In this paper, the theoretical differences and similarities of tri-level neutral-point-clamped inverters in 3-phase 3-wire and 3-phase 4-wire systems are studied. In the past researches, almost all the studies of tri-level NPC Inverter are focused on the 3-phase 3-wire applications. However, more flexible application strategy can be taken in the tri-level NPC inverter structure. With the connection of neutral wire and mid-point of d.c. linked capacitors of tri-level inverter, it turns into 3-phase 4-wire operation. In this paper, more detailed discussion is performed. Simulation and experimental results are given accordingly.