Single Phase & Three Phase Supply

An electricity generating plant produces 3 phase supply on particular voltage such as 11KV, 16.5KV etc. This 3 phase supply is transmitted from Power stations to the load centers (cities & industrial areas) & distributed inside cities & industrial areas. Transmission of power is done on a higher voltage such as 66KV, 110KV, 132KV, 220KV, 400KV, 765 KV than generating voltage. Distribution of power is done at lower voltage such as 33KV (generally for industrial area) & 11KV (in city area). This distribution voltage 11KV & 33KV is further reduced to 415V. Some loads are in industries which are designed to run on 11KV supply while mostly loads are designed to operate on 415V & 230V. Generally, 230V is called single phase supply & 415V is called 3 Phase supply.

Note– We must know the voltage level of 3-phase supply before doing any work )


Three phase supply uses 3 wires for transmission & distribution. These Three wires are identified by their colors. These 3 wires are identified as Red phase, Yellow phase & Blue phase. The voltage difference between any two phases is the voltage at which power is being used such as 415V or 11KV etc. Overall voltage across 3 phase supply is 415V or 11KV etc. Let us understand 3 phase system. There two types of 3 phase system – A) 3phase 3 wire system & 3phase 4 wire system –

  1. 3 phase 3 wire supply system – In this system, 3 wires are used for electrical system. As generating plant produces 3phase electrical power which is received & transmitted through 3 wires. It is called 3phase 3 wire system.This system is suitable for transmission of 3 phase supply because it is more economical as compared to 3-phase 4 wire system. The switchgears & equipment used in transmission are also designed for 3 phase 3 wire system.

Power transmission is done at high voltagesby 3phase 3 wire system. Step up transformer is used for transmission of power because when voltage increases the current decreases as power input to transformer is equal to power output. Hence two major advantages of 3phase 3 wire system over 3phase 4wire –

  1. It is economical as equipment & switchgears are designed for 3 wires only (not for 4 wires),
  2. It is economical as current decreases when power is transmitted on high voltage. It means less material will be used for power transfer as diameter of conductor is proportional to the current.
  1. 3 phase 4 wire supply system – This system is suitable for distribution purpose. Generally distribution of power is donein the city at 11KV & this voltage is further reduced to 415V by distribution transformer. A distribution transformer is installed to feed a particular area & whole city is fed through these distribution transformers installed at different location. Now 415V supply with 4 wires is available outside our housewhich provides both 3 phase & single phase supply. The requirement of 3 phase & single phase supply depends on the load of a particular house.

Also 11KV (or 33KV) distribution supply is available outside the premisesof industries/commercial buildings (for bulk load). This 11KV (or 33KV) supply is further reduced to 415V supply with help of distribution transformers inside the building premises for various types of load which are designed either for 415V or 230V.

Advantage of 4th wire in distribution system

  1. Single phase supply can be obtained (use any one phase & neutral wire from 3phase system). Neutral wire is obtained from star point of star connected windings of transformer.
  2. Neutral (or 4th wire) allows unbalanced current to pass through it which helps to improve performance,
  • Neutral wire helps in detection of unbalanced current or fault.
  1. As neutral is fixed & remains at ground voltage value, it helps to get better voltage regulation in fault condition in black out condition.

(Note- Output terminals of a transformer decide 3 wire or 4 wire system. If output winding is delta connected, it provides 3 wires system whereas star winding provides 4 wire system)



Generally single phase supply is 230V AC between two wires. It is used for lighting & domestic equipment purpose. There are two wires used for single phase supply, one wire is called phase wire and second wire is called neutral wire. When load is connected with single phase supply, the current flows from phase wire passes through load & returns through neutral to complete the current cycle.

One more wire is used which is connected with the body of equipment or device to prevent user from electrical shock if there is leakage of current from wire to body of equipment or device (due to failure of insulation). This third wire is called earth or ground wire.

Single phase supply is taken from three phase supply by using any one phase (R or Y or B) & neutral.

(Note: Neutral wire should be properly connected with the equipment. If it is not so, then it becomes very dangerous for the equipment because loose neutral or without neutral, the voltage across the equipment starts increasing. How it happens, is explained below-

We must remember that voltage across each phase is 415V with reference to other phase while voltage across any one phase with respect to neutral (star point) is 230V. It means it is neutral which helps to get reduced voltage i.e. 230V.  Therefore, if neutral is loosely connected with the equipment or at source end, the voltage across equipment will increase from 230 V & the equipment will get damaged when voltage crosses a certain value as equipment is designed for a particular voltage. For ex – single phase voltage equipment is designed for 230V±10% variation.)

What happens when a single phase equipment or lamp comes across higher voltage then its design voltage?

Let’s understand this by Ohm’s Law  –

According to this law –   V άI  (Voltage is proportional to Current).

Ohm’s law says when V increases, I will also increase. So, in our case when voltage increases, current will also increase. Now let’s understand this by examples –

Ex-1Supposea lampis designed for 230V & when lamp is ON, it draws a particular current& accordingly insulation on current carrying parts is done. The filament of the lamp is designed to carry this current. If current increases than design value & crosses a particular limit, the filament will get melted & lamp will be fused.

Ex-2– Suppose A Motoris designed for 230V & Its ON, the motor will draw a particular current & accordingly insulation on current carrying parts is done. The winding of motor is insulated for a particular current. When current increases beyond a particular limit, the temperature of winding will increase & this high temperature weakens the insulation between winding turns & may lead to short circuit.

Above two examples clearly indicates that loose neutral connection is harmful to the equipment.(Note- All equipment/lamps are design to operate on 5-10% higher voltagethan its design voltage.)

Difference between single & three phase supply

1 Two wires are used in single phase supply, one is phase wire & other is neutral wire.

Earthing wire is laid separately for earth fault protection.

3-ph, 3 wire & 3-ph, 4 wire system are used in 3 phase supply. In 3-ph,3 wire system, all 3 wires are phase wire &used for 3 phase load while in 3-ph, 4 wire system, 3 wires are phase wires & 4th wire is neutral wire. 4th wire system is used to make single phase supply for domestic/small load purpose. Earthing wire is laid separately for earth fault protection
2 It is used for domestic purpose like lighting, domestic appliances (less than 1 Kw) etc It is used for industrial equipment or large equipment like 3-ph motors, furnaces etc
3 For same load, the size of single phase motor is larger than 3 phase motor Size  of motor is smaller for same load
4 For same load, single phase equipment is less efficient For same load, three phase equipment is more efficient
5 For the operation of 1-ph induction motor, an auxiliary winding is required to get required starting torque No auxiliary means are required for starting torque for the operation of 3-ph Induction motor
6 The p.f is higher in single phase motor P.f is lower in three phase supply for the same rating i.e output & speed
7 Power transmission & distribution are costly Power transmission is cheaper as 3/4th weight of the copper is used at a given voltage & distance.