or lights, for example, rather than electric heating/AC). Of course not overloading the supply doesn't improve the situation much
    – Chris H
    Commented Nov 9, 2023 at 14:10
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    "Why split to 2 x 120V when you used this to join 2 x 120V to a non-working 240V?" A lot of 240V appliances split to 2 x 120V internally to power things like indicator lights and electronics. That's why neutral is present on the socket. Commented Nov 9, 2023 at 14:29
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    Note that some generators designed for hot switchover (like from UPS systems) have a mechanism for tweaking the phase timing to synchronize it since lots of electronics will have a bad day if you just throw the transfer switch without. So if you had two of those you wanted to wire together you could synchronize them enough to work. But really this is the kind of thing you can only use safely when you already know enough about all the circuits and loads that you'd be in a position to wire it up properly anyway.
    – Perkins
    Commented Nov 9, 2023 at 17:55
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Leave that hunk of Chinese unlisted junk in China. Do not be the fool that orders and becomes the importer of it.

It's connecting 15A plugs to a 30A or 50A outlet. So you'll trip the breakers a great deal.

You'd have to have outlets on 2 different phases that you could find and reach to get 240V. You're combining neutrals from different circuits, which is not allowed. You get 0 volts hot to hot if you plug it into outlets on the same phase.

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Possible short between receptacles.

The cord assumes Neutral will always be on the right prong (marked orange).
But if one of the receptacles is wired wrong way, you will create an dead short.
Practice says, you will always find a receptacle wired the wrong way if you look.

Also, the electric code surely contains a rule saying multiple plugs on single cord are illegal.

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    This, then, is probably more effective for checking receptacles for bad wiring than the "magic 8-ball" light-up receptacle testers! Plug it in to any two random receptacles. If you get a breaker pop, one of em's wired backwards. Power's already off, so take the two of 'em apart and figure out which one it is. Only half sarcastic.
    – FreeMan
    Commented Nov 9, 2023 at 13:12
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North American domestic power supplies are typically derived from a centre-tapped transformer with the centre tap acting as neutral (required to be grounded (earthed) in the home distribution panel (aka fuse box)). The two outer taps are each at a potential of 120 volts to the neutral, but being derived from a single transformer secondary winding are at a potential of 240 volts to each other.

Larger appliances such as stoves are connected across these two 120 volt lines and so run at 240 volts. Due to the construction of household distribution panels, alternate circuit breakers are on alternate 120 volt lines. By using adjacent circuit breakers which are mechanically linked, a 240 volt supply is available. 240 volt rated outlets (sockets) are easily available and have a different configuration to 120 volt outlets.

For many years I have used one of these 'combined' 240 volt outlets to power old UK tools. Note that both hot wires are at 120 volts to ground (earth) and both are protected by a circuit breaker - in my case 20 amps. A short circuit between either hot wire and ground will trip its respective breaker.

Now to the device shown. As distribution panels have alternate connections to one or other of the 120 volt lines it is possible to plug this device into outlets (sockets) coming from two different hot lines and thus provide 240 volts at its combined socket.

Although a change in rules has recently occurred, most kitchen outlets (they are known as duplex, having two outlets in one box) are wired separately with each one on a different 120 volt hot line. The wiring between the distribution panel and this 'combo' outlet uses a three wire + ground cable, with a single neutral wire going to both outlets and the other two wires going to the two different 120 volt hot sides.

This device could be plugged into one of these 'combo' outlets so providing 240 volts for some European or UK 240 volt devices.

This arrangement is not inherently dangerous. Of course the wiring in this device must be suitable for the load current of the 240 volt device.

If someone mistakenly plugs both 120 volt plugs into circuits derived from the same 120 volt hot line, the combined outlet will have a zero voltage across it instead of 240 volts, although both will be at 120 volts compared to ground. There remains the possibilty that an outlet has been incorrectly wired with hot and neutral reversed. In that case plugging the second plug into an outlet will result in a loud bang, possibly some smoke and one of the circuit breakers will have tripped. A simple North American outlet tester 'available at all good hardware stores' will confirm correct connections.

The device is likely not approved for use in North America and I have no idea if approval could even be obtained. Just on the lack of an official stamp of approval, I would suggest that the device should not be used.

Your alternative, if you really need 240 volts, is to get a licensed electrician to install a 240 volt outlet. You will need two spare, adjacent slots on the distribution panel and the circuit breakers must be the mechanically linked design. The device will need a new North American 240 volt plug, available from larger hardware stores.

A footnote: North America uses a different AC frequency to Europe/UK which may affect some devices. For example record players used the mains frequency to set the speed, so do not 'cross the pond' easily. An alternative is a single phase converter, which can derive 240 volts from 120 volts at the correct frequency for the device and without any special wiring.

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    @FreeMan: what leads you to think it's ChatGPT based? (unless that's some proprietary knowledge :-) The content seems correct, and it contains personal anecdotes. Is ChatGPT that good? Commented Nov 9, 2023 at 20:31
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    @FreeMan my reply was not ChatGPD derived - if that was what you were saying. All from personal knowledge. Regards anita2R.
    – anita2R
    Commented Nov 9, 2023 at 20:35
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I am not in the US but I did electrical engineering at uni. I would not use this.

First, multiple circuits in your house are likely to be the same phase, so the voltage between them will be zero.

