Electrical Symbols and Quantity Units
Electrical circuit symbols with drawings and meanings, plus a table of electrical quantities, their letter symbols and SI units.
Circuit symbols
| Symbol | Name | What it means |
|---|---|---|
| Resistor (IEC) | Opposes current. The rectangle is the IEC form; the zigzag is the common North American form. | |
| Resistor (zigzag) | The same component drawn the ANSI/IEEE way. Check which form a drawing set uses. | |
| Capacitor | Two plates with a gap. Stores energy in an electric field. No polarity. | |
| Polarised capacitor | Electrolytic type. The straight plate and the plus sign mark the positive side. | |
| Inductor | A coil. Stores energy in a magnetic field. | |
| Transformer | Two coils sharing a core. The two vertical lines are the iron core. | |
| DC cell or battery | The long plate is positive and the short plate is negative. Several pairs in a row mean a battery. | |
| AC source | A circle with a sine wave: an alternating voltage source. | |
| Earth (ground) | A connection to the general mass of earth, used as a reference and for protection. | |
| Diode | Conducts from the triangle side (anode) to the bar side (cathode) only. | |
| LED | A diode with arrows pointing away: it gives off light. | |
| Zener diode | A diode with a bent bar. Used to hold a voltage in reverse breakdown. | |
| Switch (SPST) | Single pole, single throw. The lever opens or closes the circuit. | |
| Fuse | A protective element that melts and opens the circuit on overcurrent. | |
| Lamp | A circle with a cross: a lamp or indicator light. | |
| Motor | A circle with the letter M. A G in the circle means a generator. | |
| Ammeter | Measures current. Connected in series with the load. | |
| Voltmeter | Measures voltage. Connected in parallel across the part being measured. | |
| NPN transistor | A bipolar transistor. Base on the left, collector at the top, emitter (with the arrow) at the bottom. |
Quantities, letter symbols and units
| Quantity | Symbol | SI unit | Unit symbol |
|---|---|---|---|
| Voltage | V | volt | V |
| Current | I | ampere | A |
| Resistance | R | ohm | Ω |
| Conductance | G | siemens | S |
| Real power | P | watt | W |
| Apparent power | S | volt-ampere | VA |
| Reactive power | Q | volt-ampere reactive | var |
| Energy | E or W | joule | J |
| Charge | Q | coulomb | C |
| Capacitance | C | farad | F |
| Inductance | L | henry | H |
| Impedance | Z | ohm | Ω |
| Reactance | X | ohm | Ω |
| Frequency | f | hertz | Hz |
| Period | T | second | s |
| Angular frequency | ω | radian per second | rad/s |
| Power factor | cos φ or pf | no unit (0 to 1) | — |
| Resistivity | ρ | ohm metre | Ω·m |
| Magnetic flux | Φ | weber | Wb |
| Magnetic flux density | B | tesla | T |
| Illuminance | E | lux | lx |
V, I, R and P are italic in print. Unit symbols are upright, and a unit named after a person is written with a lower-case name (volt, ampere) and a capital-letter symbol (V, A).
Metric prefixes used with these units
| Prefix | Symbol | Factor |
|---|---|---|
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| milli | m | 10⁻³ |
| micro | µ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
Frequently asked questions
Why do some symbols have two forms?
Two drawing conventions are in use. The IEC style draws a resistor as a rectangle; the ANSI/IEEE style draws a zigzag. Both mean the same component. Use the one your drawing standard asks for.
How do I tell polarity on a symbol?
On a battery the long plate is positive. On a polarised capacitor the straight plate and a plus sign mark the positive side. On a diode current flows from the triangle to the bar.
Where do I find the full list?
The complete sets are in the drawing standards (for example IEC 60617 and IEEE 315). This page shows the common symbols and does not reproduce those standards.
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