第二個係460-480V Line to Line，277V Phase voltage。(三相四線)
Since the residential power line voltage coming into all U.S. homes is rated at 220V, it must be reduced inside the home to 110V. The power is brought in as three phase 220V AC, which means it comes in as two 110 volt lines and a central neutral tap line. With this system you can tap each leg to provide 110 volts where it is needed. At the same time you can connect both 110 volt lines to an appliance along with the neutral leg to supply a full 220 volts where needed. In this manner your home gets all of the power it needs for all of your appliances and fixtures.
A variation is the 240 V delta 4-wire system, also known as a high-leg or red-leg delta. This is a three-phase 240 V delta connected system, in which one winding of the transformer has a center tap which is connected to ground and used as the system neutral. This allows a single service to supply 120 V for lighting, 240 V single-phase for heating appliances, and 240 V three-phase for motor loads (such as air conditioning compressors). Two of the phases are 120 V to neutral, the third phase or "high leg" is 208 V to neutral.
Multiwire systems split more than two ways are possible with both AC and DC but have the significant disadvantage that no matter which point is tied to ground some of the wires will have a higher earth relative voltage than the utilisation voltage; therefore, such systems are not used in normal power distribution.