Master Single Line Drawing Symbols: ANSI vs IEC Reference Chart

Single line drawing symbols (also called one-line diagrams) compress a complex three-phase power system into a single, readable schematic using standardized glyphs. The direct answer: if you are engineering or maintaining a system in North America, use ANSI/IEEE 315 symbols; if you are in Europe, Asia, or designing for international export, use IEC 60617. This reference maps the critical symbols, decodes legacy prints, and provides a concrete decision path for your next schematic.

The Master Single Line Drawing Symbols Reference Table

Because single line diagrams omit the physical wiring of individual phases, the symbols must convey both the component type and its protective function. In North American practice, these symbols are almost always paired with ANSI device numbers to specify the exact relay or switching function.

Component	ANSI/IEEE 315 Shape	IEC 60617 Shape	ANSI Device No.	Practical Meaning & Application
Generator	Circle with 'G'	Circle with 'G'	-	Prime mover converting mechanical to electrical energy. Usually denotes the utility feed or an on-site backup genset.
2-Winding Transformer	Two overlapping circles	Two overlapping circles	-	Steps voltage up/down and provides galvanic isolation. Delta/Wye configurations are noted via text adjacent to the symbol.
Circuit Breaker	Square with cross/latch	Rectangle with cross/latch	52	Auto-interrupts fault currents. Protects downstream equipment. Contains internal trip mechanisms (thermal/magnetic or solid-state).
Disconnect Switch	Square with break/latch	Rectangle with break/latch	89	Manual isolation only. Zero fault interrupting capability. Must only be opened when the circuit is de-energized by a breaker.
Fuse	Rectangle with center line	Rectangle with center line	-	Sacrificial overcurrent protection. One-time use. Often paired with a disconnect switch in medium-voltage applications.
Current Transformer (CT)	Circle with 'CT' or two circles	Circle with 'CT'	-	Steps down high primary current to a safe 5A or 1A secondary for metering and protective relays.
Ground / Earth	Three descending horizontal lines	Three descending horizontal lines	-	Safety earth connection. References the system neutral or equipment chassis to zero potential per NFPA 70 (NEC) Article 250.
Motor (AC)	Circle with 'M'	Circle with 'M'	-	Induction or synchronous motor load. Usually accompanied by a starter symbol (contactor + overload relay).

Regional Standards: ANSI/IEEE vs. IEC vs. Old UK

Using the wrong regional standard on a jobsite can lead to catastrophic misinterpretations, particularly when distinguishing between protective and isolating devices.

ANSI/IEEE 315 (North America)

Dominant in the US, Canada, and parts of Latin America. It relies heavily on geometric shapes (squares for breakers, circles for transformers) and the aforementioned ANSI device numbers (e.g., 52 for AC circuit breaker, 87 for differential protective relay). If you are pulling permits in a US municipality, the AHJ (Authority Having Jurisdiction) will expect ANSI formatting.

IEC 60617 (International)

The standard across Europe, the UK, Australia, and most of Asia. IEC 60617 tends to use more rectangular blocks and relies on alphanumeric reference designations (e.g., 'Q1' for a breaker, 'F1' for a fuse) rather than ANSI device numbers. The physical shapes for basic components like transformers and grounds are nearly identical to ANSI, but the control circuit logic symbols differ significantly.

Legacy UK (BS 3939)

Largely superseded by IEC standards, but you will still encounter BS 3939 symbols in older British industrial plants and marine vessels. It uses distinct, older graphical representations for relays and contacts that look like a hybrid of early ANSI and IEC. If you are retrofitting a pre-1990 UK facility, keep a BS 3939 cheat sheet on hand.

The 'Rows People Get Wrong' Notes

Even experienced electricians and junior engineers misread specific rows on a single line diagram. Here is where the mistakes happen:

• Circuit Breaker (52) vs. Disconnect Switch (89): This is the most dangerous confusion. A breaker has an arc chute and trip unit designed to safely interrupt thousands of amps of fault current. A disconnect switch is just a physical air gap. If you open a disconnect switch under a heavy load, you will draw a massive, sustained arc that can result in an arc flash incident. Always verify the symbol's latch/cross mechanism before assuming it can interrupt current.
• Current Transformer (CT) vs. Potential Transformer (PT): On faded prints, the 'C' and 'P' can look identical. Context is your clue: CTs are in series with the line (measuring load current), while PTs (also called VTs - Voltage Transformers) are in parallel with the line (measuring bus voltage for metering). Never open-circuit a CT secondary while energized; it will generate lethal high voltage.
• System Ground vs. Equipment Ground: A single line diagram usually shows the system ground (neutral-to-earth bond at the transformer). It rarely draws the equipment grounding conductor (EGC) to every motor. Do not assume the EGC is absent just because the green wire isn't drawn on the one-line.

Safe Interpretation of Faded or Missing Markings

When working in older facilities, you will frequently encounter 40-year-old blueprints where UV exposure, oil stains, or poor scanning has obliterated the fine details of a symbol.

Contextual Deduction Rules:

1. Look at the upstream/downstream devices. If the faded symbol is directly feeding a large motor starter, it is almost certainly a circuit breaker or a fused switch, as motors require overcurrent protection.
2. Check the busbar layout. Devices located on the main horizontal busbars are typically main tie breakers or feeder breakers. Devices hanging off a secondary bus are usually distribution disconnects.
3. Trace the control wiring. If the schematic shows control power (120VAC) routing into the device's trip coil, it is a circuit breaker. Disconnect switches do not have trip coils.

Decision Path: Which Standard Should You Use?

Do not mix standards on a single project. Use this decision tree to lock in your standard before opening your CAD software or drafting a new schematic.

Condition / Project Scope	Action / Standard Selection	Concrete Default Pick
Project is located in the US or Canada, and will be inspected by a local AHJ.	Use North American geometric shapes and ANSI device numbers.	ANSI/IEEE 315
Project is located in the EU, UK, Australia, or requires CE marking.	Use rectangular IEC blocks and alphanumeric reference designations (Q, F, K).	IEC 60617
Designing an OEM machine for global export (e.g., shipping a packaging line from the US to Germany).	Defer to the destination country's standard to ensure local maintenance teams can read it.	IEC 60617 (Global default)
Maintaining or expanding an existing legacy plant in the US.	Match the existing documentation to avoid dual-standard confusion in the facility's drawing archive.	ANSI/IEEE 315 (Match existing)
Maintaining a pre-1990 industrial facility in the UK.	Reference the legacy standard for interpretation, but draft new additions in the modern standard.	BS 3939 (Read) / IEC 60617 (Write)