Single-Phase Power vs Three-Phase Power - Differences

Single-Phase Power vs Three-Phase Power - Differences

Electric Power System

The principal elements of an electric power system are the generating stations, the transmission lines, the substations, and the distribution networks. The generators produce the electricity, the transmission lines move it to regions where it is consumed, and the substations transform it for industrial, commercial, and residential use.

Finally, the distribution networks carry the electricity to the customers.

Most AC power is generated as three-phase power.

Both three-phase and single-phase devices can be powered from a three-phase supply. A three-phase circuit is a combination of three single-phase circuits. The current, voltage, and power relations of balanced three-phase AC circuits can be studied by applying the rules that apply to single-phase circuits.

The sine waves of three-phase voltage are separated by 120 electrical degree because they are generated by three separate sets of armature coils in an AC generator. These three sets of coils are mounted 120 electrical degrees apart on the generator’s armature. The coil ends could all be brought out of the generator to form three separate single-phase circuits, but they are conventionally interconnected so that only three or four wires are actually brought out of the generator.

Single-phase AC voltage with zero power factor has both voltage and current sine waves in phase, so they cross the zero line together twice in each cycle.

Similarly, a plot of three-phase voltage sine waves, also with zero power factors as shown in Fig. 1, has all three voltage and current waves crossing the zero line twice each cycle together. Each of its three phases, V1, V2, and V3, is separated by 120 electrical degrees.

Power supplied to each of the three phases of a three-phase circuit also has a sinusoidal waveform, and the total three-phase power supplied to a balanced three-phase circuit remains constant.

Figure 1 - Three-phase voltage waveforms are separated by 120 electrical degrees.

Figure 1 - Three-phase voltage waveforms are separated by 120 electrical degrees.

Ok, let’s conclude something…

As a result, there are two practical reasons why three-phase power is superior to single-phase power for many applications:

1st reason Three-phase machines and controls can be smaller, lighter in weight, and more efficient than comparable single-phase equipment. More power is supplied to them in the same period than can be supplied by a single-phase power circuit.

However, the trade-off for this advantage is that three-phase machines and controls are more complex and expensive.

2nd reason Only about 75 percent as much copper wire is required for distributing three-phase power as is required for distributing the same amount of single-phase power.

Resources: N. Sclater, J. E. Traister – Handbook of electrical design

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on


  1. Asiimwe Asaph
    Apr 17, 2015

    Am doing a diploma in biomedical engineering at ECUREI in Mengo Kampala Uganda and am in my first year seeking for a scholarship. thanks

  2. […] Other Resources:    Wikipedia Definitions , Electrical Engineering Portal […]

  3. izaz ali
    Feb 13, 2015

    why three phase is preperad over single phase

  4. Nura Abdullahi
    Jan 20, 2015

    Please why in a power system supply if you are Using 3 phase supply then neutral phase fail, over current occur but if it’s single phase supply then neutral phase fail low current occur? Why is it this differences

  5. priyan
    Aug 13, 2014

    while in distribution feeders i see several houses take single phase supply. but how it is taken with as if several houses doesnt use as much power in other phase causing unequal load? , how the return path carrying return current transfer into three phase nuetral?

    • zeckrey j
      Jan 15, 2015

      hi priyan, just sharing idea here, first of all the unequal load will produce current on neutral wire for 3 phase, recall equation for 3 phase Itotal= I1<0 + I2<120 + I3<120 = 0 for balance 3 phase load "I1 = I2 = I3".
      now, answering your question balancing the load for each house so that the resultant current in neutral is difficult, but it can be minimize by controlling the incoming MCB of each house.
      if your are going to supply a single phase supply to a 3 house using 3 phase supply 40A 4pole, you have to control the incoming MCB size of each house which is all should be equal. in this case i will take 40A as the MCB, hence, from the equation, no matter how worst the unbalance occur, the worst unbalance current at neutral only limit to 40A only which is save if you size your neutral wire to withstand 100% of the phase current.
      reason i use 4 pole mccb in the incoming 3 phase is to limit the current on neutral wire in case of harmonic current.

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