Super simple problems you just can't lick are horrible. Here's the situation:
HAZMAT shed, 3 phase, three 120V circuits. Incandescent light going through bulbs. Voltage varied at the light like 109-150 volts while the line to ground volt on all three . Lost the neutral within 10 feet of the panel. Could not find it. Everything appeared wired and metered correctly. Even rung out the wiring, nothing was coming back bad.
After a few hours of pulling out what little hair I had left and cosidering a career change because I couldn't fix something so simple and after starting over for like the 20 time, I FINALLY figured it out. Typically on a 3 phase label, the three legs are on the same horizontal plane and rip the neutral lug is off set. How ever, this particular panel, two of the 3 legs were on the same horizon plane while the 3rd leg was offset. Adding to the confusion, the neutral busbar going to the terminal bus for the circuit neutrals was covered by the panel guts just like the ungrounded bus so at first glance, the neutral terminating looked like termination for one of the phases. As a result, one of the legs and neutral were cross wired when it was connected years prior.
At first thought, this don't sound right and it did not sound right to me either. I dug deeper in the panel,verified the panel was indeed terminated wrong, scratch it all out out on paper, and even talked to the users over the years because it was never a problem, until now. It turned out, the planet was align all these years and someone desired to start using the heater I think which changed the condition which threw the voltage at the light in a tizzy.
Whomever wired it initially apparently assumed the three legs on the same plane and wired it as such.
