The generic circuit symbol for a DC power supply is a circle enclosing a plus and minus sign, while an AC power supply is a circle with a sine wave. However, in 12V, 24V, and 48V renewable energy and backup power systems, the term "power supply" encompasses battery banks, solar arrays, inverters, and UPS units. Using the wrong schematic symbol—or misinterpreting a faded physical terminal label on a charge controller—can lead to reversed polarity faults, compromised equipment grounding, or failed AHJ inspections.
Below is the exact reference for power supply symbols across global standards, followed by a decision matrix to lock in the correct standard for your next schematic or wiring diagram.
The Master Power Supply Symbol Reference Table
This table maps the schematic symbols used in CAD software (like KiCad, Altium, or AutoCAD Electrical) to the physical terminal markings you will find on the actual hardware.
| Component / Source | IEC 60617 Symbol Description | IEEE 315 / ANSI Symbol Description | Physical Terminal Markings | Practical Application Notes |
|---|---|---|---|---|
| Generic DC Source | Circle with '+' and '-' inside | Circle with '+' and '-' inside | +V, -V, VCC, GND | Implies a regulated, stiff supply (e.g., bench PSU or AC-DC brick). Does not model internal resistance or voltage sag. |
| Battery (Single Cell) | One long line (+), one short line (-) | One long line (+), one short line (-) | +, - | Used for single 1.5V/3.2V/3.7V cells. The long line always represents the positive terminal. |
| Battery Bank (Multi-cell) | Single long/short pair with cell count/voltage noted, or multiple pairs | Multiple long/short pairs separated by dots | BATT+, BATT-, Temp, BMS Comm | For a 48V LiFePO4 bank, use a single pair with a designator (e.g., '48V 100Ah') to avoid schematic clutter. |
| Solar PV Array | Circle with two inward-pointing arrows, plus/minus | Square or circle with inward arrows | PV+, PV-, RS485 | Arrows indicate light-to-electricity conversion. A dashed box around multiple cells denotes a full string/array. |
| AC Mains Source | Circle with a sine wave inside | Circle with a sine wave inside | L1, L2, N, PE (or GND) | Used for grid-tie inputs or generator feeds. Always pair with a disconnect switch symbol in single-line diagrams. |
| Inverter (DC to AC) | Rectangle with DC/AC notations, or specific converter symbol | Rectangle with AC sine wave on output side | DC+, DC-, AC OUT L/N/PE | Represents units like a Victron MultiPlus. Must show both DC input and AC output paths with respective grounding. |
| UPS (Uninterruptible) | Rectangle with battery and AC wave symbols inside | Block diagram style with internal battery | AC IN, AC OUT, Batt+/- | Used for server rack or critical load backup. Distinguish between online (double-conversion) and offline topologies in notes. |
Regional Standards: IEC 60617 vs. IEEE 315 vs. Legacy
A common misconception is that the US National Electrical Code (NEC / NFPA 70) dictates schematic symbols. It does not. The NEC governs physical wire colors, ampacity, and installation methods, but it explicitly defers to engineering standards for schematic documentation. Here is how the regional breakdown actually works:
| Standard | Region / Authority | Key Characteristics | When to Use |
|---|---|---|---|
| IEC 60617 | Global, EU, UK, Australia, Modern CAD defaults | Highly modular, uses alphanumeric designators (e.g., 'G' for generator, 'B' for battery). Preferred by modern solar and EV engineers. | International projects, EU CE-marking documentation, KiCad/EasyEDA default libraries. |
| IEEE 315 / ANSI Y32.2 | United States, Canada, US Military | Older, more illustrative style. Often uses specific reference designators (e.g., 'BT' for battery, 'E' for miscellaneous power supply). IEEE Std 315 remains the US baseline. | US industrial control panels, military contracts, legacy US facility documentation. |
| NFPA 79 | US Industrial Machinery | Adapts IEC symbols for US industrial machinery electrical panels. NFPA 79 bridges the gap between IEC and US practices. | Building custom automated machinery or industrial battery testing rigs in the US. |
| BS 3939 (Legacy) | Old UK (Withdrawn) | Replaced by IEC 60617, but still found on schematics inside machinery manufactured before the late 1990s. | Only when reverse-engineering or maintaining legacy UK equipment. |
The "Rows People Get Wrong" Notes
When designing schematics for hybrid inverters or 48V battery systems, three specific symbol confusions cause recurring engineering and safety errors:
Using a generic DC source (circle with +/-) to represent a 48V LiFePO4 battery bank is a critical error in fault-current analysis. A generic DC source implies infinite stiffness and zero internal resistance. A battery symbol (parallel lines) tells the engineer and the protection software that voltage will sag under heavy inverter surge loads (e.g., starting a well pump) and that fault current is limited by the battery's internal chemistry and BMS discharge limits.
