Read orthographic views, scales, dimensions, tolerances, schematics, GD&T callouts and floor plans, then sketch and model parts in free CAD tools.

Technical Drawing & CAD Basics

## Why it matters

Electricians install from drawings, mechanics troubleshoot machines from them, and electrical engineers design with them. The same circuit can be drawn several ways, each for a different job. Knowing which kind of drawing you are holding and what its symbols mean is the first step in any electrical task.

> Reading diagrams is safe; working on circuits is not. Troubleshooting energised equipment requires training, qualification and your employer's procedures (see Workplace Safety & Tools).

## Four kinds of electrical drawing

| Drawing | Shows | Used for |
|---|---|---|
| **Schematic** | how components connect electrically, using symbols, laid out for clarity, not physical position | understanding and troubleshooting a circuit |
| **Wiring (connection) diagram** | physical layout: terminals, wire colours and numbers, where each wire lands | building or rewiring a panel or device |
| **Ladder diagram** | control logic between two vertical power rails, one rung per circuit | motor controls, relays, PLC logic |
| **One-line (single-line) diagram** | power distribution with one line per circuit, regardless of phases | services, switchboards, transformers |

Building plans also have **electrical floor plans** with symbols for receptacles, switches and lights (lesson 6).

## Common schematic symbols

Symbols follow standards such as IEEE 315 / ANSI Y32.2 (US) and IEC 60617 (international), so the same part can look different on a European print.

- **Battery / DC source:** long and short parallel lines; the long line is positive.
- **AC source:** a circle with a sine wave.
- **Resistor:** a zigzag (US) or a small rectangle (IEC).
- **Capacitor:** two parallel plates; one curved for polarised types.
- **Inductor / coil:** a series of loops.
- **Diode:** a triangle pointing to a bar (current flows in the direction of the triangle); an **LED** adds two arrows pointing out.
- **Switch:** a line hinged away from a contact point. **Normally open (NO)** is drawn open; **normally closed (NC)** is drawn closed.
- **Push button:** a bridge contact with a plunger; NO and NC versions.
- **Fuse:** a rectangle with a line through it, or an S-curve. **Circuit breaker:** a curved contact.
- **Lamp:** a circle with an X.
- **Motor:** a circle with M.
- **Transformer:** two coils side by side with lines (core) between them.
- **Ground / earth:** a set of decreasing horizontal lines. Chassis ground and signal ground have their own variants.
- **Relay:** a coil (often a circle or rectangle with a label like CR1) plus contacts elsewhere on the drawing with the **same label**.

### Wires and junctions

A **dot** where lines meet means the wires are connected. Lines that simply cross with no dot are **not** connected. (Some older drawings use a small hump to show a crossing.)

### Labels and reference designators

Each component has a **reference designator**: R1, C3, F2, M1, CR1, S1. Wires often have **wire numbers** that match the labels on the real wires, which is how you trace a schematic onto a panel.

### Shelf state

Schematics show contacts in their **de-energised, at-rest ("shelf") state**. A relay's NO contact is drawn open even though it closes when the coil is powered.

## Worked example: a simple DC circuit

A 12 V battery's positive terminal goes through fuse **F1** (5 A), then switch **S1**, then lamp **L1**, and back to the negative terminal.

Read it as a loop: + → F1 → S1 → L1 → −. With S1 closed, current flows and the lamp lights. If L1 is dark with S1 closed, the likely suspects in order are: blown F1, faulty S1, failed lamp, open wiring. The schematic tells you where to test, with the right meter and under the right procedure.

## Worked example: a start/stop ladder rung

Two vertical rails, L1 (left) and L2 (right). One rung, from left to right:

`L1 — [STOP: NC push button] — [START: NO push button] — (M coil) — L2`

with an **M auxiliary NO contact** drawn in parallel around the START button.

- Press START: current flows through STOP (closed) and START to the motor starter coil M. The motor runs.
- The M auxiliary contact closes and "seals in" the circuit, so the motor keeps running when you release START.
- Press STOP: the NC contact opens, M drops out, and the seal-in contact opens.

This three-wire control circuit appears in almost every industrial machine and is a classic exam and interview question.

## Common mistakes

- Treating a schematic as a map of where parts physically sit.
- Reading crossing lines as connected.
- Forgetting that contacts are shown in shelf state.
- Missing that a relay's coil and contacts can be on different pages; follow the label.

## Recap

- Schematic = logic; wiring diagram = physical connections; ladder = control logic between rails; one-line = power distribution.
- Know the core symbols; dots mean connected; contacts are drawn de-energised.
- Trace circuits as loops from source to load and back.


Read common electrical symbols and trace a simple schematic, wiring diagram and ladder diagram.

