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Single line diagram symbols
34 symbols for electrical one-line diagrams: switching devices, transformers, protection, metering, busbars and loads, each drawn the ANSI way and the IEC 60617 way. Free to download as SVG, or as printable charts.
How to read a single line diagram
A single line diagram shows a three-phase power system with one line for all three phases. Read it from the top down, the way the power flows:
- Sources sit at the top: the utility supply, generators and transformers from a higher voltage.
- Busbars are the heavy horizontal lines, each with its voltage and rating. See busbar.
- Switching devices sit between the busbars and each feeder: circuit breakers, disconnect switches and fuses.
- Protection is drawn beside them: CTs and VTs feeding relays marked with device numbers.
- Loads sit at the bottom: motors, drives, distribution boards and capacitor banks.
Next to each symbol you will find its tag and ratings: transformer kVA and vector group, breaker frame and trip rating, cable size, CT ratio. On a project these are the tags the commissioning team tests and energises, one by one.
For the full walkthrough, with the ratings explained and substation, solar PV and data centre one-line diagram examples, see the single line diagram guide.
One-line diagram symbols for substations
A substation one-line diagram uses the same symbols, arranged by bay: incoming and outgoing lines, the busbars and bus couplers, power transformers, and on every bay a circuit breaker between disconnectors, with earthing switches, CTs, VTs and surge arresters.
The busbar arrangement (single busbar, double busbar, ring or breaker and a half) is what makes each substation one-line look different. The single line diagram guide compares them.
Which standards SLD symbols follow
- IEC 60617: the international library of electrical symbols, with a number for every symbol.
- IEEE 315 (ANSI Y32.2): the US symbols, still the reference for US one-lines.
- IEEE C37.2: the ANSI device numbers for protection and switchgear.
Compare them on IEC vs ANSI symbols. For process drawings, see the P&ID symbols.
Using these symbols
Every symbol on this page was drawn by Deskely. You are free to use them in your own drawings, documents and training material.
Frequently asked questions
What is a single line diagram?
A single line diagram (SLD), also called a one-line diagram, shows an electrical power system with one line standing for all three phases. It shows the sources, busbars, transformers, breakers, protection and loads, and how they connect.
Which standards define single line diagram symbols?
IEC 60617 internationally, and IEEE 315 (ANSI Y32.2) in the US, where protective functions are also named with the device numbers of IEEE C37.2. Every project issues its own legend sheet, which takes precedence.
What is the difference between IEC and ANSI symbols?
Mostly the switching devices and relays. ANSI draws a breaker as a square marked 52 and a relay as a circle with a device number; IEC draws a switch blade with a cross and a relay as a box marked I>.
What do the numbers next to relays mean?
They are ANSI device numbers from IEEE C37.2: 50/51 is overcurrent, 87 differential, 27 undervoltage, 86 a lockout relay. See the full list of ANSI device numbers.
Are one-line diagram symbols the same as single line diagram symbols?
Yes. One-line diagram and single line diagram are two names for the same drawing, and use the same IEC 60617 or IEEE 315 symbols.
Can I use these symbols in my own drawings?
Yes. The symbols on this page were drawn by Deskely and are free to use in your own drawings, documents and training material.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016
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