Hello, I am an Electrical Engineer for the Marine Corps, and I ran into an interesting situation that I honestly do not fully understand. I was setting up a small power grid for another shop recently to operate 4 systems, using "house power," which means we are uusing power from the building, standard 3 phase power L1, L2, L3, and grounded neutral. We have it set up to have a 300-amp cable going from the load studs on the building power box going to a 100kw "turtle" (Basically, a circuit breaker box), which then uses 4 60amp cables running from the turtle to the gear. I ran into a problem because on the gear itself there is a phased indicator lamp which according to the manual 'will illuminate when phases A, B, and C are in phase, and will not illuminate when they are not in phase.' These lamps were not illuminating and was interrupting power from actually going to the gear even if the main cb was closed (I am assuming it is a protective device to prevent the gear from being damaged by voltage spikes, because it is sensitive intel/comm gear.). After I looked at the wiring (another engineer had set it up so I had to check it over, I wanted to see if i understood the problem) So at the turtle, I simply switched phases B and C (to my knowledge, you can switch any 2 phases and it will put it back in-phase) and voila, the indicators illuminated and the gear worked fine. So I knew that somewhere the wiring was messed up. So I switched back B and C on the turtle, and checked all the wiring. Sure enough, on the 300amp cable running from House Power to the 100kw turtle, the guy who had initially attached the cable had attached it wrong. They had it installed A-B-C Black-White-Red on the House power, but A-B-C Red-White-Black on the turtle; phases A and C were flipped. I switched it up and it worked fine. So, finally to my question. I don't understand why it mattered if the gear was hooked up backwards to the House Power because each load stud was still putting out 120V 60Hz. Why does this put the power "out-of-phase." If anyone can help me understand this, that would be great! thanks!