Reference Designator Letter Codes
Per ANSI Y32.2 / IEEE 315 convention. J and P aren't defined by pin gender — J is the more fixed half of a connector pair (chassis receptacle), P is the more movable half (cable plug), regardless of male/female pins.
| Letter | Component |
|---|---|
| A | Assembly/sub-assembly |
| AR | Amplifier |
| BR | Bridge rectifier |
| BT | Battery |
| C | Capacitor |
| CB | Circuit breaker |
| D | Diode |
| DS | Display/indicator lamp/LED |
| E | Terminal (misc. electrical part) |
| F | Fuse |
| G | Generator |
| HW | Hardware |
| J | Jack connector (fixed/receptacle half) |
| K | Relay |
| L | Coil or inductor |
| M | Meter |
| P | Plug connector (movable/free half) |
| PS | Power supply |
| Q | Transistor |
| R | Resistor |
| RN | Resistor network |
| RT | Thermistor |
| RV | Varistor |
| S | Switch |
| T | Transformer |
| TB | Terminal board/block |
| TC | Thermocouple |
| TP | Test point |
| U | Integrated circuit/inseparable assembly |
| V | Photoelectric cell (also vacuum tube, older usage) |
| VR | Voltage regulator |
| W | Conductor/wire/cable |
| X | Socket (fuseholder, lampholder, etc.) |
| Y | Crystal or oscillator |
The modern industrial/plant-engineering successor is IEC/EN 81346, which uses a different (partly overlapping) letter scheme — e.g. it groups resistors and inductors both under "R."
Ground Symbols
| Symbol | Ground | Drawn as | Reference |
|---|---|---|---|
| Earth | Stepped bars, longest on top | IEC 60417-5017 | |
| Chassis / frame (may float relative to earth) | One bar with three diagonal strokes | IEC 60417-5020 | |
| Signal / common | Downward-pointing triangle | US schematic convention, no formal IEC symbol |
Common Schematic Symbols — ANSI/IEEE 315 vs. IEC 60617
| Component | ANSI/IEEE 315 (US) | IEC 60617 (International) |
|---|---|---|
| Resistor | Zigzag line (3–4 peaks) — evokes coiled resistance wire | Plain empty rectangle, no internal detail |
| Capacitor (non-polarized) | Two parallel straight lines, small gap | Same — effectively identical |
| Capacitor (polarized) | One straight line (+), one curved/bowed line (−) | Same curved-plate convention |
| Inductor | 3–4 solid semicircular loops in a row | Same loops, or a plain rectangle in some libraries |
| Switch (SPST) | Two terminal marks with a pivoting line, shown open | Same, contacts drawn as short bars rather than dots |
| Relay coil | Circle, or a small box like an inductor (usage varies) | Rectangle (box) — the biggest visual difference cited |
| Diode | Triangle pointing at a perpendicular bar (cathode) | Identical |
| LED | Diode symbol with two small arrows pointing away (emitted light) | Identical |
| Transistor (NPN/PNP) | Base line into a bar; emitter arrow out=NPN, in=PNP | Same geometry |
| Fuse | Wavy/sine line inside or in place of a small rectangle | Plain rectangle, no internal wavy line |
| Battery/cell | One long thin line (+), one short thick line (−) | Identical convention |
| Transformer | Two coil-loop groups facing each other, parallel lines = iron core, dot marks matching polarity | Same in full schematics; one-line diagrams use two overlapping circles instead |
IEC 60617 favors abstract, uniform "box" shapes for most passive parts; ANSI/IEEE 315 favors pictorial shapes that evoke the physical component. Electrically the symbols are always equivalent — the difference is purely graphical convention by region.
Line Type Conventions
| Line Type | Meaning |
|---|---|
| Solid | A physical electrical conductor actually present in the circuit |
| Dashed | A mechanical linkage (e.g. a relay coil actuating its contacts), or a future/optional/not-yet-installed connection |
| Dotted | Bounds a shielded cable/twisted-pair group, or marks an enclosure/panel boundary a symbol crosses |
| Dash-dot | A boundary or field of interconnect (e.g. a chassis/mounting-plate boundary), less universal than the others |
| Heavy/bold | A higher-power or higher-current conductor (main bus, primary feed) vs. lighter signal/control wiring |
| Junction dot | A filled circle at an intersection means the wires are electrically connected |
| Crossing, no dot | Wires cross but do not connect — they "hop over" each other |
One-Line (Single-Line) Diagram Basics
A one-line diagram simplifies a balanced three-phase power system by drawing one line to represent all three phases, emphasizing power flow and equipment ratings (voltage, kVA, breaker size) over full point-to-point wiring detail.
| Device | Typical Symbol |
|---|---|
| Transformer | Two overlapping circles, sometimes with a core-line between |
| Circuit breaker | A break in the line with a curved arc/bracket over it; nested chevrons denote a drawout/withdrawable breaker |
| Disconnect switch/isolator | A simple angled line at the break point (no arc) — isolation only, no overcurrent interruption rating |
| Generator | Circle labeled "G" |
| Motor | Circle labeled "M" |
| Bus/busbar | A single heavy line that multiple feeder lines tee off of |
| Fuse | A small rectangle in the line |
ANSI/IEEE practice commonly tags one-line symbols with standardized function numbers from ANSI/IEEE C37.2 (e.g. 52 = AC circuit breaker, 89 = disconnect/auxiliary switch) alongside or instead of a plain reference designator.