Lesson page
Lesson 5: Electrical Calculations and Units
Read the lesson, then answer the multiple-choice practice set below. You get instant right or wrong feedback with explanation.
Electrical Calculations Exam Practice
Lesson Content
This lesson is about exam technique as much as math. Unit conversion is a common place where beginners lose points.
Japanese Key Term
電力（でんりょく / power）
Hiragana
でんりょく
English Meaning
Power
Simple Explanation
Many exam problems are really unit problems. If you can convert, substitute, and keep track of scale, the calculation becomes much easier.
Detailed Explanation
A Class 2 exam problem may combine amps, watts, volts, and ohms in one question. It may also ask about breaker capacity, wire size basics, or voltage drop. Do not rush. Identify the known values, select the formula, and check the units after calculation.
Key Vocabulary
- kW (kilowatt)
- W (watt)
- A (ampere)
- V (volt)
- Ω (ohm)
Formula
1 kW = 1000 Wn1 A = 1000 mAnVoltage drop is often estimated from current, resistance, and cable length.
Worked Example
Convert 2.5 kW to watts: 2.5 × 1000 = 2500 W.
Exam Tips
When the question has mixed units, convert first and then calculate.
Common Exam Trap
Do not leave answers in the wrong scale. A 2500 W device is not 2.5 W.
Site Reality in Japan
On site, correct unit handling matters for load planning, breaker sizing, and equipment selection.
Practice Question
Convert 750 mA to amperes.
Answer
0.75 A
Explanation
Divide by 1000: 750 mA = 0.75 A.
1. What exam skill does this pair of notations test?
[Single-line] DB -> SW -> Lamp [Multi-line] L -> SW -> Lamp -> N
The candidate must translate between simplified and detailed conductor representations correctly.
2. If Outlet 1 fails open at its load, Outlet 2 is usually:
L ---+---(Outlet 1)
+---(Outlet 2)
N ---+---(Outlet 1/2)
In a parallel branch arrangement, one load open-circuit does not necessarily interrupt other branches.
3. Primary purpose of the PE line here is:
PE ------------------------(Metal case) L,N -----------------------(Appliance)
PE is for protective earthing and fault-current safety, not normal operating current.
4. With SW1 open, the expected state is:
L --[SW1]--+--(Lamp A)--N
+--(Lamp B)--N
SW1 is upstream of both branch lamps, so opening it removes supply to both.
5. If the node above has no dot at crossing in exam notation, you should assume:
L ----(Node)----[SW]----(Lamp)
N ------------------------(Lamp)
Standard exam diagram logic treats crossings as non-connections unless explicit junction indication is given.
6. Most likely question theme for this hierarchy is:
DB -> Main breaker -> Branch breaker -> Lighting branch
Hierarchical protective chains are used to test protection role and branch isolation understanding.