Why Correct Cable Sizing Is Critical
Selecting the wrong cable size is one of the most common — and most dangerous — mistakes in electrical installation. An undersized cable can overheat, damage insulation, cause voltage drops that reduce equipment performance, or in the worst case, start a fire. This guide walks through the key factors you must consider when choosing cable size.
Step 1: Determine the Design Current (Ib)
The design current is the maximum current the circuit will carry under normal operating conditions. For a single-phase load:
I = P ÷ V
Example: A 2,300 W load on a 230 V single-phase circuit:
I = 2,300 ÷ 230 = 10 A
For three-phase loads: I = P ÷ (√3 × VL × PF)
Step 2: Select a Protective Device Rating (In)
The protective device (fuse or circuit breaker) rating must satisfy: Ib ≤ In ≤ Iz, where Iz is the cable's current-carrying capacity. Choose the next standard rating above the design current (e.g., 10 A design current → 16 A breaker).
Step 3: Apply Correction Factors
The tabulated current capacity of cables assumes specific installation conditions. Apply correction factors for:
- Ambient temperature (Ca) — higher temperatures reduce capacity
- Grouping (Cg) — cables bunched together run hotter
- Thermal insulation (Ci) — cables in insulation are less able to dissipate heat
Corrected capacity: Iz = Tabulated value × Ca × Cg × Ci
Step 4: Check Voltage Drop
A voltage drop that is too large causes equipment to malfunction. The standard limit is typically 5% of the nominal voltage for the total installation (often 3% for lighting, 5% for power).
For a single-phase circuit, voltage drop is approximately:
ΔV = (2 × L × I × R/m) / 1,000
Where L = cable length (m), I = current (A), R/m = conductor resistance per metre (mΩ/m).
Common Cable Sizes and Current Ratings (Reference)
| Cross-Section (mm²) | Single-Core in Conduit (A) | Typical Application |
|---|---|---|
| 1.5 | 16 A | Lighting circuits |
| 2.5 | 25 A | Socket outlets, small appliances |
| 4.0 | 32 A | Shower, cooker circuits |
| 6.0 | 40 A | Large cookers, EV chargers |
| 10.0 | 57 A | Sub-main distribution |
| 16.0 | 75 A | Large sub-mains |
Note: These are general reference values only. Always consult applicable standards and manufacturer datasheets for your specific installation.
Step 5: Verify Short-Circuit Capacity
The cable must withstand the maximum prospective short-circuit current for the time it takes the protective device to operate. This is especially important on sub-main cables close to the supply transformer.
Practical Checklist
- ✅ Calculate design current (Ib)
- ✅ Select protective device rating (In)
- ✅ Determine installation method and apply correction factors
- ✅ Choose cable with Iz ≥ In (after correction)
- ✅ Check voltage drop is within acceptable limits
- ✅ Verify short-circuit rating is adequate
- ✅ Document your calculations for inspection
Key Takeaway
Cable sizing is not simply a matter of picking "the next size up." It requires systematic calculation considering load, installation environment, distance, and protection. When in doubt, always consult the relevant national standard (in Japan: JIS C 3605 and related standards) and have qualified personnel verify your design.