Cable Sizing Calculator IEC 60364-5-52

International Cable Sizing Standard

This Cable Sizing Calculator can calculate minimum active, neutral, and earth cable sizes in compliance with the international standard IEC 60364-5-52. It covers all cable types, installation methods, and correction factors in the standards.

This cable sizing standard applies to circuits up to 1,000 V AC or 1,500 V DC.

The calculator follows the cable sizing procedure outlined in IEC Standard 60364-5-52, which is discussed below in detail.

Assumed conditions:

• Ambient air temperature: 30°C
• Soil temperature: 20°C
• Soil thermal resistivity: 2.5 K·m/W
• Standard depth of burial: 0.7 metres (700 mm) The tables' current-carrying capacities are based on this standard burial depth. If the actual depth differs, a correction factor for depth of laying must be applied.
• Installation and grouping: Based on typical International practices

Cable Sizing Overview

The minimum size of a low-voltage power cable is determined for a particular installation based on its ability to satisfy current-carrying capacity, voltage drop, and short-circuit performance/rating.

The minimum cable size will be the smallest cable that satisfies the three requirements.

⚠️There is also a minimum cable conductor size to meet mechanical strength requirements.

1. Current-carrying Capacity

Current carrying capacity, often called ampacity, is the maximum amount of electric current a cable can safely conduct without exceeding its temperature rating. This limit is determined by the cable's conductor material, insulation type, installation method, ambient temperature, and external factors. Constant constant current (100 % load factor) and constant ambient conditions are assumed.

ℹ️ The current-carrying capacity dictates the active cable size for large load currents.

2. Voltage Drop

Voltage drop is the reduction in voltage that occurs as current travels through a cable due to the conductor's inherent impedance. This drop is influenced by factors such as the phases of the supply, the length of the cable, the cross-sectional area, the material, the operating temperature of the conductor, the amount of current flowing, and the power factor of the load it supplies.

ℹ️ The voltage drop dictates the active cable size for long route lengths.

3. Short-circuit Rating

Short circuit rating is the maximum current a cable can safely carry for a very short duration during a fault condition, such as a short circuit, without sustaining damage. The minimum cable size to withstand the short-circuit current is determined based on the actual short-circuit current and the specified period (fault clearing time of the protection obtained from the time-current curve).

ℹ️ The short circuit rating (rarely) dictates the active size for short cable runs carrying a small load current and where the supply’s fault level is high.

Minimum Conductor Size for Mechanical Strength

Conductors must have sufficient mechanical integrity to withstand handling, installation stresses, and environmental conditions without sustaining damage.

Table 52.2 specifies the minimum cross-sectional area of conductors.

Cable Sizing Coordination with Protective Device

A protective device’s primary function is to protect cables from thermal damage due to overload and/or short circuits. Below is the image showing the damage caused by an improperly coordinated circuit protective device and power cables.

The required coordination between the cables and the circuit protection is shown in the figure and explained in the table below.

The following parameters are defined:

Cable Temperature Limits by Insulation Type

The current carried by a power cable system heats it. The conductor temperature is limited to avoid overheating the adjacent insulation layers.

The temperature limits of insulated cables, such as Thermoplastic PVC (up to 70 °C continuous, 160 °C short circuit), XLPE (up to 90 °C continuous, 250 °C short circuit), Mineral with Thermoplastic PVC Insulation (up to 70 °C continuous, 250 °C short circuit), and Mineral Bare (up to 105 °C continuous, 250 °C short circuit), are crucial for ensuring safety, performance, longevity, and regulatory compliance.

The maximum operating temperature of cable insulation determines a cable’s current-carrying capacity by setting the maximum allowable temperature rise during operation. The table below provides the temperature limits of cable conductors of common cable insulation types.

⚠️ The current carry capacity tables of IEC 60364-5-52 are based on the ‘maximum permissible conductor temperatures' given in Table 52.1.

Correction Factors

The international standard IEC 60364-5-52 provides correction factors in Tables B.52.14 to B.52.21.

Refer to these tables for a complete set of correction factors from IEC 60364-5-52.

It’s simple to use correction factors:

• Obtain the current rating based on the standard conditions.
• Determine the factors from the correction factor tables for your installation conditions.
• Multiply all correction factors and then multiply this value by the cable's standard (tabulated) current rating to obtain the derated cable current rating.

Cable sizing calculators may include all relevant correction factors from IEC 60364-5-52

Voltage Drop Calculations

Excessive voltage drop can cause equipment malfunction and failures. Therefore, the standards dictate maximum permissible voltage drop limits. Voltage drop often significantly impacts the minimum cable size and must be calculated accurately.

✅ Accurate voltage drop calculations will result in smaller cable sizes.

Accurate Voltage Drop Equation

Accurately calculating voltage drops results in lower voltage drops, which leads to smaller cable sizes and saves money. The accurate voltage drop equation for three-phase systems is shown below.

