Expected VAC at Sub-Panel?

9 years ago

First time posting…please bear with me…

I am trying to establish that my sub-panel is good before I
go looking for two open neutrals in two multi-wire circuits. I recorded a lot of measurements (voltage and
resistance) at the subpanel, but have no point of reference for what is
normal. With all 6 wires removed from
the breakers and the breakers OFF, I get VAC readings from 0.17 to 1.69VAC from
the breaker to NEUTRAL. These numbers
change 0.105 to 1.34VAC when I measure from breaker to GROUND. Are the readings normal? Are they of any concern?

When I flip the breaker ON, I get the readings I expect (121
at for the breakers on the B terminal, and 117 for the breakers on the A
terminal). The sub-panel is a Murray LC008
with two 12-3 romex cables creating 4 (20amp) circuits and one double pole
breaker (each shows 40amp) for my 220v air compressor.

Any help would be greatly appreciated!

Thanks,

Ron

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Comments (3)

Ron Natalie
9 years ago
When you connect a high impedence meter to a disconnected circuit (what you have when you have the breaker off), you can often read a few induced volts. More if you have a long unterminated wire that you're trying to read.

Any of the hot legs (either the main bus or the breakers when switched on) to neutral or ground should be around 120V. Between the two main buses should be 240.

Neutral to ground should be zero.

An open neutral in a MWBC is very serious. What makes you think you have one of these. The panel end is real easy to check. The neutral (white) wire should be landed with the neutrals. The ground (bare or green) should be landed with the grounds. The only other thing to check is that the neutral and the ground bar in your subpanel are isolated from one another.
Ron B
Original Author
9 years ago
last modified: 9 years ago
Thank you Ron Natalie.

I confirmed Neutral to Ground is 0.000 V

I confirmed A+B buses at 243.0 V

I confirmed that I get around 120V at buses and breakers,
BUT THAT ONLY OCCURS when the MWBC’s are disconnected from the breakers (the
double pole breaker doesn’t have any affect, but I disconnected it anyway).

When the MWBC is connected (either one or both), I get funky
voltages of 132V on one leg and 107V on the other leg (at the breakers). My stating “open neutral” is based on my code
book example where it shows an example of an open neutral, but their figure shows
200V on one leg and 40V on another leg (I was thinking the imbalance of voltage
was key rather than the numbers…maybe I’m wrong?). Also based on a few youtube videos. Realistically, it might be something else.

Thanks, Ron
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Ron Natalie
9 years ago
You can get wild voltages with long unterminated runs.

Pencil voltage testers give a lot of false positives as well.

If you get a solid 120V from hot to neutral while the power is on, the neutral is not likely open. With the power off you're not really testing things.