3-Phase Supply Symbol
Definition: The 3-Phase Supply symbol represents a three-phase alternating-current power source in electrical schematics and wiring diagrams, denoting a system with three conductors (L1, L2, L3) carrying sinusoidal voltages mutually displaced by 120°, standardised under IEC 60617 and ANSI Y32.2 / IEEE 315-1975, and used to indicate the mains supply connection for industrial motors, distribution panels, and three-phase loads.
Also known as: three-phase supply, 3-phase mains, 3-phase power source, three-phase source, three-phase AC supply, 3φ supply, poly-phase supply.
What the 3-Phase Supply symbol means
The 3-Phase Supply symbol identifies the point at which three-phase AC power enters a circuit or distribution system. A three-phase supply delivers three sinusoidal voltages of equal magnitude, each phase shifted 120° from the next, allowing continuous power delivery, balanced loading across all three phases, and efficient transmission at high power levels. The symbol marks connections for conductors L1, L2, and L3 (sometimes labelled A, B, C or R, S, T in different regions).
In industrial and commercial wiring diagrams, the 3-Phase Supply symbol appears at the origin of motor control circuits, variable-frequency drive (VFD) inputs, transformer primaries, and distribution switchboards. It distinguishes a three-phase source from a single-phase or DC supply, ensuring that connected equipment is rated and wired for poly-phase operation.
How to identify the 3-Phase Supply symbol
The 3-Phase Supply symbol is most commonly drawn as three parallel lines (representing the three phase conductors L1, L2, L3) emerging from a source block or a circle containing a sine wave or the letter 'G' (for generator) with three output terminals. In schematic shorthand it may appear as three horizontal lines labelled L1, L2, L3 at the left edge of a one-line diagram, or as three stacked AC source symbols each 120° phase-shifted. Some standards show a circle with three evenly-spaced radial lines at 120° intervals to represent the rotating-phasor nature of the supply.
Function in a circuit
A three-phase supply provides balanced, high-efficiency AC power to equipment that would be impractical to operate on single-phase supply. The three phase conductors allow three-phase induction motors to self-start without auxiliary windings, enable delta and wye transformer configurations for voltage transformation, reduce neutral current in balanced loads to zero, and deliver three times the power of a comparable single-phase system at the same conductor current, lowering distribution losses and conductor costs.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-11 (power sources and supply symbols) defines the graphical symbol for a three-phase AC source as a circle enclosing three-phase sine-wave notation or the standard AC source symbol applied three times. IEC 60038 specifies standard voltages: 400V line-to-line (230V phase-to-neutral) at 50Hz for European low-voltage systems. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 represents a three-phase supply as three individual AC voltage source symbols connected in wye or delta configuration, or as a simplified three-line feed into a one-line diagram. IEEE C37-series standards govern three-phase protection and switching device symbols. |
| Key difference | IEC practice more commonly uses a single composite circle symbol with three-phase notation, while ANSI/IEEE practice typically shows three discrete AC source circles in a wye or delta arrangement in detailed schematics, or three parallel horizontal lines in one-line diagrams. Functional meaning is identical in both systems. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
Typical values
Europe/IEC: 400V line-to-line, 230V phase-to-neutral, 50Hz. North America: 480V or 208V line-to-line (industrial/commercial), 120/208V (commercial wye), 60Hz. Industrial heavy: 690V (IEC), 600V (North America). Current ratings depend on load; distribution transformers range from 15 kVA to several MVA.
