Earthing Cable Sizing Calculator (IEC/IEEE)

This calculator determines the minimum required cross-sectional area for protective earthing (grounding) conductors based on fault current, fault duration, and conductor material properties. It applies the adiabatic equation as per international standards like IEC 60364-5-54 and principles from IEEE Std 80.

Fault Current Parameters

Conductor Material & Temperature

Earthing Cable Sizing Results

Parameter Value

A Comprehensive Guide to Earthing (Grounding) System Design

Properly sizing a Protective Earth (PE) conductor, or "earthing cable," is one of the most critical aspects of electrical safety design. Its sole purpose is to provide a low-impedance path for fault current to flow back to the source, allowing a protective device (like a circuit breaker or fuse) to operate almost instantly, de-energizing the circuit and preventing electric shock or fire. An undersized earthing conductor can act like a fuse itself—vaporizing during a fault and leaving the system dangerously live and ungrounded.

This calculator uses the adiabatic equation, the internationally accepted method described in standards like IEC 60364-5-54 and IEEE Std 80, to determine the minimum required size. This guide explains the principles behind the calculation.

1. The Core Principle: The Adiabatic Equation

The term "adiabatic" assumes that during the very short duration of a fault (e.g., 0.1 to 1 second), all the heat generated by the fault current (I²R losses) is absorbed by the conductor itself, with no time to dissipate into the surrounding environment. This is a worst-case scenario calculation.

The formula is:

$$ S = \frac{I \times \sqrt{t}}{k} $$

2. Deep Dive: The All-Important 'k' Factor

The '$k$' factor is not just a simple constant; it is calculated from the fundamental thermal and electrical properties of the conductor material. The calculator derives this value for you using the full formula:

$$ k = \sqrt{ \frac{Q_c \times \ln \left( \frac{\beta + T_f}{\beta + T_i} \right)}{\rho_{20} \times (\beta + T_i)} } $$

This formula precisely calculates how much current a conductor can withstand before its temperature rises from $T_i$ to $T_f$ in one second.

Typical 'k' Values (from IEC 60364-5-54)

While the calculator computes the exact '$k$' value, standards provide tables for common scenarios. These values are what you would use for a quick manual calculation.

Material Insulation / Condition Initial Temp ($T_i$) Final Temp ($T_f$) '$k$' Factor
Copper PVC Insulation 30°C 160°C 143
Copper XLPE/EPR Insulation 30°C 250°C 176
Copper Bare (Bolted joints, limit 300°C) 30°C 300°C 192
Aluminum PVC Insulation 30°C 160°C 95
Aluminum XLPE/EPR Insulation 30°C 250°C 116
Steel Bare (Bolted joints, limit 300°C) 30°C 300°C ~58

3. Key Parameters Explained

Prospective Fault Current ($I$)

This is the maximum root mean square (RMS) current that would flow in the event of a "bolted" short circuit (a solid, zero-impedance connection) at the point of installation. This value is critical and must be accurate. It is typically determined by:

Fault Duration ($t$)

This is the total clearing time of the upstream protective device (circuit breaker or fuse). A common misconception is to use the device's instantaneous trip time (e.g., 0.02s). However, many breakers have a "short-time" delay to ensure coordination, so a time of 0.1s, 0.2s, or even 0.5s is often more realistic and conservative. As you can see from the $\sqrt{t}$ in the formula, a longer duration significantly increases the required cable size.

Initial ($T_i$) and Final ($T_f$) Temperatures

  • Initial ($T_i$): This is not always ambient temperature. If the PE conductor is part of a multi-core cable, its initial temperature is assumed to be the cable's maximum operating temperature (e.g., 70°C or 90°C). For a separate, bare earthing conductor, ambient temperature (e.g., 30°C) is a safe assumption.
  • Final ($T_f$): This is the critical limit. It is NOT the melting point of the metal. It is the temperature at which the conductor's *insulation* is permanently damaged. For PVC, this is 160°C. For XLPE, it's 250°C. For bare conductors, the limit is often set by the jointing method (e.g., bolted joints may fail above 300°C).

4. Practical Considerations & Conductor Types

The formula gives a minimum size based on thermal withstand. Codes often add further requirements:

Mechanical Strength

Many standards (like IEC 60364) mandate a minimum size for mechanical robustness, regardless of the calculation. For example, a PE conductor may be required to be at least 2.5 mm² (if mechanically protected) or 4 mm² (if unprotected) simply so it doesn't break easily, even if the formula calculates a smaller size.

Corrosion

An earthing conductor buried in soil must resist corrosion for decades. This influences material choice. Bare copper is excellent but expensive. Galvanized steel is common for earth grids (like in substations) but is less conductive. Copper-clad steel offers a balance of conductivity and corrosion resistance.

Cable vs. Flat Bar/Strip

Choosing between a round cable and a flat bar (busbar) depends on the application:

  • Cables (Round): Highly flexible and easy to install in conduits and trays. They are the standard choice for equipment grounding and PE conductors within circuits.
  • Flat Bars/Strips: Offer a larger surface area for their cross-section, which is excellent for heat dissipation. They are mechanically very strong and are the preferred choice for main earthing busbars (MEBs), substation earth grids, and carrying very high fault currents (e.g., > 50kA).

5. IEEE Std 80 vs. IEC 60364

While this calculator uses the same fundamental adiabatic equation found in both standards, it's important to know their primary focus:

  • IEC 60364: Primarily focuses on low-voltage (LV) and medium-voltage (MV) electrical installations *within buildings and facilities*. Its goal is the safety of personnel and property from shock and fire.
  • IEEE Std 80: Primarily focuses on high-voltage (HV) *AC substation grounding*. While it uses the same formula for conductor sizing, its main concern is the design of the entire earth grid to manage dangerous "step" and "touch" potentials during a fault.

For sizing a PE conductor for a motor, panel, or transformer within a commercial or industrial building, the IEC 60364 approach (as used by this tool) is the standard method.