Technical Drawing, Schematics and Trade Calculations
Technical Drawing, Schematics and Trade Calculations tests your ability to read wiring diagrams and symbols, follow DIN terminal numbering, and calculate current, voltage drop, cable size and fuse rating. You must use the diagram key and show the calculation, not guess.
Core rules
- Learn common symbols: battery, fuse, relay coil and contact, switch, lamp, motor, ground and connector.
- DIN 72552 terminal numbers include 30 battery positive, 15 ignition positive, 31 ground, 85 and 86 relay coil, 87 output and 87a changeover.
- Wiring diagrams show physical routing; schematics show electrical function; block diagrams show system flow.
- Wire colours and codes vary by manufacturer; always use the diagram key, not guesswork.
- Trade calculations include Ohm's law, power, voltage drop, cable size and fuse rating.
Worked example
A 20 A load is 6 m from the battery, using 2.5 mm² copper cable with resistivity 0.0175 Ω·mm²/m. Positive run resistance R = ρL/A = 0.0175 × 6 / 2.5 = 0.042 Ω. Drop = 20 × 0.042 = 0.84 V. If the return is a 6 m wire instead of the chassis, total length is 12 m and drop is 1.68 V, which is excessive for a 12 V circuit. Include the return path in calculations.
Traps
Forgetting the return path. Reading relay pin 87a as the main output. Mixing mm² and mm diameters. Using the wrong wire colour code. Calculating with 12 V when the circuit runs at about 14 V charging voltage.