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I am getting help from two friends, both of whom work as professional electricians and HVAC installers. I'm hearing, from them, two different methods for checking AC voltage.

I have power coming from our power meter, through a generator transfer switch, to two electrical panels in the house. We can ignore one - it's house only. The 2nd has a large breaker in it (100 Amps) and, from there, heavy duty wire to take it out of the house to a j-box where it connects to three 4/0 cables (and a ground) that run about 500', to our barn. (And the barn also has its own earth ground as required by code.) We're having trouble, suddenly, with the power in the barn.

We're checking the power coming in to the panel in the barn. (The master switch on the panel is turned off, so nothing in the barn is on.) The three 4/0 cables go directly to the panel there. The wires have colored tape to label them. One leg has no tape (black), the other has red tape, and the neutral wire is white. While both electricians are talking about total voltage, measuring the voltage from the red leg and black leg, one is also looking at the voltage from white to red and white to black.

The first one has said you never measure voltage from one leg (like red or black in this case) to neutral because it could go through motors. (I'm not clear about this, unless it's a reference to the generator back at the power company - which is on the other side of several transformers, right?) The other says it's okay to do measure from neutral to one leg. (Like measuring from white to red or white to black in this case.) I'm a bit confused because I know when I put an outlet checker into an outlet and get a reading, it's reading voltage between hot and neutral and that seems like it's the same thing as measuring, in the panel, from the source, from red (or black) to white.

Is there a problem with measuring from one leg to neutral to check voltage? If so, why is it a problem?

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    "you never measure voltage from one leg (like red or black in this case) to neutral because it could go through motors." This is complete nonsense.
    – J...
    Mar 16, 2022 at 14:37
  • @J...: I thought so, but sometimes you find that, if you're a DIYer, there's things that you thought made sense that work differently.
    – Tango
    Mar 16, 2022 at 14:42

2 Answers 2

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Measuring from one leg to neutral to check voltage is correct, not a problem. I measure from each leg to neutral, then I measure across the two legs. If the voltage from each leg to neutral is different you probably have a neutral connection problem. The sum of these two voltages should be equal to the phase to phase voltage, all within a volt or so as I do not know your equipment. Do the measurement again with a load turned on, first on one phase and the other and see if you get a voltage change. Do the same thing at the source breaker they should be the same except the voltage with the load may be a volt or two higher. Check the voltage between neutral and ground, it should be about zero. Do this again with a load on only one of the phases then the other. The reading should be the same. Preferably the load is a few amps.

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  • I'm posting another question (thinking it through now) about the bigger issue. First, I wanted to clarify this issue. Electrician A is electrician first, then trained in HVAC. B is HVAC, but then learned electrical, so I think B, when he talked about motors, is talking from an HVAC background and thinking about the ventilation fans. We're getting 237VAC or less - it goes up and down - at the barn. The total from black to white is 125, maybe 127 sometimes, and the total from leg to leg is always the sum of the voltage for measuring them separately. (That's without load.)
    – Tango
    Mar 16, 2022 at 2:41
  • You're readings are a bit high but that is OK. The difference is also important that is why I measure each phase with and without a load (several amps) the other no load. Your readings should stay about the same if you get a big difference you have a bad neutral connection.
    – Gil
    Mar 16, 2022 at 3:00
  • I'm writing up the whole thing and I'll provide a link to my full question once I have it written out. Basically the voltage on the red goes up and down! When it's up, it's enough. When it's down - lights dim, HVAC stops, and it's a pain.
    – Tango
    Mar 16, 2022 at 3:05
  • Here's the link to my question about the full situation - with a possible solution that makes sense, but that I keep thinking makes too much sense and could be a problem.
    – Tango
    Mar 16, 2022 at 3:23
  • I believe this is the link Tango forgot to include in the previous comment: diy.stackexchange.com/questions/246167/… Mar 16, 2022 at 13:07
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You need to measure hot-neutral to make sure you don't have a "lost neutral".

If you aren't checking neutral, then you're just presuming everything in the neutral wiring is OK. But that attitude is prevalent -- in fact, people who have lost neutrals have a great deal of trouble convincing some professionals that they really have a problem! They're seeing 135V/105V on the two phases, and the electrician and power company are like "you're measuring it wrong, see? 240V across the outer poles."

If neutral wiring were that unimportant, it wouldn't exist.

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  • Thanks! I'm checking neutral. I find the total (from leg to leg) is always in line with the total from neutral to each leg. But what you say explains why the person telling me you don't measure neutral got the idea - it sounds like it's a popular myth or belief. I talked with one of the inspectors who has always been a big help to me. He said with aluminum wiring if there's a nick in insulation, when water gets in there, it can take a couple years before it goes bad - which fits with when the barn was built. (The supply lines are 4/0 aluminum.)
    – Tango
    Mar 16, 2022 at 18:54

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