Electrical symbols and schematics

How to Read a Schematic

Schematic Diagrams & Symbols, Electrical Circuits - Resistors, Capacitors, Inductors, Diodes, & LEDs

## Why it matters

Plus/minus tolerances control size, but they do a poor job of controlling **form** (is it flat?), **orientation** (is it square?) and **location** (is the hole where it should be relative to the surfaces that matter?). **Geometric Dimensioning and Tolerancing (GD&T)** solves this with a compact symbolic language. Mechanical engineers, machinists, inspectors and maintenance mechanics all meet it on prints.

In the US the standard is **ASME Y14.5** (current edition 2018); internationally, ISO GPS standards such as ISO 1101 cover similar ground with some differences. Check which one the title block references.

## The feature control frame

A GD&T requirement is written in a **feature control frame**, a row of boxes attached to the feature:

`[ ⌖ | Ø0.2 Ⓜ | A | B | C ]`

Read it left to right:

1. **Geometric characteristic symbol** (here ⌖, position).
2. **Tolerance zone**: Ø means the zone is a cylinder 0.2 mm in diameter; no Ø means a zone between two parallel lines or planes.
3. **Material condition modifier** (optional): Ⓜ maximum material condition (MMC), Ⓛ least material condition (LMC). No modifier means "regardless of feature size".
4. **Datum references** in order of priority: primary A, secondary B, tertiary C.

"The axis of this hole must lie within a 0.2 mm diameter cylinder at MMC, located relative to datums A, B and C."

## The characteristics, by group

| Group | Characteristics | Datum needed? |
|---|---|---|
| Form | straightness, flatness, circularity, cylindricity | no |
| Orientation | perpendicularity ⊥, parallelism ∥, angularity ∠ | yes |
| Location | position ⌖ | yes |
| Profile | profile of a line, profile of a surface | usually |
| Runout | circular runout ↗, total runout | yes |

ASME Y14.5-2018 lists 12 characteristics; older editions also had concentricity and symmetry, which you will still see on older prints.

## Datums

A **datum** is a theoretically exact reference: a plane, axis or point. The **datum feature** is the real surface on the part that establishes it, marked with a **datum feature symbol** (a letter in a box attached to a triangle on the surface, for example A).

The order in the frame matters. A typical part sits on datum A (the primary plane, three points of contact), is pushed against B (two points), and then against C (one point): the **3-2-1 principle**. Inspection fixtures copy this setup.

## Basic dimensions

Dimensions in a **rectangular box** are **basic**: theoretically exact values with no tolerance of their own. The GD&T frame supplies the tolerance. A hole located by boxed 25 and 40 from datums B and C, with a position tolerance of Ø0.2, must have its axis inside a 0.2 mm cylinder centred at exactly (25, 40).

## Worked example: bonus tolerance at MMC

A hole is sized Ø10.0 to Ø10.2 with position Ø0.2 Ⓜ to A, B, C.

- For a hole, **MMC** is the smallest size (most material left): Ø10.0.
- At MMC, the position tolerance is Ø0.2.
- If the hole is made at Ø10.1, it is 0.1 away from MMC, and that departure is added as **bonus tolerance**: Ø0.2 + 0.1 = **Ø0.3**.
- At the largest size, Ø10.2, the position tolerance becomes Ø0.4.

This makes sense functionally: a bigger hole can be slightly further out of place and a bolt will still fit. That is why Ⓜ is common on clearance holes.

## Other mechanical symbols

- **Threads:** metric `M8×1.25-6H` (8 mm nominal, 1.25 mm pitch, tolerance class 6H for an internal thread); unified inch `1/4-20 UNC-2B` (1/4 in, 20 threads per inch, coarse, class 2B internal). Threads are usually drawn simplified, with a dashed or thin line for the thread depth.
- **Surface texture:** a check-mark symbol with a value, for example Ra 1.6 (μm), meaning average roughness. A bar across the top means material must be removed by machining.
- **Welding symbols (AWS A2.4):** a reference line with an arrow; symbols below the line apply to the arrow side, above the line to the other side. A triangle is a fillet weld.
- **Chamfer:** `2 × 45°`. **Fillet/round:** `R3`.
- **Section and detail views:** a circle labelled DETAIL B, scaled up, for small features.

## Common mistakes

- Reading the datum order as alphabetical rather than as written in the frame.
- Forgetting that boxed (basic) dimensions have no plus/minus.
- Treating the Ø in the frame as the hole size; it describes the shape of the tolerance zone.
- Applying bonus tolerance when there is no Ⓜ modifier.
- Mixing ASME and ISO rules on one print.

## Official source

- ASME Y14.5 Dimensioning and Tolerancing: https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing

Checked on 2026-10-23.

## Recap

- A feature control frame reads: characteristic, zone (Ø = cylindrical), modifier, datums in priority order.
- Groups: form, orientation, location, profile, runout.
- Basic dimensions are exact; the frame gives the tolerance.
- At MMC, a hole gets bonus tolerance as it grows from its smallest size.


Recognise common GD&T callouts, datums, threads and surface finish symbols on mechanical drawings.

