A cable that looks adequate on a drawing can overheat once it is placed in conduit with several other circuits, run through a hot ceiling space, or extended across a large building. To calculate cable size in Japan correctly, you must check more than the appliance wattage. The conductor must carry the expected load safely, work with its protective device, and keep voltage drop within the limits required for that installation.
For electricians preparing for Japanese qualifications or working alongside a licensed supervisor, cable sizing is a practical skill rather than a calculation to memorise. The exact final selection should follow the current rules, project specifications, and manufacturer data. However, the method below gives you a reliable way to understand and check a cable choice.
Start with the actual load current
Cable size is selected around current, not watts alone. First identify the circuit voltage, the connected load, whether the load is continuous, and its power factor where applicable. In Japan, common supplies include 100 V single-phase, 200 V single-phase, and 200 V three-phase.
For a simple resistive single-phase load, such as a heater, use:
`Current (A) = Power (W) ÷ Voltage (V)`
A 1,500 W heater on 100 V draws 15 A. That result is only the starting point. A circuit serving a motor, air conditioner, or other inductive equipment needs its power factor considered:
`Current (A) = Power (W) ÷ Voltage (V) × power factor`
For three-phase equipment, use:
`Current (A) = Power (W) ÷ (√3 × Voltage (V) × power factor)`
Use the equipment nameplate where possible. It is more dependable than estimating from a product description, especially for motors and packaged air-conditioning equipment. Also consider starting current, demand factors, and any instructions from the equipment manufacturer.
Check Japanese cable markings before comparing sizes
One common source of confusion is that Japanese cable labels do not always express size in the same way as overseas cable schedules. Flexible and stranded conductors are often shown in square millimetres, written as sq. For example, IV 2.0 sq indicates a 2.0 mm² insulated wire.
However, VVF cable, commonly used for indoor fixed wiring, is often identified by the diameter of its solid copper conductor. VVF 1.6 mm and VVF 2.0 mm refer to conductor diameter, not 1.6 mm² and 2.0 mm² cross-sectional area. A 2.0 mm diameter solid conductor has a cross-sectional area of about 3.14 mm².
This distinction matters when reading drawings, ordering materials, or comparing Japanese products with international cable data. Confirm whether the notation refers to diameter, cross-sectional area, number of cores, or all three. For example, `VVF 2.0 mm 2C` means two solid insulated conductors with a 2.0 mm diameter, while an earth conductor may be specified separately.
How to calculate cable size in Japan
The practical sequence is straightforward, but every step affects the next one.
First, calculate the design current from the expected load. Next, select a provisional cable type suitable for the location, such as VVF for an appropriate indoor concealed wiring installation, or IV conductors in conduit. Then check the allowable current, 許容電流 (kyoyō denryū), for that cable under its real installation conditions.
Do not use an ampacity figure in isolation. Tables are based on stated conditions, such as the insulation type, ambient temperature, number of loaded conductors, installation in free air or conduit, and grouping with other circuits. A cable installed alone on a tray can dissipate heat more easily than the same cable surrounded by other loaded cables in a conduit.
After checking ampacity, choose the overcurrent protective device. In domestic and light commercial work this is often a miniature circuit breaker, 配線用遮断器 (haisenyō shadan ki), or an equivalent protective device. The breaker must protect the cable, so its rating cannot simply be chosen to prevent nuisance tripping. The relationship between design current, cable allowable current, and breaker characteristics must comply with the applicable rules.
Finally, check voltage drop, 電圧降下 (den’atsu kōka), particularly on long runs. If voltage drop is excessive, increase the conductor size even if the original cable has sufficient current capacity.
Installation method changes the answer
The installation method, 施設方法 (shisetsu hōhō), is often the reason a seemingly correct cable selection becomes unsuitable. Heat is the main issue. As conductor temperature rises, insulation ages faster and the allowable current must be reduced.
Pay close attention to cable runs in conduit, 電線管 (densenkan), ceiling voids, insulated walls, floor spaces, and vertical risers. Grouping is also critical. Several circuits sharing a conduit or trunking may require a correction factor because each loaded conductor adds heat.
Ambient temperature can have the same effect. A circuit passing through a boiler room, plant area, rooftop enclosure, or poorly ventilated roof space may need a larger cable than the same circuit in a normal indoor room. The cable’s insulation temperature rating also matters. Do not assume that all PVC-insulated cable has identical temperature performance.
Japanese internal wiring practice is guided by the Electrical Equipment Technical Standards and related interpretations, along with the 内線規程 (Naisen Kitei), or Internal Wiring Regulations. On real projects, the electrical contractor, designer, utility requirements, and client specifications can add further requirements. Use the current approved tables for the exact cable and installation arrangement rather than relying on a remembered value from a different job.
Check voltage drop on long circuits
Voltage drop is often missed on small-power circuits because the breaker does not trip and the cable does not feel hot. Yet a long cable with high resistance can leave equipment operating below its intended voltage. Lights may dim, motors may struggle to start, and sensitive equipment may behave unpredictably.
For a simple copper-conductor estimate, single-phase voltage drop can be expressed as:
`Voltage drop = 2 × length × current × conductor resistance per metre`
The factor of two accounts for the outgoing and return conductors. For a balanced three-phase circuit, the common simplified relationship is:
`Voltage drop = √3 × length × current × conductor resistance per metre`
Use the one-way route length, not the total conductor length, when applying these forms. For final design work, use the resistance or impedance values provided for the actual cable, taking operating temperature and AC effects into account where relevant. Larger conductors may be required primarily to control voltage drop rather than ampacity.
There is no sensible universal shortcut such as “always use the next size up”. The permitted voltage-drop approach can depend on the type of installation, supply arrangement, and project standard. Record the calculation and confirm the applicable limit before installation.
A worked example for a 100 V circuit
Suppose a 100 V circuit supplies a 1,200 W fixed resistive appliance. The basic load current is:
`1,200 W ÷ 100 V = 12 A`
You would then identify the intended cable type and installation. If the cable is installed in a normal indoor route with no significant grouping, consult the relevant allowable-current table for that specific cable. If it shares conduit with several loaded circuits, apply the required grouping correction. The corrected allowable current must remain suitable for the 12 A design load and the selected breaker.
Now consider length. If the appliance is at the far end of a long building, a cable that satisfies the current check may still produce too much voltage drop. Calculate the drop using the cable resistance data. If the result exceeds the project allowance, select a larger conductor and repeat both the ampacity and protective-device checks.
This is why cable selection is an iterative process. Load current sets the minimum starting point, but route length and installation conditions may determine the final size.
Common errors to avoid
Avoid selecting a cable from breaker rating alone. A 20 A breaker does not automatically mean every cable associated with a 20 A circuit is acceptable in every installation method. Equally, do not treat a cable’s catalogue ampacity as permanent if the route changes during construction.
Another frequent mistake is overlooking the neutral conductor. On a single-phase circuit, the neutral carries load current and must be considered in the circuit design. In three-phase systems, harmonic-producing loads can make neutral loading significant even where phase loads appear balanced.
Also check termination compatibility. A larger conductor is only useful if it can be terminated correctly in the breaker, terminal block, isolator, and equipment connection. Poor termination increases resistance and heat, defeating the purpose of careful cable sizing.
A good cable calculation gives you more than a number on a drawing. It gives you a defensible decision: the conductor can carry the load, the breaker protects it, the voltage at the equipment is acceptable, and the installation conditions have been accounted for. Build the habit of checking all four points, and your calculations will become safer and more reliable on every Japanese job site.