In inverter and UPS schematics, grounding symbols are not interchangeable.
- Earth Ground (Three decreasing horizontal lines): The physical rod driven into the dirt. Required for AC mains fault clearing.
- Chassis Ground (Three equal lines on a diagonal): The metal enclosure of the inverter. Must be bonded to Earth Ground.
- Signal Ground (Single horizontal line or inverted triangle): The 0V reference for BMS communication (RS485/CAN bus). Bonding this to Earth Ground will destroy the communication transceivers.
A single solar cell symbol implies an open-circuit voltage of roughly 0.5V to 0.6V. When documenting a string of panels feeding a 150V or 250V MPPT charge controller, you must use the array symbol (a dashed boundary box enclosing the cell symbol, or a single cell with an explicit 'Array' designator). Failing to do this misleads wire-sizing calculations, as the voltage dictates the insulation rating (e.g., 600V vs 1000V PV wire).
Safe Interpretation When Markings Are Faded or Missing
Physical power supplies—like Mean Well AC-DC DIN rail supplies, or the AC input blocks on OutBack Radian inverters—often have silk-screened terminal markings (L, N, PE, +V, -V) that flake off after years of UV exposure, heat cycling, or chemical cleaning.
Never guess terminal assignments based on wire color alone. A previous installer may have used black for DC+ and white for DC- in a pinch, violating standard color codes. If the physical markings are gone, use this verification procedure:
- De-energize and Lock Out: Disconnect all AC and DC sources. Verify zero voltage with a tested True-RMS multimeter (e.g., Fluke 87V).
- Identify PE (Protective Earth): Set the multimeter to continuity mode. Place one probe on the bare metal chassis or the known grounding lug. Probe the faded terminals. The terminal that reads < 1 ohm to the chassis is your PE (Earth Ground).
- Identify AC vs. DC Blocks: On an AC input block, L (Line) and N (Neutral) will show no continuity to each other or to PE. (Note: You may see a brief continuity spike that quickly opens; this is the EMI filter capacitors charging your meter's test leads. This is normal).
- Identify DC Polarity: If dealing with a DC-DC converter or BMS output where labels are missing, you must perform a live test. Re-energize the primary side, set your meter to DC Voltage, and probe the output terminals. If the red probe on a terminal yields a positive voltage reading relative to the known ground, that terminal is +V. If it yields a negative reading, it is -V.
Decision Path: Which Symbol and Standard to Use
Stop debating which standard to use. Follow this decision tree to terminate on a concrete pick for your documentation, CAD library, or panel label.
| Your Scenario / Application | Decision Condition | Concrete Pick (Standard & Symbol Style) |
|---|---|---|
| Designing a commercial solar or battery storage system in the US for AHJ inspection. | US Jurisdiction + Commercial Permit | IEEE 315 / ANSI Y32.2. Use the multi-cell battery symbol and standard ANSI ground symbols. US inspectors are trained on this legacy format. |
| Designing a system for the EU, UK, Australia, or exporting hardware globally. | International / CE / UKCA Compliance | IEC 60617. Use the modular IEC symbols with alphanumeric designators (e.g., 'G1' for Solar Array, 'B1' for Battery). Mandatory for CE technical files. |
| Building a DIY 48V home solar/UPS system and drawing a schematic in KiCad, EasyEDA, or Altium. | Hobbyist / Open Source / Modern CAD | IEC 60617. Modern CAD libraries default to IEC. Fighting the default library to force ANSI symbols wastes hours and creates mismatched netlist errors. |
| Wiring an industrial battery testing rig or automated manufacturing panel in the US. | US Industrial Machinery | NFPA 79 (IEC-adapted). Use IEC symbols but apply US NEC wire color rules (e.g., black for AC line, blue for DC control) as mandated by NFPA 79. |
By aligning your schematic symbols with the correct regional standard and verifying physical terminals when silk-screen markings fail, you ensure your 12V/24V/48V power systems are safe, inspectable, and logically sound from the CAD screen to the terminal block.
