Microwave on A leg causes B leg voltage to increase, neutral issue, but how to diagnose?

Question

I'm having an issue with flickering LEDs, the PoCo came out and upgraded my service with a brand new transformer and wiring, they did a power survey before and after and said their side is good now. Previously, they noticed some voltage fluctuations, low voltage, given my distance from the previous tranformer. This was definitely fixed as noted by 120 volts consistently inside the house after their upgrade.

Anyway, flickering issue has been reduced (it's house wide, occurs at the same time for all light circuits, suggesting neutral issue) but it still exists. I can make the flickering occur on demand now by plugging a Keurig into a GFCI beside the kitchen sink and turning it on to pre-heat.

It was during this flicker testing that I noticed a few remarkable things:

1. When I turn on the microwave (on A leg in the panel) I noticed a voltage drop on A leg from 120.5 to ~118, very reasonable and expected, HOWEVER, I am noticing a voltage increase from 120.5 to ~121.2 on B leg throughout the house. The expectation would be that the B leg does not increase in voltage.
2. I turned off the light circuit upstairs to check for a loose connection (I tested every single lightswitch for loose neutrals mind you), however, the circuit was still hot. I had to turn off an adjacent breaker for the same room's outlets in order to make the circuit safe to work on. Turning off the outlet breaker does not stop the lights from working and turning off the lightswitch does not stop the outlets working.
3. Switching the lights to be exclusively on B leg made the flickering worse than switching them over exclusively to A leg, flickering still exists, but it's more faint.

I believe #1 and #2 above are related/causing my flickering issue, I believe the flickering issue to be a tell of a loose neutral or something along those lines.

Unfortunately electricians to date have not given a lot of confidence, but, I'm not sure they found the same findings I have now. I've had 5 visits and no success on fixing the flickering.

I have checked all neutrals on the panel, they are tight, as I said before the PoCo checked their side very thoroughly. How can I begin to diagnose this? Could it be a loose neutral anywhere on the A leg, as in, in any junction box for switches or lights? I don't mind checking them all if that helps! I've spent 100 hours on this, another 20 hours is nothing.

TL;DR - With microwave on A leg drops from 120.5V-118V, but B leg increase by .7 volts from 120.5V - 121.2V. Seeing flickering in the house and noticed an unsafe junction box after turning off the correct breaker for said box. Is there a bigger neutral issue?

Thank you!

Answer 1

The flaming "burn your house down" emergency is the flickering!

Barring cheap LEDs or bad LED/dimmer matches, flickering is caused by the wires being connected and disconnected rapidly. You can create flickering by plugging in a (preferably incandescent to avoid delayed startup on LED/CFL) lamp into a socket, but floating the plug halfway in, so it just barely starts to make contact, and then vibrating the plug like mad. So you're making the lamp flicker on and off for like five minutes straight. Please go actually do that (or take my word on it).

OK, now yank out the plug and feel it. I bet it's warmer than normal. See all that booger-snot of metal damage that's on the tips of the prongs? Yeah. That is the calling card of "arc damage". Damaged contacts and violent quantities of heat. And the damaged contacts tend to produce worse contact and more arcing, so it's a death spiral in that sense.

We dislike both of those, but what spooks us is the heat. That heat can start a fire - even in a current-limited case like a series arc. (where the issue is a loose connection and loop current is limited to what the load will draw - still plenty enough to start a fire).

This sucker has nothing to do with the voltage drop, and you need to find it.

It sounds like you have considerably more skills than the average bear in identifying circuits and current flow paths generally, so I'd put those to full use in narrowing down the possible locations. I wouldn't make a habit of provoking it until you are sure it's outside your house, nor would I intentionally let it go nuts for testing. But for what it's worth, the enclosure where it's happening will be warmer than it ought to be. (By a detectable amount? Don't know.)

Find it and kill it with fire, before it kills you with fire.

Why lost neutral is a red herring.

The voltage drop you are seeing is explainable. Consider you are dealing with five voltage drops:

• A) Phase L1, from panel to microwave
• B) Neutral, from panel to microwave
• C) Phase L1, transformer to panel
• D) Neutral, transformer to panel
• E) Line and neutral of the utility's distribution wires back to the power plant.

E is undiscernable, and probably trivially small since they are distributing 9600V on at least #4AL wire. Assuming normal installations, A and B will be equal. If we assume hot and neutral service drop wires are of equal size, C and D will be equal.

Obviously, the microwave sees all four voltage drops. So

A + B + C + D = ~2.6 volts.

However a quiescent circuit on phase L1 sees the last three voltage drops.

And here's the rub: A circuit on phase L2 sees the "D" voltage drop as an anti-voltage-drop. Because neutral is being dropped farther away from it. That is, the "D" drop causes L1-N voltage to drop and N-L2 to rise by the same amount. So we know D = 0.7 volts.

Let's assume the power company dropped the same size neutral as hots, so C = 0.7 volts.

A + B + 0.7 + 0.7 = 2.6 volts.

A + B = 1.2 volts.

A = 0.6 volts.
B = 0.6 volts.
C = 0.7 volts.
D = 0.7 volts.

These are normal values, my friend.