Where the 3-Phase Supply symbol is used
- Input supply to three-phase induction motors and synchronous motors in industrial plant
- Input to variable-frequency drives (VFDs) and soft-starters for motor speed control
- Primary winding connection of delta/wye power transformers in distribution systems
- Supply to three-phase rectifier bridges producing high-current DC for electrochemical and welding processes
- Main incomer of switchboards, motor control centres (MCCs), and distribution boards
- Three-phase UPS system input for data centres and critical infrastructure
- Feed to large HVAC systems, chillers, and compressors requiring balanced poly-phase power
Example
On the one-line diagram of a pump station, a 3-Phase Supply symbol labelled '400V / 50Hz' connects through a main isolator and circuit breaker to the primary of a step-down transformer, and separately feeds a motor control centre containing VFDs for three 75 kW centrifugal pumps — the symbol immediately tells the reader that all downstream equipment is designed for three-phase operation.
Key facts
- The 3-Phase Supply symbol represents a poly-phase AC source with three conductors (L1, L2, L3) carrying voltages 120° apart in phase.
- Standard three-phase voltages are 400V line-to-line / 230V phase-to-neutral at 50Hz (IEC) or 480V / 277V at 60Hz (North American industrial).
- A balanced three-phase load draws equal current on all three phases, resulting in zero neutral current in a wye system.
- Three-phase supply allows induction motors to develop rotating magnetic fields and self-start, unlike single-phase supply which requires auxiliary capacitors or windings.
- The 3-Phase Supply symbol is defined in IEC 60617-11 for schematic diagrams and represented in ANSI Y32.2 / IEEE 315-1975 for North American drawings.
- The three phase conductors are designated L1, L2, L3 (IEC) or A, B, C (North American transmission) or R, S, T (older European convention).
- Three-phase power delivery is three times that of single-phase at the same line current and phase voltage, making it the standard for loads above approximately 5 kW.
Frequently asked questions
What does the 3-phase supply symbol look like in a schematic?
The 3-phase supply symbol is typically drawn as three parallel horizontal lines labelled L1, L2, and L3 at the input edge of a one-line diagram, or as a circle with three evenly-spaced output lines representing the rotating phasor voltages. In detailed circuit diagrams it may appear as three separate AC source circles in a wye or delta arrangement, each phase-shifted 120° from the others.
What does the 3-phase supply symbol mean?
The 3-phase supply symbol means that three-phase alternating current power is available at that point, with three energised conductors (L1, L2, L3) each carrying a sinusoidal voltage 120° out of phase with the others. It indicates the power origin for motors, drives, transformers, and other three-phase equipment downstream.
What is the standard voltage for a 3-phase supply?
In IEC regions the standard low-voltage three-phase supply is 400V line-to-line (230V phase-to-neutral) at 50Hz. In North American industrial applications the common three-phase voltages are 480V or 208V line-to-line at 60Hz. Higher transmission voltages (11kV, 33kV, 132kV, etc.) are used for medium- and high-voltage distribution.
What is the difference between a 3-phase supply and a single-phase supply symbol?
A single-phase supply symbol shows one AC source (one hot conductor and a neutral), while the 3-phase supply symbol shows three AC sources (L1, L2, L3) displaced 120°. The 3-phase symbol has three output lines versus one for single-phase, and the associated circuit equipment (motors, breakers, transformers) must be rated for three-phase operation.
Which standard defines the 3-phase supply symbol?
The three-phase supply symbol is defined in IEC 60617-11 for IEC-based schematics and in ANSI Y32.2 / IEEE 315-1975 for North American drawings. IEC 60038 standardises the voltage levels themselves (400V / 230V for low-voltage systems).
How many pins does the 3-phase supply symbol have?
The 3-phase supply symbol has three output pins: L1, L2, and L3. In a wye system a neutral (N) pin is also present, giving four conductors. A protective earth (PE) conductor is added for safety grounding, sometimes shown as a fifth connection point in wiring diagrams.
Why is three-phase power used instead of single-phase for motors?
Three-phase power is used for motors because the three 120°-displaced voltages create a continuously rotating magnetic field in the stator, enabling induction motors to self-start and run smoothly with no starting capacitors. Single-phase motors need auxiliary windings or capacitors to produce a rotating field. Three-phase motors are also more efficient, smaller for the same power output, and produce less torque ripple.
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