What is the difference between a schematic diagram and a wiring diagram?

A schematic diagram (also called a circuit diagram) shows the electrical connections between components using standardised symbols, without regard to the physical location or appearance of the components. It shows how the circuit works electrically. A wiring diagram shows the physical connections between components — where wires go, which terminals they connect to, and the physical layout of components. Wiring diagrams are used for installation and maintenance, while schematic diagrams are used for understanding circuit function and fault-finding.

What electrical symbols do UK drawings use?

UK electrical drawings use symbols based on BS EN 60617, which is the British adoption of the IEC 60617 international standard. In practice, most modern UK electrical drawings use IEC symbols, which are the same standard. Older UK drawings may use legacy BS symbols that differ from IEC equivalents. The IET Wiring Regulations (BS 7671) reference IEC 60617 for circuit symbols. Where drawings use non-standard symbols, a symbol legend (key) should always be provided.

What is a single-line diagram and when is it used?

A single-line diagram (SLD), also called a one-line diagram, represents a three-phase electrical distribution system using a single line to represent all three phases, for the sake of clarity. SLDs show the hierarchy of the distribution system — from the incoming supply (DNO cutout or transformer) through the main switchboard to sub-distribution boards and final circuit protective devices. SLDs are used extensively for commercial buildings, industrial facilities, and infrastructure projects. They show circuit breaker ratings, transformer ratings, cable sizes, and protection coordination without the complexity of showing all three phases separately.

How do I identify phases on a three-phase diagram?

In the UK, three-phase conductors are labelled L1, L2, and L3 (replacing the old red, yellow, and blue colour coding, which was changed in 2004 to align with European harmonised colours). On diagrams, phases may also be shown as R, S, T (older convention) or A, B, C (American convention on some imported equipment). The neutral conductor is labelled N and the protective earth conductor is labelled PE. Combined neutral and earth conductors (PEN conductors, used in TN-C systems) are labelled PEN. Always check the diagram's legend to confirm the labelling convention used.

What does a star connection look like on a wiring diagram?

A star (Y) connection shows three load components or windings connected at one end to a common neutral point, with the other end of each connected to a phase conductor (L1, L2, L3). The neutral point may or may not be connected to the system neutral, depending on the system configuration. On a diagram, star connections are often represented by the letter Y or a Y-shaped symbol. Balanced star loads draw equal current from each phase. Unbalanced star loads (common in building services) result in neutral current.

How do I read relay and contactor diagrams?

Relay and contactor control circuit diagrams typically show the coil and contacts separately. The coil is the element that energises when current flows; the contacts are shown elsewhere in the circuit, controlled by the coil. Normally open (NO) contacts are shown as a gap between two lines — they open the circuit until the coil is energised. Normally closed (NC) contacts are shown with a diagonal line across the contact gap — they close the circuit and open when the coil is energised. Each contact symbol is labelled with the relay or contactor reference (e.g., K1) to link it back to its coil.

Why do some wiring diagrams show conductors in different colours?

Conductor colours on wiring diagrams represent the harmonised cable core colours specified in BS 7671 and IEC 60446. In the UK: brown = line/phase L1, black = line/phase L2, grey = line/phase L3, blue = neutral, green-and-yellow = protective earth. On older drawings, the pre-2004 UK colours may be shown: red = phase, black = neutral, green-and-yellow = earth. Diagrams using non-standard colours should have a colour legend. When connecting to equipment with conductors marked in non-harmonised colours, confirm polarity using a test instrument — never rely on colour alone where there is any ambiguity.

TECHNICAL GUIDE

How to Read a Wiring Diagram: Symbols, Single-Line Diagrams and Three-Phase Explained

Understanding electrical wiring diagrams is a core skill for any electrician or electrical engineer. This guide explains the difference between schematic, wiring, and single-line diagrams; IEC vs BS symbol sets; common symbols for switches, motors, transformers, and relays; how to read single-line and three-phase diagrams; and practical tips for working from drawings on site.