ℹ️ For DC voltage drop calculations, the value for reactance X is zero.

Voltage Drop Limits

The voltage drop between the point of supply for the low-voltage electrical installation and any point in that installation must not exceed a percentage of the nominal voltage of the point of supply, as given in Table G.52.1 (summarised below).

When the cable system length exceeds 100 m, these voltage drop limits may be increased by up to 0.005 % per metre beyond 100 m, without this supplement being greater than 0.5 %.

A greater voltage drop may be accepted for motors or other equipment with high inrush currents during starting periods.

Unbalanced Multiphase Circuits

In an unbalanced system, current will be flowing through the neutral conductor as illustrated by the phasor diagram in the figure below:

There are two ways of dealing with voltage drop for unbalanced three-phase circuits:

Conductors in Parallel

Running conductors in parallel is a standard method of increasing a circuit's current-carrying capacity without resorting to excessively large conductors. This technique involves using two or more conductors in parallel to share the electrical load, but it requires careful consideration to ensure safety, efficiency, and compliance with standards.

Using multiple smaller conductors in parallel significantly boosts the circuit's overall current-carrying capacity. Smaller conductors are easier to install, route through conduit, and navigate around obstacles. Parallel conductors provide redundancy; if one conductor fails, the others can continue to carry the load, albeit at a reduced capacity.

⚠️ Parallel conductors require more cables and installation costs than larger or fewer parallel cables.

Requirements

To avoid overheating one or more parallel cables, it is essential to ensure that the grouping of cables does not affect the cooling of each parallel cable or group and that there is equal load current sharing between them. This is achieved as follows:

• Conductors must be made of the same material and have a cross-sectional area.
• Cables follow the same route and have the same length.
• Each parallel cable or group of conductors is effectively bonded at both ends.
• The relative position of active and neutral conductors in and between parallel groups considers the mutual impedances (i.e. separation is symmetrical and consistent, and the phase arrangement is optimal).

ℹ️ IEC 60364-5-52 requires that parallel sets of cables must have a minimum size of 50 mm2 for copper and 70 mm2 for aluminium conductors.

Optimal Phase Arrangements

Proper phase arrangements of multiple single-core cables of multi-phase systems are essential for ensuring balanced load current sharing. The figure below from the standard AS/NZS 3008.1 references the optimal phase arrangements for up to four parallel circuits.

Neutral Cable Sizing

The neutral conductor in IEC 60364-5-52 installations is intended to carry current under normal conditions and must be sized appropriately for the system and load conditions.

Single-phase or multi-phase circuits with phase conductors up to and including 16 mm² copper or 25 mm² aluminium require the neutral conductor to be the same size as the phase conductor.

For multi-phase circuits with phase conductors larger than 16 mm² copper or 25 mm² aluminium, the neutral conductor may be smaller than the phase conductors if the maximum expected neutral current, including any harmonic components, does not exceed the current-carrying capacity of the neutral conductor, and the neutral is protected against overcurrent. The minimum neutral size in this case is 16 mm² copper or 25 mm² aluminium.

⚠️ When there are substantial harmonics, the neutral conductor size may need to be larger than the active conductors. In the case of a multi-core cable, the neutral currents may dictate the active size.

Where there are substantial third harmonic currents (typically when the third harmonic current exceeds 33% of the phase current), the neutral conductor must be at least the same size as the phase conductor, as the neutral current can exceed the phase current in these conditions.

For circuits feeding predominantly balanced multi-phase loads, the neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component, and may be smaller than the phase conductors if this requirement is satisfied.

Summary Table:

Protective Earth Cable Sizing

In low-voltage electrical installations, the protective earth (PE) conductor ensures safety by providing a low-impedance path for fault currents, enabling protective devices to operate promptly. IEC 60364-5-54 specifies requirements for sizing PE conductors to handle thermal and mechanical stresses during faults.

How to Size the Earth Cable

The PE conductor must satisfy three criteria:

1. Earth fault loop impedance: Ensure impedance is low enough for protective devices to disconnect within safe time limits.
2. Current-carrying capacity: Withstand fault currents without overheating (using adiabatic or simplified methods).
3. Mechanical strength: Meet minimum cross-sectional area (CSA) requirements.

Simplified Method

PE conductor size is determined by the phase conductor’s CSA (copper or aluminium):

Notes:

• Minimum PE size: 2.5 mm² Cu (if mechanically protected) or 16 mm² Cu (unprotected or exposed to corrosion).
• Aluminium PE conductors are not allowed.
• TN systems: PE conductors must match phase conductor size if ≤16 mm² Cu.
• TT/IT systems: PE size may be reduced for low fault current levels.
• PEN conductors must be ≥10 mm² Cu or 16 mm² Al.