In North America we use 120V wall outlets, but sometimes 240V is needed for an application.

Usually in a residential house in order to reduce load on the circuits they either run two different phases of AC into the house for different outlets, or with single phase they use one line in the circuit connected to neutral for one outlet, and another line in the circuit (the return) connected to neutral to another outlet, giving a 240V potential difference across the live wires of two different outlets in your house. Assuming your house has these two different outlets right next to each other, as often occurs in kitchens and bathrooms, how safe would it be to have a plug that rather than plugging into the neutral and live of a single outlet, plugs into the live of two different outlets to harvest this 240V potential difference? I know you would have the disadvantage of not being able to polarize your plug, but aside from that would there be any safety concerns for running a device with this method?

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    \$\begingroup\$ I can't imagine that adjacent outlets in a room would not be wired on the same branch, making it impossible to do what you suggest. Outlets in different rooms might be on different branches from the breaker box but still on the same phase. All neutrals connect together and to ground at the breaker box. I'm not sure you really understand how residential wiring is done. \$\endgroup\$ – Elliot Alderson Jun 29 '18 at 18:19
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    \$\begingroup\$ Do not do this, it's illegal. Wire a 240v circuit to a double pole breaker in the switchboard. \$\endgroup\$ – Jack Creasey Jun 29 '18 at 18:21
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    \$\begingroup\$ @ElliotAlderson Actually, standard double sockets are usually wired to two different drops with different phases. That is why the receptacle has removable bridge on the back. \$\endgroup\$ – Maple Jun 29 '18 at 18:24
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    \$\begingroup\$ I have seen NEMA 5-15 duplex receptacle split into one switched and one unswitched outlet, so user can plug a lamp controlled by a wall switch. But I've never seen two different phases wired to the same duplex receptacle. Doesn't that violate the US NFPA electrical code? \$\endgroup\$ – MarkU Jun 29 '18 at 18:41
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    \$\begingroup\$ @Maple I just finished building a house and the two outlets in a standard receptacle were not on separate phases or breakers. Doing so means running twice as much wire, twice the labor to wire a receptacle, and uncertainty over which breaker controls which outlet for no benefit. The little bridge tab on the outlet is so you can make one of the outlets a switched outlet and the other non-switched. If wiring two drops was "standard" the outlets wouldn't need the tab at all. This is in the U.S., BTW \$\endgroup\$ – Elliot Alderson Jun 29 '18 at 20:12

In North America the most common residential power service is 240 volts divided into two 120-volt groups of circuits using a center tapped transformer as shown below. That provides 120 volts for ordinary wall outlets and 240 volts for large appliances like water heaters, clothes dryer, electric cooking stoves and central air-conditioning systems. The neutral is grounded near the meter. The there are two "hot" lines at 120 volts with respect to neutral.

Normally the 240 volt circuits have a double-pole circuit breaker that opens both hot lines in the event of a fault.

It is possible to obtain 240 volts from two 120-volt outlets that are connected to opposite sides of the transformer as for the 240-volt load shown below. That would require a connection that would violate electrical codes. The safety problem is that a fault could cause only one of two breakers to trip leaving the faulted circuit connected. That is shown by representing the load as a coil or heating element that shorts to ground on one end. In the illustration a high fault current is shown flowing to ground through circuit breaker 2. The fault current flowing through the remainder of the load may not be high enough to trip circuit breaker 1, so the faulty circuit could remain energized with 120 volts applied.

In addition, if the fault connection is burned open, current 2 can flow through the 240-volt load connected in series with the normal loads on circuit 2.

It is possible to connect two 120-volt circuits and a 240 volt circuit through a double-pole circuit breaker. That would insure that any fault would open both of the hot lines. That type of circuit may be permitted in some jurisdictions under some circumstances.

Additional Issues

Note that the 120/240-volt, split-phase service illustrated above is the most common 120-volt service in North America. There is also a 120/208 volt service that is primarily used in individual units of multi-unit residential buildings and in some commercial buildings. With that service the line-to-line voltage is 208 volts and the 120-volt receptacles are connected to neutral of the three phase service and one or the other of two phases of the three phase service. It is also possible that some residences are supplied with three phases. In that case, 208 volts can be obtained similarly with the same problems.

Note also that important points are covered in other answers:

  1. Making the connection to two receptacles live with plugs has the risk that as soon as the first plug is plugged in, the other plug has live 120 volts on the exposed prongs.

  2. The commercial device described by Glenn Willen addresses the risks described. The literature for that device also points out something that can prevent the connection from working. Ground fault protection devices work by detecting the slightest inequality between the currents carried by a hot line and the neutral serving that circuit. Drawing using hot lines in two circuits will therefore trip any ground fault device in one of the circuits. Since code requires two circuits to serve kitchen counter receptacles and ground fault protection for kitchen receptacles, the most likely receptacle to be found for use in this way will not work. Code also requires bathroom receptacles to be ground fault protected but allows one circuit to serve more than one bathroom. Outdoor receptacles also must be ground fault protected and are likely to all be on one circuit. I believe the same is true of basement receptacles.

