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I have a smart meter that is complaining the voltage is too high. When I use my AC volt meter on the two lines, I get 229. But when I do just L1 to N or L2 to N i get 129 and 131. I thought the voltages were supposed to add up, so I would expect each one to be only 115. Is something wrong?

Edit: added photos. Note that while the panel is 3 phase, there are only 2 phases coming in and connected. The first photo is the community box where all the main power meters reside. The large cables go to the transformer and the smaller cables go to the individual meters. From the meters to the houses. The 2 lines are labeled A and B.

This is mexico, not US.

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DAB
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  • They should add up. Are you checking everything at the same spot? The only answer I can see I would be that you’re checking two different neutrals and they aren’t the same. – DoxyLover Feb 01 '23 at 17:56
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    Measure neutral to ground and see if you get anything other than zero volts. – Chris O Feb 01 '23 at 17:58
  • Did you measure with main breaker off or on? Really need to test when off to verify i is not a user equipment caused issue. With main breaker off "ANSI C84.1 2020 American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hz)" designates the utility supply should not exceed 127v. But in any circumstance the utility should normally come out to verify if the cause of their alarm is their problem without charging you. – NoSparksPlease Feb 01 '23 at 19:20
  • please post a picture of your panel and smart meter (you may cover the serial number) – P2000 Feb 01 '23 at 19:40
  • @ChrisO confirmed N to ground = 0 – DAB Feb 02 '23 at 14:10
  • @p2000 added. The smart meter is blinking green/red which the manual says is over 130v. My volt meter agrees it's about 129/131 – DAB Feb 02 '23 at 14:16
  • @DAB excellent. well, your meter is indeed suitable for 208 and 240, which means that your "circuito 1 fase A" and "fase B" could be 2 phases from a 3 phase system, and the numbers from the answer by manassehkatz applies. But it's hard to distinguish between 3 phase and a faulty 2 phase without an oscilloscope or phase tester, so you'd have to call the utility company to confirm. But a further strong hint is that the phases are called A and B and not L1 and L2, which suggest indeed 2 of a 3 phase system. – P2000 Feb 02 '23 at 14:29

4 Answers4

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TL;DR 208V System

This is characteristic of 3-phase power distribution. It is very common in large commercial and industrial use, and also in apartment buildings in many places. The typical numbers are 208V line-to-line and 120V line-to-neutral. Bump everything up by 9% and you get line-to-line 227 and 131 line-to-neutral - very close to your figures.

9% is a little on the high side. The normal range would be 5% +/-, so a max of 126V for line-to-neutral. If you are using an analog meter or a very cheap/out-of-calibration digital meter then use a quality digital multimeter (or change the batteries!) and you may get better results. Or your power may just be a little on the high side.

See this article or this article for some more detailed explanations.

Update: OP is in Mexico. Mexico standard is both 120V/240V (same as US) and 127V/220V 3-phase. Starting from 127V, 131V is 3% up, and 229V is 4% up from 220V. All very close to OP's original readings. And knowing that the standard for the area is 127V, the smart meter should be programmed for +/- 5% = 120V - 134V.

manassehkatz-Moving 2 Codidact
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    Good eye..... not the obvious lost neutral at all! – Harper - Reinstate Monica Feb 01 '23 at 19:04
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    Nice catch - 2/3 of a 208Y/120 would fit right in. – Chris O Feb 01 '23 at 19:05
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    As an ordinary person looking at a panel, is there a way/something to tell if it is part of a three phase(208v) or a split phase(240v) system? – crip659 Feb 01 '23 at 23:53
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    @crip659 Not that I know of, other than checking voltage. But breakers will be the same, appliances mostly the same - a typical oven or clothes dryer will be rated to work with both 208V and 240V. – manassehkatz-Moving 2 Codidact Feb 01 '23 at 23:57
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    That's a good catch. – JACK Feb 02 '23 at 13:29
  • I edited the OP with an attachment of main cables coming into the community area where the meters are. The large cables go to the transformer and the small ones go to each house's meter just below. This is Mexico, but it's residential. I have a split phase solar inverter connected, but even when i disconnect the voltage is the same. If this really is 3 phase won't this cause more problems? – DAB Feb 02 '23 at 14:09
  • Also in your articles it refers to the phases as A, B and C. Note that my photo says A and B, but everything that talks about 2 phase is usually L1 and L2. is this confirmation that i'm actually using 2 phases of a 3 phase? – DAB Feb 02 '23 at 14:21
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    L1 and L2 are generic terms for "hot Line #1" + "hot Line #2". Which can have different designations depending on jurisdiction, type of system, etc. 3-phase power is usually (in my experience, again may vary in some places) A, B, C. So for anything that uses only 2 phases, it gets split around - i.e., in your community you are part of the AB source panel and there should be similar panels serving from BC and AC. – manassehkatz-Moving 2 Codidact Feb 02 '23 at 15:44
  • That 127V and 3% make it tight on the over-voltage detector, set at 130V. – P2000 Feb 02 '23 at 16:05
  • So is the meter just flashing a warning of over-voltage of one of the phases, but the meter is working and power is properly delivered? – Jim Stewart Feb 02 '23 at 20:04
  • How common is it in 3-phase for the phase to neutral voltage to vary slightly between the phases? How common is it for the phase angles to deviate from 120 deg? – Jim Stewart Feb 02 '23 at 20:06
  • One phase at 129 V and another at 131 V relative to neutral should give 225 V phase to phase. Why is the OP measuring 229 V? Is the phase angle deviating from 120 deg or could this be an error in his meter? Do loads in a customer's system change phase angle? – Jim Stewart Feb 02 '23 at 20:17
  • I don't think loads should change the phase angle. I suspect either phases not quite exactly correct or measurement issues. But this comes closer than any other scheme I can come up with. – manassehkatz-Moving 2 Codidact Feb 02 '23 at 21:39
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Based on the updated question with supplied pictures, your meter is indeed suitable for 208 and 240V, which means that your "circuito 1 fase A" and "fase B" are possibly 2 phases from a 3 phase system.

