Overview of the 第二種電気工事士 Written Exam
Japan's Second Class Electrician (第二種電気工事士) exam is the gateway qualification for anyone working on residential and light commercial electrical installations. The written (筆記) exam contains 50 multiple-choice questions, of which roughly 10–15 involve direct calculations. Passing this section is often where candidates lose marks — but with the right approach, it's very manageable.
Which Formulas Are Tested?
The exam consistently tests a core set of calculations. Memorise these:
- Ohm's Law: V = I × R
- Power: P = V × I = I² × R = V² ÷ R
- Series resistance: RT = R1 + R2 + ...
- Parallel resistance: 1/RT = 1/R1 + 1/R2 + ...
- Voltage drop: ΔV = 2rIL (for single-phase 2-wire circuits)
- Power factor: PF = P ÷ (V × I)
- Energy: W = P × t
Voltage Drop Calculations — A Common Trap
Voltage drop questions are frequently mishandled because candidates forget the factor of 2 for single-phase 2-wire circuits (current travels out AND back through two conductors).
Formula: ΔV = 2 × r × I × L
Where r = resistance per unit length of one conductor (Ω/m), I = current (A), L = length (m).
Worked Example
A single-phase 2-wire circuit is 30 m long. The conductor resistance is 0.008 Ω/m per conductor, and the load draws 15 A. Find the voltage drop.
ΔV = 2 × 0.008 × 15 × 30 = 2 × 3.6 = 7.2 V
If the supply is 100 V, this represents a 7.2% drop — which would fail the standard limit and requires a larger cable.
Power Factor Questions
In the exam, power factor problems often involve finding either true power, apparent power, or reactive power given two of the three values.
Example
A single-phase motor operates at 200 V, draws 10 A, and has a power factor of 0.8. What is the real power consumed?
P = V × I × PF = 200 × 10 × 0.8 = 1,600 W = 1.6 kW
Parallel Resistance: The Two-Resistor Shortcut
For exactly two resistors in parallel, use the shortcut formula instead of the reciprocal method:
RT = (R1 × R2) ÷ (R1 + R2)
Example: 6 Ω and 12 Ω in parallel: RT = (6 × 12) ÷ (6 + 12) = 72 ÷ 18 = 4 Ω
This shortcut saves significant time under exam conditions.
Exam Strategy Tips
- Read units carefully — answers may be in W, kW, A, or mA. A missed unit conversion is a lost mark.
- Eliminate obvious wrong answers first — in multiple choice, two of the four options are often clearly unreasonable.
- Sketch the circuit before calculating — even a rough diagram prevents mistakes on complex networks.
- Memorise the VIR and PIV triangles — they quickly show you which formula to use.
- Practice with past papers (過去問) — the exam reuses similar question structures year after year.
Recommended Study Order
| Week | Focus Area |
|---|---|
| 1–2 | Ohm's Law, Power formulas, unit conversions |
| 3–4 | Series and parallel resistors, Kirchhoff's Laws basics |
| 5–6 | Voltage drop, power factor, single/three-phase calculations |
| 7–8 | Past paper practice — timed exam conditions |
Final Advice
Calculation questions are reliable marks — unlike knowledge-based questions where memory can fail you, the correct answer is always derivable from a formula. Invest time in mastering the formulas above, practise with past papers, and you can realistically expect to score full marks on the calculation section.