Got a call about a rooftop A/C unit not working. Open the panel and find it is three-phase, with those tell-tale blanks every few spaces. The A/C is on a 3-phase breaker. With my meter I get the following readings, on both the main lugs and the A/C breaker: For all three phases I get 120v to ground. Phase A to Phase B I get 240v. Phase A to Phase C I get 240v. But Phase B to Phase C I get NOTHING. I expected to get 208v. Is this a problem at the transformer? How can B have voltage to ground, but not to another phase?
