Cable Sizing Methodology — Multi-Standard Approach

Cable sizing is the process of selecting a conductor cross-sectional area that safely carries the design current while meeting voltage drop limits and fault current withstand requirements. This methodology applies across four major international standards:

• AS/NZS 3008.1.1:2017 — Electrical installations: Selection of cables (Australia/New Zealand)
• BS 7671:2018+A2 — Requirements for Electrical Installations (IET Wiring Regulations, UK)
• IEC 60364-5-52 — Low-voltage electrical installations: Selection and erection of electrical equipment — Wiring systems (International)
• NEC/NFPA 70:2023 — National Electrical Code (United States)

While the fundamental engineering principles are identical across all standards, each defines its own set of installation methods, reference conditions, derating factor tables, and permissible voltage drop limits. A multi-standard cable sizing tool handles these differences by applying the correct tables and methodology for the selected standard.

The design current is the maximum sustained current that the circuit is expected to carry under normal operating conditions. For simple resistive loads:

Single-phase:

I_b = P / (V × PF)     — (Eq. 1)

Three-phase:

I_b = P / (√3 × V_L × PF)     — (Eq. 2)

Where:

For motor circuits, the design current must account for starting conditions and service factor per the applicable standard (e.g., NEC 430.6, AS/NZS 3000 Clause 4.7).

The protective device (circuit breaker or fuse) must have a nominal rating equal to or greater than the design current:

I_n ≥ I_b     — (Eq. 3)

Standard protective device ratings follow preferred values: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630 A.

The selected rating must also satisfy the condition that the cable's current-carrying capacity, after all derating factors, is at least equal to the protective device rating.

The installation method defines how the cable is physically installed, which directly affects heat dissipation and therefore the cable's current-carrying capacity.

Each standard defines its own set of reference installation methods:

Common installation methods include: cable in conduit, cable on tray, cable in free air, cable buried direct, cable clipped direct, and cable in trunking. Each method has a different base current rating due to the thermal environment.

Derating factors reduce the cable's current-carrying capacity to account for real-world installation conditions that differ from the standard reference conditions.

If the ambient temperature differs from the reference temperature (typically 30°C for air, 20°C for ground), a temperature derating factor applies:

k₁ = √((T_max - T_a) / (T_max - T_ref))     — (Eq. 4)

Where T_max is the cable's maximum operating temperature (e.g., 75°C for PVC, 90°C for XLPE), T_a is the actual ambient temperature, and T_ref is the reference ambient temperature.

Standard references:

• AS/NZS 3008 — Table 22 (air), Table 23 (ground)
• BS 7671 — Table 4B1
• IEC 60364 — Table B.52.14, B.52.15
• NEC — Table 310.15(B)(1)

When multiple cables are installed together, mutual heating reduces each cable's current-carrying capacity. The grouping derating factor depends on the number of circuits, their arrangement, and the installation method.

Standard references:

• AS/NZS 3008 — Table 22 (columns vary by method)
• BS 7671 — Table 4C1 to 4C5
• IEC 60364 — Table B.52.17 to B.52.21
• NEC — Table 310.15(C)(1)

For example, with 6 single-core cables on an open tray, the grouping factor might be 0.73, meaning each cable can only carry 73% of its standalone rating.

Cables passing through or in contact with thermal insulation require additional derating because the insulation impedes heat dissipation. Factors range from 0.5 (fully enclosed in insulation) to 0.89 (one side in contact with insulation).

This factor is particularly relevant for domestic installations where cables route through insulated walls and ceilings.

The minimum current-carrying capacity required from the cable is:

I_z ≥ I_n / (k₁ × k₂ × k₃ × ...)     — (Eq. 5)

where I_n is the protective device rating and k₁, k₂, k₃ are the applicable derating factors.

The cable is then selected from the standard's current rating tables to find the smallest cross-sectional area whose tabulated current rating meets or exceeds I_z.

Key cable selection tables:

After selecting the cable size based on current capacity, the voltage drop must be checked to ensure it remains within allowable limits.

ΔV = (I_b × L × Z_c) / 1000     — (Eq. 6)

ΔV% = (ΔV / V_supply) × 100     — (Eq. 7)

Where Z_c is the cable impedance (mΩ/m), L is the route length (m), and I_b is the design current (A).

Typical voltage drop limits:

If the voltage drop exceeds the limit, the cable must be increased to the next standard size. This is the most common reason for cables being larger than the current-carrying capacity alone would require, especially for long cable runs.

The earth fault loop impedance must be low enough to ensure the protective device operates within the required disconnection time (typically 0.4s for socket circuits, 5s for fixed equipment).

Z_s ≤ U₀ / I_a     — (Eq. 8)

Where U₀ is the nominal line-to-earth voltage and I_a is the current that causes the protective device to operate within the required time.

If the calculated loop impedance exceeds the maximum, either the cable earth conductor must be increased or an RCD must be used to achieve the required disconnection time.

The final cable selection is the largest of the sizes determined by:

1. Current-carrying capacity (with derating)
2. Voltage drop limit
3. Earth fault loop impedance
4. Short circuit withstand (I²t)
5. Minimum size for mechanical strength

A professional cable sizing report documents all intermediate calculations, derating factors with their source table references, and clearly indicates which criterion was the governing factor for the final cable selection.

Professional cable sizing tools automate this process by iterating through multiple standard tables, applying derating factors, and documenting every intermediate step with full clause references for audit verification.