Difference Between a Single Phase & Three-Phase Electrical Box

By Bjorck DiMarco, eHow Contributor

 
 
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Three-phase current travels over every massive electrical grid.

Three-phase electric power is the most common method of distributing alternating electrical current over a grid. Three-phase electric is used to power large motors and other heavy machinery. It's more economical in these situations because it uses less conductor material to transmit power than single-phase or two-phase systems, which transmit electricity at the same voltage. Single-phase is used when you need to take high voltage current and plug in something more delicate, like your computer. For that to happen, three-phase power has to be converted into single-phase outlets.

1. Three-phase Electric Power

   • Three-phase power works because the three circuit conductors are all carrying currents of the same frequency but which reach their peak values at different times. For example, when the first conductor is at its peak output, the second conductor is one-third of a cycle behind and the third conductor is two-thirds of a cycle behind. This means at any given time, one of the three conductors will be at its peak output and the current will flow consistently. This delay between the phases also creates a rotating magnetic field in an electric motor.

   Three-Phase Electrical Box

   • Three-phase power is found in high tension wires and electrical boxes that drive heavy machinery. However, in residential use, three-phase power boxes are found in two places. The first is at utility poles along residential street. The second is in the breaker box in a home. Older homes that don't require any 240-volt outlets, often used to power stoves and other large appliances, sometimes don't have three-phase power coming into the home.
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   Single Unit Residences

   • A breaker box is generally located on the lowest floor, often near the furnace. If you open the front panel of the breaker box, you will notice some of the breakers are linked together and cannot be turned off or on independently. These breakers are two-phase breakers, or the breakers which control the 240-volt outlets in your home. The other breakers, which can be turned on and off one at a time, control 120-volt outlets. When a licensed electrician brings three-phase current from the street into your house, he or she will have three-phase wires which are being split into single-phase (120 volt) and two-phase outlets (240 volt). The breaker box is the main safety feature which prevents outlets from becoming overloaded. If you try to pull too much current from a single 120-volt outlet, for example, the breaker will "trip" or click into the off position. The overload problem has to be addressed before you can reset or turn the breaker back on.

   Multi-unit Residences

   • In some multiple-unit residential buildings, three-phase power is supplied to the building's main breaker panel, but individual units have only single-phase power. The currents returning from the individual apartments to the main breaker panel share the neutral wire, which has a voltage of zero. Returning currents in a single unit residence work the same way. The system will run efficiently unless one of the phases is out of balance. If the system becomes out of balance you'll notice a drop in the efficiency of the 120-volt outlets.

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