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Most of us know that in the world of electricity, current through the wires carry electricity that light our bulbs and run our appliances. The type of current that is supplied from electricity grid is Alternating Current (or AC). In a single-phase supply, a single alternating current in supplied through a single wire, whereas in a three-phase system 3 wires carry alternating current with a definite time offset in between the voltage waves.
In India a single-phase meter supply is a 230V supply through two wires and 3-phase meter supply is 415 V supply through 4 wires and in the house the line can be divided to give 230V at individual point. The basic difference between the two is that a three-phase connection can handle heavy load while a single-phase cannot.
To give an analogy to help you understand the difference, let’s take an example of a highway. If the highway is a single lane highway, only a few two wheelers may go on it in parallel, or if we try to squeeze, we may put two cars running in parallel. But beyond that things will not move, whereas if we have a 3-lane highway, lot of vehicles can move together in parallel. Even on a single-phase highway, number of vehicles that can run together also depends on the size of vehicles. A car and a two-wheeler may easily go in parallel on a single-lane highway, but a truck may just have to be left alone.
Similarly consider single-phase as a single lane highway and three-phase as a multilane highway. There is a limit to the load that a single phase can handle and typically that number is set to 7.5 kW (or 7500 watts or 10 Horse Power). So if sum of wattage of all the appliances that you are running at a time is more than 7.5 kW, then you need a three-phase connection. And you can get to 7.5 kW if you have three 1.5 ton ACs and a water heater all running together. Or you have a machine that has a motor that is more than 10 HP. If the load is less than 7.5 kW, then a single-phase connection can easily handle it.
