# 120/208 VAC Service

Discussion in 'Electrical Engineering' started by J.B. Wood, May 22, 2013.

1. ### J.B. WoodGuest

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,

2. ### J.B. WoodGuest

Hello, again, and I should have said "receptacle" vice "plug".

3. ### Tom BiasiGuest

What's the voltage between L1 and L2?

4. ### J.B. WoodGuest

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,

5. ### Tom BiasiGuest

I was just asking to see if you determined (measured) the 120° correctly.
No offense intended.
Tom

6. ### Rich.Guest

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.

7. ### J.B. WoodGuest

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,

8. ### Guest

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).

It has to do with wye and delta transformation and the differences
between them with respect to where "ground" is and where "neutral" is.