Electrical Codes

The electrical codes in the USA and Canada are quite long and complex. They are revised and updated every few years. The USA and Canadian codes are very similar, but not identical. In the USA each individual jurisdiction has its own electrical code; some jurisdictions simply adopt the latest edition of National Electrical Code (NEC) as soon as it is published while others adopt the latest edition after careful evaluation with added provisions and exemptions. Each jurisdiction has its own provisions for permits, inspections etc. For those reasons an answer here can never completely and accurately describe what is or is not permitted.

Electrical codes generally require that all devices and materials used in a building wiring system must be acceptable to the authority having jurisdiction. That usually means everything must be listed and labelled by an independent testing laboratory such as UL, CSA or ETL. Appliances plugged into receptacles may not be required to be listed and labelled in all jurisdictions.

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    \$\begingroup\$ I'd also be suspicious of the phase relationship of two random single-phase circuits. \$\endgroup\$ – chrylis -on strike- Jul 1 '18 at 2:51
  • \$\begingroup\$ The phase relationship with residences that have a 120/240 volt, split-phase service as illustrated. That is strictly a single-phase service. In the minority of residences that have two phases of a three phase service with 120 volts line-to-neutral, the line to line voltage is 208 volts rather than 240 volts. Otherwise, there is no problem. I will revise my answer. \$\endgroup\$ – Charles Cowie Jul 1 '18 at 12:14

Here is the danger of doing what you want. If you were to connect your 240 volt load, plug in one of the 120 volt plugs and grab the prongs of the other plug in your hand, you would be exposed to the 120 volts from the first, conducted through the load. You could be electrocuted!

Making a male plug hot is why it is so dangerous.

  • \$\begingroup\$ -1 I don't think you understood the question. \$\endgroup\$ – workoverflow Jul 1 '18 at 12:14
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    \$\begingroup\$ @workoverflow: I think he did understand it. I've seen similar questions asked here before. If some sort of dual plug such as Maple suggested is used then it might be half-OK but the temptation to the uninitiated is to use two independent plugs with only a single wire to each of the live pins. \$\endgroup\$ – Transistor Jul 1 '18 at 12:54

That would be foolish. There are sockets made for 240V AC for various loads.
Get one of those installed.

enter image description here

  • \$\begingroup\$ Not to mention that OP probably has one of these for stove and dryer already \$\endgroup\$ – Maple Jun 29 '18 at 18:53
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    \$\begingroup\$ So these plugs utilize what I was saying about using two hot wires? I see in the diagrams for 240V that there are two hot wires in the circuit. \$\endgroup\$ – user180969 Jun 29 '18 at 19:17
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    \$\begingroup\$ I always wondered why every US/NEMA locking plug I have ever come across looked different. \$\endgroup\$ – KalleMP Jun 30 '18 at 20:41

There is a commercially-available device designed to safely facilitate this: http://www.quick220.com/ It has safety interlocks to ensure that nothing is powered unless both circuits are plugged in and live. I am not associated with the company and I've never used their product, but it seems to be the only commercial offering in this space for home use.

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    \$\begingroup\$ Hmm, interesting device. It has two inputs but does not indicate if they need to be on separate hot phases . For all we know it just needs them on two fuses to draw 20A from each fuse and then has a SMPS to generate the 230V. Have you used one? I think you are also skirting on the fringes of code compliance when you plug it in. Nice to see someone has the guts to market one though. \$\endgroup\$ – KalleMP Jun 30 '18 at 20:34
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    \$\begingroup\$ @KalleMP I wouldn't be shocked about the code compliance, but at some point I read the manual, and it does use two phases. It has some indicators to help you detect when you've got it right. \$\endgroup\$ – Glenn Willen Jul 1 '18 at 2:49

The safest way of doing this is to find double outlet which already has two phases on different sockets.

Buy 6-Way splitter, drill a hole in it and attach to your cable as if it is regular plug. All safety and electrical codes aside, at least this way you won't mess up the polarity when plugging in. And you'd have reliable ground wire connection.

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  • \$\begingroup\$ The incoming supply from the grid is already 240Vrms, not 240VDC. I think you are assuming 2 phases of a 3 phase system, but USA residential power is 240Vrms center-tapped or split phase. \$\endgroup\$ – MarkU Jun 29 '18 at 18:35
  • \$\begingroup\$ This no not very much safer than using two plug leads (it eliminates the chance of having one plug lead loose when the other is already pluigged into a live outlet) and requires adjacent and alternately phased outlets which seems rather the exception than the rule. It is certainly a practical way of mechanically supporting 6 outlets in a suitable double outlet. \$\endgroup\$ – KalleMP Jun 30 '18 at 20:38

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