Three-phase power is what separates residential guys from commercial/industrial, and understanding wye vs delta configurations is foundational. Wye (Y) configuration: three phase conductors and a neutral, where each phase conductor is connected to the neutral point. Line-to-neutral voltage is your phase voltage (120V in a 120/208V system, 277V in a 277/480V system). Line-to-line voltage is phase voltage x 1.732. In a 480V system, line-to-line is 480V, line-to-neutral is 277V. This is why you can run 277V fluorescent or LED lighting on 480V wye.
Delta configuration: no neutral (in most cases), three phase conductors forming a triangle. Common in older industrial installations. The “high leg” delta (240V delta with a center-tapped phase) is the weird one - you get 120V from two legs to neutral, 240V line-to-line, but 208V from the high leg to neutral. The high leg must be identified with orange tape and you can’t use it for 120V loads. This configuration trips people up every time.
Troubleshooting three-phase: if you have one leg low, start at the transformer. Single-phasing (loss of one phase) is catastrophic for three-phase motors - they’ll try to run on two phases, overheat, and destroy the windings. VFDs often have built-in single-phase detection. If a motor is failing repetitively, verify all three phases before condemning the motor.