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13 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1There are three main types of electrical diagram used in the UK: schematic (circuit) diagrams showing how components are connected electrically, wiring diagrams showing physical connections between components, and single-line (one-line) diagrams used for power distribution systems.
2The UK traditionally uses BS symbols derived from BS EN 60617 (now aligned with IEC 60617). Most modern UK electrical drawings use IEC symbols, which are the international standard. Both symbol sets appear on UK drawings, and electricians need to recognise both.
3A single-line diagram (SLD) represents three-phase power systems using a single line per phase for clarity. It shows the distribution hierarchy from supply to load, protective devices, transformer ratings, and bus arrangements. SLDs are essential for commercial and industrial electrical work.
4Reading a wiring diagram requires identifying the supply point, tracing the circuit from supply through the protective device to the load, identifying switching arrangements, and noting the conductor colours and sizes specified.
5Three-phase diagrams show all three phases and neutral (and sometimes earth). Understanding star (Y) and delta connections, phase labelling (L1, L2, L3), and how three-phase loads are distributed across phases is essential for commercial and industrial work.

01 · Technical Guide

Types of Electrical Diagram: Schematic, Wiring, and Single-Line

Understanding the difference between types of electrical diagram is the first step to reading them confidently. Each type serves a different purpose and is used in different contexts.

Schematic diagram (circuit diagram) — shows how components are connected electrically, using standardised symbols. Does not show physical layout. Used for understanding circuit operation and for fault-finding. Components are represented by symbols, not their physical appearance.
Wiring diagram — shows the physical connections between components, including which terminals wires connect to, cable routes, and sometimes the physical location of components. Used for installation and maintenance. More practical but less clear than schematics for understanding circuit logic.
Single-line diagram (SLD) — represents three-phase power distribution systems using one line per circuit for clarity. Shows the full distribution hierarchy from incoming supply through switchboards to final circuits. Used in commercial and industrial electrical engineering.
Block diagram — shows the functional blocks of a system and how they relate to each other, without detailed component or connection information. Used for system overviews and communication with non-technical stakeholders.

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02 · Technical Guide

IEC Symbols vs BS Symbols: Which Standard Applies?

UK electrical drawings use symbols from two overlapping standards. Understanding which applies and how they relate prevents confusion when reading older or internationally produced drawings.

IEC 60617 (current standard) — the International Electrotechnical Commission standard for graphical symbols for electrical diagrams. The UK adopted this as BS EN 60617. Modern UK electrical drawings use IEC 60617 symbols. This is what you will encounter on current drawings from UK and international engineers.
Legacy BS symbols — older UK drawings (pre-2000s) may use BS 3939 symbols, which differ from IEC 60617 in some respects. The most commonly encountered differences are in switch and relay symbols, motor representations, and transformer winding notation. When reading older drawings, check for a legend.
Always check the drawing legend — any drawing that uses non-standard or project-specific symbols should include a symbol legend. If no legend is provided, assume IEC 60617 for modern drawings and BS 3939 for older ones. When in doubt, clarify with the designer.

03 · Technical Guide

Common Electrical Symbols Explained

The following are the most frequently encountered symbols on UK electrical drawings, based on IEC 60617 / BS EN 60617.