Second, even if you have two phases on two circuits (sometimes electrical companies need to balance the load) it will be about 210v, not 220v

Third, the neutral will be live, which I understand is not correct for US 220v and certainly isn't code for official 220v countries. This could cause interference or even electrocution

I wouldn't use these cables if you paid me

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    This is mostly not correct. US residential wiring is nearly always done so that half of the circuits are on one phase and the other half are on the other phase. Most residential wiring is two-phase, so you get the full 110V + 110V = 220V out the other end, and neutral will be cold. Commented Nov 9, 2023 at 14:32
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    But in some apartment buildings, a unit gets two phases of three-phase power, so in that circumstance, you're right that you'll only get 210V, but then that's the same as you'd get from a "220V" wall socket in that building, so most, if not all, consumer electronics can tolerate that variance. Commented Nov 9, 2023 at 14:34
  • Neutral will only go live if something else is also wrong. Commented Nov 9, 2023 at 14:41
  • And this is also why you'll find that most US appliances have a fairly wide voltage tolerance. If you get out into the rural areas where transmission lines are longer you'll find many places where the houses closest to the transformer station are 115V or even 120V, while the house at the end of the line may be as low as 105V, or 100V. Not common for them to let it go down to 90V anymore, but a bunch of my old tools from the 40s and 50s are labeled to work 90-120V, 50-60Hz.
    – Perkins
    Commented Nov 9, 2023 at 18:03
  • @A.R. note that "if something else is also wrong" is something that happens a lot more often than one might initially think. Between "handymen" making their own electrical replacements/modifications/additions with zero understanding/training, and underpaid contractors, and just plain human error, there are so many places "something else" could be wrong.
    – Doktor J
    Commented Nov 9, 2023 at 18:57
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All other answers already conclude to never use such a gizmo, so I won't go into why one should not use it.

However, I would like to bring up some thoughts on this strange contraption.

For me, this is a physical equivalent of a conspiracy theory: there's a tiny hint of facts, but overall, the more you think about it, the more it falls apart. Only believing in it makes it work, thinking it through makes it fall apart.

Let me elaborate:

To a moderately interested lay-person, "connecting two 110 V circuits creates a 220 V circuit" may sound vaguely convincing. That's the kernel of truth. However, the qualifier that the two 110 V outlet need to be of a different phase, is the practical death blow to the thing.

Who knows, which outlets in ones house/flat/room are on which phase?

tldr: I rest my case.

If, however, you have the means to determine which outlet is on which phase, respectively which outlet are on different phases, you're most likely informed enough to never even tough this thing with a long pole; and moreover, you're most likely better equipped to never actually have a need for such a thing.

So, what is the most likely scenario of use? People go about and try different 110 V outlets until the thing requiring 220 V works. Can the 220 V thing get damaged in the course of this? I don't know. May the process of trying different outlets or using this thing at all represent a hazard, probably. Could it be, that for smaller installations, e.g., a small flat, that all outlets are on the same phase, and as such the thing is useless to the user? maybe. It's certainly not unheard of, that all outlets in a room may be on the same phase.

This device belongs into museums, schools and textbooks under the category "this is a bad idea, and here's why". Note that nothing is entirely useless, it may at least serve as a bad example.

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  • Not likely to damage the 220V if you plug both ends into the same phase since then the potential between the two hot wires will be 0 and between either hot and neutral will be 110V, just as intended. Only way you damage it is if it's using some 110V and some 220V internally in such a way that the 110V stuff powering up without the 220V stuff also running would break it. Which would be kind of an odd design I think. Bigger problem is that you only get 15A, or maybe 20 through the circuits you've plugged into, and most 220V things are on 220V because they need far more than that.
    – Perkins
    Commented Nov 9, 2023 at 18:08
  • @Perkins I agree I would be pretty surprised if a device was designed in a way where where it becomes damaged if 110V were available but not 220, because that would also imply damage if the hot hot not used for 120 were lost, such as by a wire coming loose, contact wearing out, circuit breaker common trip mechanism malfunctioning, etc. I agree that current is a major concern. Devices that only need 15 or 20A at 220V have special plugs (NEMA 6-15 and 6-20), and there are not a lot of them. Some commercial vacuums use em. I've not seen a lot else. Commented Nov 9, 2023 at 20:05
  • @KevinCathcart There are a ton of them actually. Practically any UK home appliance for example. But, once again, they have their own plug. Something like this for using those might actually make sense in some circumstances, but I think I'd at least want a switch in the mix to make sure there were no live prongs hanging out by mistake.
    – Perkins
    Commented Nov 9, 2023 at 20:59
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    Correct me if I'm wrong, but "connecting two 110 V circuits creates a 220 V circuit" i EXACTLY how heavy-load circuits are provided in homes in the USA. (Admittedly done in a more sensible manner than this!)
    – MikeB
    Commented Nov 10, 2023 at 21:45
  • @MikeB Yup, that's how it works, so it's theoretically usable if you can find two outlets that are on opposite phases within reach of the cord and the device you're plugging in doesn't actually need more than 15A. Situationally useful, but also a lot of ways to electrocute yourself or burn your house down. Put it in the box of things to be used only by experts or madmen.
    – Perkins
    Commented Jan 10 at 5:46

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