Residential split-phase: 120V and... 210V??

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I've just installed a residential power monitoring system that puts a CT around your two incoming lines. The control box that takes readings from the CTs ties into a 2-pole breaker to power itself, and then also measures and reports voltage on that circuit.

It had been reporting ~105V, which seemed very low, so I took a voltmeter to the lines and noticed that line-to-neutral I have ~123V, but line-to-line I have ~213V (I'm guessing the device is just halving the ~213V it's seeing to get ~105V).

My question is: is this normal? I expected that my line-to-line would be exactly double what my line-to-neutral voltage is. Any ideas why this might not be the case?

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Senior Member
What is the neutral voltage? You need to take some voltage measurements to a common reference point, like by using gar's screwdriver-in-the-ground method.


Senior Member
Are both L/N voltages 123V? If one L/N were 123, and the L/L vlots are 213, then you would expect the other L/N to be the remainder, namely 90V. If thats the case then it all adds up. The theres just the question of why your supply is a bit assymetric and low voltage.

On the other hand, if both L/N are 123, and L/L is not 246V, then thats a bit odd.


Staff member
I agree with the two smart guys above. :)

I also wonder what are the accuracy specifications of the power meter and the tester you are using?

jim dungar

Staff member
. so I took a voltmeter to the lines and noticed that line-to-neutral I have ~123V, but line-to-line I have ~213V
Everything is fine, except you voltages are about 3% high. You must be in a large multi-metering environment, you have two hots and a neutral from a 3-phase wye distribution system (some utilities call this a network service).

A normal 208Y/120 service has L-L = 1.732x the L-N voltage , so in your case 123V x 1.732 = 213V.

Your "power" software needs to be set up for this type of service, or its results may be meaningless.


Wow I can't believe the responses. Very helpful!

After I made the post I did some more reading, and found that some residential services DO get 120/208V. I had thought/assumed all residential was 120/240V split-phase.

Yes - both line-to-neutrals are ~123V (~121.5V morning).

And correct - I do live in a large apartment building.

I guess I'll contact the manufacturer of the meter - I know that it's possible to run the control box off of just one leg, in which case it would report 120V. But since my service is part of a three-phase (correct?) - I would perhaps need the three-phase version they're working on? (bottom of their FAQ)

Perhaps next week I'll grab a clampmeter from work and make my own calculations on my load to see if the meter is correct.

Thanks very much.

Smart $

Esteemed Member
100412-0821 EST

A 120/208 3W source is certainly not single phase. However, single phase loads can be run on it.

Blame the IEEE, not me ;)

While you certainly can power single phase loads with it, you can't directly power a standard 3? load on it.
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