Switch (single-pole) — a line with a diagonal break, typically with a small arc or angle. The diagonal indicates the switch can break the circuit. A two-pole switch has two of these in parallel, connected by a dashed line (indicating they operate together).
Circuit breaker (MCB) — a rectangle with a diagonal line inside (or in some conventions, the standard switch symbol with a specific annotation). On SLDs, circuit breakers are shown as a square with a diagonal break, sometimes with the rating annotated (e.g., "32A B-type").
Transformer — two circles side by side (representing the primary and secondary windings) connected to the circuit. Star and delta connections are indicated by Y and triangle symbols respectively. Transformer ratios (e.g., 11kV/415V) are typically annotated alongside.
Motor — a circle with the letter M inside. Three-phase motors typically show three input lines. DC motors may show a circle with M and arrows indicating direction of rotation. The motor rating (kW) and supply voltage are usually annotated.
Relay coil — a rectangle or circle labelled with the relay reference (e.g., K1, CR1). The coil is the electromagnetic component that operates when energised. Relay contacts (controlled by the coil) are shown separately in the circuit, linked by the same reference.
Fuse — a rectangle with a line through it (sometimes shown as a small rectangle or diamond). The fuse rating is annotated. BS 88 HRC fuses, BS 1361 fuses, and BS 3036 rewirable fuses all use the same generic symbol but may be annotated differently.
Earth symbol — three horizontal lines of decreasing length, stacked vertically (or sometimes a single vertical line terminating in three shorter lines). The protective earth (PE) symbol is connected to all exposed conductive parts and metalwork.

04 · Technical Guide

Reading Circuit (Schematic) Diagrams

A circuit diagram shows the electrical path from supply to load. Reading it systematically, from supply through protective device to load and back to supply, is the most reliable approach.

Step 1: Identify the supply — find the supply terminals (usually at the top or left of the diagram). Note the voltage and frequency. On a control circuit, this may be a 230V supply or a stepped-down 24V or 12V supply from a control transformer.
Step 2: Identify protective devices — fuses, MCBs, or RCBOs in series with the circuit. Note their ratings. These define the maximum current the circuit can safely carry.
Step 3: Trace the current path — follow the line from supply through the protective device, through any switching devices, through the load component, and back to the neutral or return. Identify all series and parallel elements.
Step 4: Identify control elements — switches, relays, and contactors that control when the load operates. On control circuits, a series of normally open and normally closed contacts creates logic conditions that must all be satisfied before the load can operate.

05 · Technical Guide

Reading Wiring Diagrams

A wiring diagram shows which physical terminals are connected to which conductors. It is the most directly useful diagram type for installation and fault-finding on site.

Terminal numbering — terminals on components are numbered or lettered in the wiring diagram. Wire references (e.g., W1, W2, or using a wire numbering system) link the terminal on one component to the terminal on another. Follow the wire reference to find where it goes.
Cable references — cables are typically labelled with a reference (e.g., C1, C2) that matches the cable schedule. The cable schedule specifies the cable type, size, core colours, and length. Cross-reference the wiring diagram with the cable schedule for full information.
Multi-core cable representation — a thick line with a number indicating the number of conductors (e.g., a line with "4c" means a 4-core cable). Individual cores within the cable may be shown branching from the main line to their respective terminals.

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06 · Technical Guide

Reading Single-Line Diagrams (SLDs)

Single-line diagrams (SLDs) are the primary tool for communicating the structure of a power distribution system. They are used in commercial buildings, industrial facilities, data centres, and any installation with a distribution hierarchy more complex than a simple domestic consumer unit.

Read from top to bottom — SLDs typically show the incoming supply at the top (DNO cutout, transformer, or site incoming point) and cascade downward through the distribution hierarchy to final circuits and loads. The "parent" board is above its "child" sub-boards.
Busbar representation — horizontal lines represent busbars (copper bars distributing supply to multiple circuit breakers). The main LV busbar in a switchboard is shown as a thick horizontal line with circuit breakers connecting to it vertically.
Protective device ratings — each circuit breaker, fuse, or switch on the SLD is annotated with its rating (e.g., "400A 4P ACB" for a 400-amp 4-pole air circuit breaker). Cable sizes between boards are also annotated (e.g., "4 x 240mm CWA SWA").
Transformer representation — on SLDs, transformers are shown as two circles side by side with winding notation (e.g., Dyn11 for delta primary, star secondary, 11 o'clock vector group). The primary and secondary voltages and transformer rating (kVA) are annotated.

The electrical design guide covers SLD production and interpretation in more detail for those working in commercial electrical design.

