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I have access to three 110 V outlets, representing 3 phases of a 240 V outlet (phase A, B, C). I am thinking of using two of these three phases (A and B) as my hot and neutral to give around 208 V.

I am attaching a wiring diagram below. Please advise on whether this is okay, given that a few junction are now exposed and unwired.

enter image description here

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    \$\begingroup\$ This is totally not OK. Plugging one end of the cable will lead to the other male plug having exposed mains on it. \$\endgroup\$
    – Wesley Lee
    Jul 22, 2022 at 1:24
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    \$\begingroup\$ @Hearth - true, I misread that diagram (twice). But anything non-standard with two male plugs is a big nono in any country. A random person picks it up there is no way of telling what the cable does short of having a multimeter and so on. \$\endgroup\$
    – Wesley Lee
    Jul 22, 2022 at 1:44
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    \$\begingroup\$ @Hearth: if there is a load plugged into the receptacle, it will effectively connect the two Hot lines together, so if only one plug is connected to a 120 V source, that 120 V will make its way to the other plug. \$\endgroup\$ Jul 22, 2022 at 2:03
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    \$\begingroup\$ It is definitely not as good as installing a NEMA 6-15 receptacle. But as long as everything is wired correctly it should be OK, I guess. Definitely not code compliant and not a standard practice. But it should be OK for screwing around and whatnot. One thing to keep in mind is that two receptacles may be on different circuit breakers, so if either one blows, you will not have power at the device. But you may still have a hot wire. \$\endgroup\$
    – user57037
    Jul 22, 2022 at 2:30
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    \$\begingroup\$ Okay in the sense of "works electrically" or the sense of "definitely won't accidentally shock you even if something goes wrong?" The building code cares a lot about the second one \$\endgroup\$ Jul 22, 2022 at 11:01

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Do not do this. Full stop. It's not safe, and it shouldn't be necessary. You can always use a step-up transformer, if the load is 2kVA or less. Otherwise, pull a 208V circuit from the panel. Since this is likely in a business environment, you really want to call a licensed electrician to do the work.

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You have an incoming three-phase 208 V utility supply, distributed as single-phase 120 V (utilizing all three phases).

enter image description here

You need a 208 V single-phase outlet.

The standard safe method would be to have two lines from the panel terminated in a 250 V single-phase receptacle.

The non-standard method suggested by you would be unsafe since one exposed pin of a disconnected 120 V plug would be live via the load.

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The best way to accomplish this would be to install an actual 208V three phase outlet, and then you could make a proper power cord that plugs into it and brings out two hot phases for 208V and one earth ground. Then the female IEC connector would be within its rating of 250 VAC maximum.

I would advise against making a kluge as shown in @HYQ's sketch, but it would work. If you do make this, perhaps for a quick test of some 208-240 VAC equipment, make sure it is clearly labeled and only used long enough to perform the test.

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    \$\begingroup\$ "only used long enough to perform the test." aka destroy/dismantle the cable afterwards \$\endgroup\$ Jul 22, 2022 at 22:47
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Do not do this.

In theory, yes, it is the same as plugging in to a real 208/240V receptacle. But practically, there are several differences that make it unsafe. Other answers have mentioned some of these, but there is another important one:

In mains power distribution it is crucial for all of the wires in a circuit to be in the same cable or conduit. This ensures physical proximity of the current-carrying conductors, which allows their magnetic fields to cancel out to zero (or close enough).

If wires aren't in proximity (either due to being run improperly, or due to the proposed "cheater cable" in the question connecting to hots on totally different circuits), the wires will emit significant net magnetic fields, which will lead to vibration and heating (which lead to fires) as well as inducing unwanted currents into other nearby conductors.

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    \$\begingroup\$ Yes, there are magnetic fields, but significant is relative. Vibration and heating sounds excessive to me. \$\endgroup\$
    – Jens
    Jul 24, 2022 at 21:16
  • \$\begingroup\$ Meh, 60hz noise is a fact of life. If electricians cared about EMI they’d use twisted pair. \$\endgroup\$
    – Navin
    Jul 28, 2022 at 7:03

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