The answer is very simple. A 3-phase 3-wire system does not contain a neutral. This occurs when you have 3-phase loads such as motors that do not require a neutral. A 3-phase 4-wire system has a neutral. A neutral is required to obtain either 120 or 277 volts in a 3-phase system. Both of these scenarios can occur on wye and delta systems. I don't know what you mean by transferring a range of power. By the way, kerry k is completely wrong. Every system needs a ground. Just so you know, I am the one who gave you thumbs down. L1,L2,L3 -3phase 3 wire for balanced Load (3phase x 220v or 380V or 400V , running a 3 phase load like motor, Machine supply 3 wire is enough .3phase 4 wire(L1,L2,L3, NEUTRAL) can be used for balanced load or Un balanced load or both In addition of above load single phase Load (110V or 220V )can be used like lighting, window AC etc, the 4th wire is called Neutral (Star point).In other way Generation (11KV or 15.75kv )- transmission 230KV-400KV ) with 3 wire enough in HV side - .In utility point 4 ( 230 , 380 or 415 V)wire required in LV side.