07 · Technical Guide

Reading Three-Phase Electrical Diagrams

Three-phase diagrams show all three phase conductors, neutral (if present), and earth. They are used for motor starters, three-phase distribution boards, and three-phase load connections.

Phase labelling — UK drawings label phases L1, L2, L3 (or the older R, Y, B using red, yellow, blue colours). On three-phase diagrams from European manufacturers, phases may be labelled L1, L2, L3 or 1, 2, 3. American-convention drawings may label phases A, B, C. Always check the legend.
Star (Y) connection — three loads or windings connected at a common neutral point. Each load connects between a phase conductor and neutral. Phase voltage (230V in a 400V system) is applied across each load. Used in balanced and unbalanced resistive and inductive loads.
Delta connection — three loads connected in a closed triangle between phase conductors (no neutral connection). Line voltage (400V in a 400V system) is applied across each load. Used in three-phase motors and transformers. Higher current per winding than star configuration.
Star-delta starter — a common three-phase motor starting arrangement. The motor starts in star (reduced voltage across each winding, lower starting current) and is automatically switched to delta after a set time (full voltage, full power). Shown on diagrams as two contactors (K2 star, K3 delta) plus the main contactor (K1).

The single phase vs three phase guide explains the practical difference between supply types and when three-phase is required for UK installations.

08 · Technical Guide

Practical Tips for Reading Electrical Drawings on Site

Reading electrical drawings on site in a practical installation context requires a different approach to reading them at a desk. The following habits save time and reduce errors.

Start with the drawing schedule — a drawing schedule lists all drawings in the package and their revision status. Always confirm you have the current revision of each drawing before working to it. Installing to a superseded drawing is a common cause of rework.
Cross-reference SLD to board schedule — the SLD gives the big picture; the board schedule gives the detail (circuit numbers, protective device ratings, cable sizes, load descriptions). Read both together.
Mark up drawings as you work — use a pencil or marked-up PDF to record what you have installed, connections made, and any deviations from the drawing. Your mark-ups form the basis of as-fitted (red line) drawings.
Always verify with instruments — never assume a circuit is correct based on the drawing alone. Use a multifunction tester to verify polarity, continuity, and insulation resistance after installation. Drawings can contain errors, and site conditions can deviate from design.

09 · Technical Guide

For Electricians: Diagrams, Testing, and Certification

The ability to read and work from electrical diagrams is a core competency for any qualified electrician. It underpins accurate installation, effective fault-finding, and complete certification.

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After installation, the circuit details from your drawing feed directly into your test documentation and certificate. Use the Elec-Mate certificate app to enter circuit information, test results, and board details on your phone before you leave site.

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Frequently Asked Questions About Reading Electrical Diagrams

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How to Read a Wiring Diagram: Symbols, Single-Line Diagrams and Three-Phase Explained

Key Takeaways

Types of Electrical Diagram: Schematic, Wiring, and Single-Line

Get the BS 7671 A4:2026 Cheat Sheet — free

IEC Symbols vs BS Symbols: Which Standard Applies?

Common Electrical Symbols Explained

Reading Circuit (Schematic) Diagrams

Reading Wiring Diagrams

Try Elec-Mate free for 7 days

Reading Single-Line Diagrams (SLDs)

Reading Three-Phase Electrical Diagrams

Practical Tips for Reading Electrical Drawings on Site

For Electricians: Diagrams, Testing, and Certification

Record Your Circuits Accurately

Study Centre for Electrical Theory

Certify your installation work with Elec-Mate

Frequently Asked Questions About Reading Electrical Diagrams

What is the difference between a schematic diagram and a wiring diagram?

What electrical symbols do UK drawings use?

What is a single-line diagram and when is it used?

How do I identify phases on a three-phase diagram?

What does a star connection look like on a wiring diagram?

How do I read relay and contactor diagrams?

Why do some wiring diagrams show conductors in different colours?

Continue reading

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Single Phase vs Three Phase

Continuity Testing R1+R2

Transformer Installation Guide

EICR Certificate App

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