120/208 VAC Service

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Hello, all. Has anyone ever come across a twist-lock plug that just has
3 current-carrying wires L1, L2 and neutral (N) such that L1-N is 120 V,
L2-N is 120 V and L1-N and L2-N are 120 electrical degrees apart? IOW,
there is no L3 wire as you would normally(?) expect. Thanks for your
time and comment. Sincerely,
• posted
Hello, again, and I should have said "receptacle" vice "plug".
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What's the voltage between L1 and L2?
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Hello, and given the 120 degree difference the line-to-line voltage would be 120 * sqrt(3) = 208 V. We have what would essentially be a 4-wire wye 120/208 VAC service but the L3 wire is absent. So again I don't know if this 3-wire service is common. I wouldn't think so but I'm not an electrician/electrical contractor. Sincerely,
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I was just asking to see if you determined (measured) the 120° correctly. No offense intended. Tom
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IIRC, I ran into it in Santa Monica. It might have been 240V rather than 120V, but the idea is the same. As long as you do not need a symmetrical neutral, it is no problem. It still would be a good idea to have a protective ground,
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You didn't specify an amperage, but for a 15-amp circuit you would use a NEMA L6-15R, for 20 a NEMA L6-20R, for 30 a NEMA L6-30R. This will give you your 3 terminals. X & Y would be the hots and G would be the neutral or ground wire.
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Hello, and I did measure the L1-L2 voltage with a VOM and it indicated 207.8 VAC. The L1-N and L2-N measured 119.7 VAC. The remaining slot on the twist-lock receptacle doesn't appear to be connected to anything. I'm also assuming the slot I identified as neutral (N) isn't a ground wire since for that case the receptacle would be intended as only a 208 VAC single-phase outlet. Sincerely,
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Is that the third leg, just dead?
Why would you assume that there is a neutral but not safety ground? Why not 208? Maybe I missed something in the discussion above but it makes no sense to have two phases of a three phase 'Y', with no safety ground. Single phase 208V w/ground doesn't seem to be nearly as far-fetched. Can you open the receptacle? Perhaps there is a clue in there (a ground would normally be either green or bare).
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I lived in a part of New York City that typically had 2 phases in each house. I don't know how much of NYC is wired that way, but in Queens the areas known as Floral Park (adjacent to, but not the same as Floral Park in Nassau County) and Glen Oaks we had a nominal 208V AC.
You had to be careful when buying air conditioners, electric stoves, electric driers, and some power tools in Nassau County because you had to make sure that you got stuff wired for 208V, not 240V.
The stuff my family owned used plugs with two phases plus ground. We didn't have an electric stove or electric drier, but they would typically need 208V and 120V. I don't know if they had a separate circuit for clocks and lights or if they used Ground as neutral or had a separate neutral line. Many houses didn't have grounded 120V outlets and I'd bet that many houses didn't have Ground for 208V and 240V outlets, no matter what the code was when the house was built.
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It has to do with wye and delta transformation and the differences between them with respect to where "ground" is and where "neutral" is.
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Clocks ran synchronous with power frequency, so that voltage difference would not matter, save for the fact that 240 into a 208 synchronous "motor' might fry it. But it would still run at the same rpm.

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