Cable Sizing Guide

1. Why Cable Sizing Matters

Incorrect cable sizing is the leading cause of electrical fires in commercial and industrial installations. A cable that is too small overheats under load, degrades insulation, and can ignite surrounding materials. Conversely, an oversized cable wastes copper, increases installation cost, and adds unnecessary weight to cable trays.

Every cable in an electrical installation must be sized to satisfy three independent criteria:

1. Current Carrying Capacity (Ampacity) — the cable must safely carry the design current without exceeding its thermal rating
2. Voltage Drop — the cable must deliver adequate voltage at the load terminals (IEC limit: 3% lighting / 5% power)
3. Short-Circuit Withstand — the cable must survive fault current for the duration of the protective device clearing time

The final cable size is the largest cross-section that satisfies all three checks.

2. Step-by-Step Cable Sizing Procedure (IEC 60364)

Step 1: Determine Design Current (I_b)

The design current is the maximum sustained current the circuit will carry under normal operating conditions. For motor circuits, include starting current multipliers. For circuits with harmonic distortion, apply appropriate derating.

• Single-phase: I_b = P ÷ (V × PF)
• Three-phase: I_b = P ÷ (√3 × V_L × PF)

Step 2: Select Protective Device Rating (I_n)

Choose the next standard protective device rating above I_b. The relationship must satisfy: I_b ≤ I_n ≤ I_z, where I_z is the cable's current carrying capacity.

Step 3: Identify Installation Method

IEC 60364-5-52 defines installation methods that determine the cable's heat dissipation capability:

Step 4: Apply Derating (Correction) Factors

The tabulated current capacity must be reduced by multiplying all applicable correction factors:

I_z = I_tab × C_a × C_g × C_i × C_s

Step 5: Verify Voltage Drop

After selecting a cable based on ampacity, verify the voltage drop is within limits:

Vd = (mV/A/m × I_b × L) ÷ 1000

Where L is the one-way cable route length in metres. IEC limits are typically 3% for lighting circuits and 5% for power circuits, though local regulations may differ.

Use our free Voltage Drop Calculator to check this instantly.

Step 6: Check Short-Circuit Withstand

Use the adiabatic equation: S = √(I²t) ÷ k

Where S is the minimum conductor cross-section (mm²), I is the fault current (A), t is the disconnection time (s), and k is a material constant (copper XLPE = 143, PVC = 115).

3. Cable Sizing Chart — IEC 60364 (Copper, PVC, Method C)

Values are for single circuit, 30°C ambient, PVC/PVC copper conductors clipped direct (Reference Method C per IEC 60364-5-52 Table B.52.4).

4. NEC Cable Sizing (AWG / kcmil)

The National Electrical Code (NEC) Article 310 provides ampacity tables for North American installations. Key differences from IEC include:

• Wire gauge uses AWG (American Wire Gauge) instead of mm²
• Ambient temperature base is 30°C for most conductor types
• Correction factors found in NEC Table 310.15(B)(1) and 310.15(C)(1)

5. Common Cable Sizing Mistakes

1. Forgetting derating factors — Grouping 6 cables together reduces capacity by up to 43%
2. Using wrong installation method — Method A1 vs Method E can differ by 80% in ampacity
3. Ignoring harmonics — Triple-N harmonics in the neutral require separate neutral sizing
4. Not checking voltage drop — A cable passing ampacity may fail voltage drop on long runs
5. Mixing standards — Never combine IEC derating factors with NEC ampacity tables

6. Related Calculators