Mechanical and GD&T symbols

What is GD&T? A Simple Explanation

Understanding GD&T

## Why it matters

Buildings and civil works are built from sets of drawings, sometimes hundreds of sheets. Electricians lay out boxes and runs from floor plans, civil engineers design grading and utilities on site plans, and mechanics and engineers use them to find equipment rooms and access routes. Reading the set quickly, and knowing which sheet to open, is a core site skill.

## How a drawing set is organised

Sheets are grouped by discipline, usually with a letter prefix (the US National CAD Standard uses this convention):

| Prefix | Discipline |
|---|---|
| G | general (cover, index, code summary) |
| C | civil (site, grading, utilities) |
| L | landscape |
| S | structural |
| A | architectural |
| M | mechanical (HVAC) |
| P | plumbing |
| E | electrical |
| FP | fire protection |

A sheet like **E-201** is electrical, floor plans series, sheet 1. Start with the **sheet index** on the cover and the **general notes and legend** sheets, which define the symbols used in that set.

## Floor plans

A **floor plan** is a horizontal section: imagine cutting the building about 4 feet (1.2 m) above the floor and looking down.

- **Walls:** thick lines (cut), often with hatching or wall-type tags that refer to a wall schedule.
- **Doors:** a gap in the wall with a thin arc showing the swing, and a door tag that refers to a door schedule.
- **Windows:** thin lines within the wall, with window tags.
- **Stairs:** a series of lines with an arrow and **UP** or **DN**; a break line where the cut passes through.
- **Room names and numbers.**
- **Dimension strings:** usually the overall dimension on the outside, then openings and walls inside it. Check the general notes for whether dimensions are to the face of stud, the centre of walls or the face of finish.
- **Grid lines:** lettered and numbered column lines in bubbles (A, B, C... and 1, 2, 3...) for locating everything: "the panel is at C-4".
- **North arrow** and **scale** (often 1/8" = 1'-0" or 1/4" = 1'-0").
- **Section and detail markers:** a circle split in two, with the detail number on top and the sheet number below. "3 over A-501" means detail 3 on sheet A-501.

### Electrical floor plan symbols (typical)

- **Duplex receptacle:** a circle with two short parallel lines; **GFCI** written next to it for ground-fault protected outlets.
- **Switch:** an **S**; **S3** is a three-way switch.
- **Ceiling light:** a circle, often with a fixture letter that refers to the lighting fixture schedule.
- **Panelboard:** a rectangle, often filled, with its name (LP-1).
- **Home runs:** an arrow with a circuit number to the panel.

Always check that set's legend: symbols vary between firms.

## Site plans

A **site plan** shows the building in its surroundings, at an engineering scale such as 1" = 20'.

- **Property lines** with bearings and distances (for example N 45°12' E, 150.00').
- **Setbacks** and **easements** (areas where you may not build, or where utilities have rights).
- **Existing contours** (dashed) and **proposed contours** (solid), with elevations.
- **Spot elevations** at corners, curbs and drains, and the **finished floor elevation (FFE)** of the building.
- **Utilities:** water, sanitary sewer, storm drainage, gas, electric and telecom lines, with structures such as manholes and catch basins.
- **Paving, parking, sidewalks**, and the **benchmark** that elevations refer to.
- **North arrow**, scale, and legend.

### Worked example: slope from contours

Two proposed contours, 101.0 and 100.0 (1 ft interval), are 25 ft apart measured on the plan.

Slope = rise / run = 1.0 / 25 = 0.04 = **4%** falling away from the building. Steeper ground shows contours closer together.

### Worked example: finding a detail

The site plan shows a catch basin with a marker "5 / C-501". Open sheet C-501 and find detail 5, which shows the basin's construction.

## Reading order for a new set

1. Cover sheet and index.
2. Legends, abbreviations and general notes for your discipline.
3. Site plan to understand the building's position, access and utilities.
4. Floor plans for your discipline, using grid lines to locate work.
5. Sections and details referenced from the plans.
6. Schedules (doors, fixtures, panels, equipment).
7. Revision clouds and addenda: check you have the latest issue.

## Common mistakes

- Using symbols from memory instead of the set's legend.
- Missing a revision cloud (a scalloped outline marking a change).
- Confusing existing and proposed contours.
- Scaling a dimension that is written elsewhere (or one that was resized when printed).

## Recap

- Sets are organised by discipline prefix; start with the index and legends.
- Floor plan = horizontal cut at about 4 ft: walls, doors, windows, stairs, grids, dimensions, section and detail markers.
- Site plan: property lines, setbacks, contours, elevations, utilities; slope = rise / run.


Read the main features of an architectural floor plan and a civil site plan.

Site and floor plans

How to Read a Site Plan - The Basics

How to Read House Plans – Beginner’s Guide (Explained Simply)

Read electrical schematics and wiring diagrams, mechanical and GD&T callouts, and site and floor plans.