A further strong hint is that the phases are called A and B and not L1 and L2, which suggest indeed 2 of a 3 phase system.

What further complicates the matter is that your utility could be supplying 2 phases from a 127V / 220V 3 phase system (see also the answer by @manassehkatz). If this is the case, your smart meter is not compatible with these voltages.

In a three phase 120V system, the phase-phase of any 2 of the 3 is 208V and phase to neutral is 120V (+/-10%).

In a 3 phase 220V system, the phase-phase is 220V (+/-10%), and the phase to neutral is 127V (+/-10%). Note, this is not the same as a 3-phase 380V supply in the European 220/380 system.

Your meter does not have labeling for a 127/220 system. This does not mean that your meter will necessarily fail to work, but it's possible that the over-voltage alarm itself is set based on the 120/208 levels and associated tolerances.

Here are some diagrams of the voltages.

This first 3-phase "Y" configuration is likely what you have, without phase C. Other consumers/sites in your complex or neighbourhood possibly have other pairs, like A/C and B/C, since this would balance the load. (You could ask around)

enter image description here

Your AGF meter indeed warns if the voltage is over 130V:

enter image description here

If you have a solar power inverter installed, disconnect it to first independently confirm the correct interaction between the utility line voltages and the smart meter.

This open delta is another possibility, if B to neutral is 120V while A to neutral is 208V.

enter image description here

Further, this is a 2 phase 120V/240V system. Notice the labeling is L1 / Line1 and L2 / Line 2:

enter image description here

It's hard to distinguish between a 3 phase and a faulty 2 phase system by measuring voltages alone. You need an oscilloscope or a phase tester.

Alternatively you can call the utility company to confirm.

Can a faulty L1/L2 240V system give voltages that you quote?

If you have loads running on L1 and you measure L2 to a bad neutral of L1 you might indeed measure a high voltage. That's because a bad neutral of L1 will be somewhat "floating" the opposite polarity of L2, and hence the higher reading.

Same applies vice-versa, when measuring L1 to a bad neutral of L2.

It's possible you have an inadequate neutral at the panel.

Measuring neutral to ground near the panel or meter might not reveal this problem, since this is where they are tied together. If either is failing, the voltage across would still be (near) zero.

P2000
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Assuming that North America is wired the same way as down under. 3-phase has 3 sine waves of the same amplitude, 120 degrees apart.

When you measure line to line, the sine waves do not add up as they are out of phase.

The sum is sqrt(A1*A1 + A2*A2 - 2*A1*A2*cos(120°)) which comes to 225

There is a generic mathematical formula here Sum of sin waves with same frequency and different amplitudes and phase

Jacob Krall
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Rohit Gupta
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  • It is 225 V phase to phase, if each phase to neutral is near 130 V. Is that the usual phase to neutral voltage in Australia? Of course when A1 = A2 = A, the formula reduces to A*sqrt (3). – Jim Stewart Feb 02 '23 at 18:22
  • Note that assuming the phase angle is very close to 120 deg, the expression reduces to sqrt (A1*A1 + A2*A2 + A1*A2). From this you can readily see where the sqrt (3) comes from if A1=A2. And A1 and A2 don't need to be very close to just use their average. For phases as different as 125 V and 135 V the exact expression gives 225.22 V and 130 V * sqrt 3 gives 225.17 V. – Jim Stewart Feb 02 '23 at 19:52
  • From the formula in this answer the phase angle would have to be close to 124 deg to get 229 V phase to phase if the phases are 129 V and 131 V to neutral. Does that ever happen? – Jim Stewart Feb 02 '23 at 21:12
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    @JimStewart you have to take the measurements as approximate. – Rohit Gupta Feb 02 '23 at 21:20
  • I would expect a modern digital meter to get closer agreement than 229 V measured, 225 V expected, but I don't do this sort of measurment regularly. My Fluke multimeter says true RMS. I wonder what meter the OP was using. (I am heading for my 79th birthday and am entering 2nd childhood where I frequently find expectations are unfulfilled.) – Jim Stewart Feb 02 '23 at 21:25
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    Yet, the OP measured 129 and 131. – Rohit Gupta Feb 03 '23 at 02:23
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Something is off, either your measuring device/technique or your electrical service isn't what you think it is. You should be getting 120 Volts line to neutral and 240 Volts line to line. Try measuring a few different times to see if these readings vary a lot. Try a different meter. You would expect a slight voltage drop if appliances are kicking in but your line to neutral readings, if correct, are way too high. Your line to line readings are too low if it's a steady reading.

If the reading remain high with different meters, contact your power company and ask them to install a 24 or 48 hour chart. They should do that at no charge.

Based on the new information from the OP, this being in Mexico where their three phase service can be 127/230V, the readings are within the 5+-%.

Reqular residential services would only require two of the three hot legs, neutral and ground. The selection of which two legs is made to balance the total load on the transformer.

JACK
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