---
title: "Cable sizing model"
slug: "cable-sizing-model"
updated: 2026-03-24T12:48:40Z
published: 2026-03-24T12:48:40Z
---

> ## Documentation Index
> Fetch the complete documentation index at: https://kb.solargis.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Cable sizing model

**In this document**

This document describes the sequential methodology for the **Solargis cable sizing model**, used for choosing and sizing optimum medium and high voltage power cables within a PV power plant design. It details the technical criteria, mathematical equations, and international standards used to ensure reliable electrical performance through a structured calculation process.

## Overview

The **Solargis cable sizing model** assists PV designers in selecting the most effective medium voltage (MV) and high voltage (HV) cables by simulating real-world technical and thermal conditions. Configuring these parameters is vital for minimizing energy transport losses and ensuring system safety. For a detailed explanation of technical terms used in this methodology, please explore our [Glossary](/v1/docs/solargis-glossary).

The model is built upon the following international standards and industry references (linked in the [Further reading](/v1/docs/cable-sizing-model#further-reading) section):

- **IEC 60287 series**: Standards for calculating current ratings and active power losses.
- **IEC 60502 series**: Specifications for power cables with extruded insulation for rated voltages up to 30 kV.
- **IEC 60038**: Defined international standard voltages.
- **IEC 60228**: Standards for conductors of insulated cables.
- **Electric Cables Handbook by G.G. Moore (BICC Cables)**: Used for thermal coefficients and inductive reactance calculations.

## Power cable types and formations

Before calculations begin, the designer must first define the physical cable configuration within the Solargis cable sizing model from a range of XLPE-insulated options designed for ground burial:

- **MV cables**: These can be selected as single-core (aluminum or copper) in trefoil or flat formations, or as three-core cables that are either armored or unarmored. The rated voltages are $U_{0}$/$U$($U_{m}$) = 1.8/3 (3.6) kV to 18/30 (36) kV.
- **HV cables**: These are specified as single-core aluminum or copper cables, arranged in either trefoil or flat formations. The rated voltages are $U_{0}$/$U$($U_{m}$) = 26/45 (52) kV to 289/500 (550) kV.

where:

- $U$: The rated voltage between conductors for which the cable is designed.
- $U_{0}$: The rated voltage between conductor and earth or metallic screen for which the cable is designed.
- $U_{m}$,: The maximum value of the "highest system voltage'' for which the equipment may be used. More information in IEC 60038.

![](https://cdn.document360.io/ae2d502f-6c0d-4865-a68e-43ad8da61149/Images/Documentation/image-D9GU2VYC.png)

***Figure 1****: Comparison of three single-core cables in trefoil formation (left), three single-core cables in flat formation (middle), and one three-core cable (right).*

#### Supported installation configurations

- **Single-core cables** (aluminum or copper) in trefoil or flat formations.
- **Three-core cables** (aluminum or copper), which may be armored or unarmored.

> **Note**: Cable types are specified by the number of parallel circuits, conductor material, cross-section, insulation, voltage rating, and armoring. For example: 5 x (3 × 1c) Cu 95 mm2, XLPE, 18/30(36) kV, unarmored.

![](https://cdn.document360.io/ae2d502f-6c0d-4865-a68e-43ad8da61149/Images/Documentation/image-1INOGLPJ.png)

***Figure 2:****Five parallel three-phase circuits consisting of three one-core cables in trefoil formation.*

## Technical criteria for cable sizing

With the cable type defined, the Solargis cable sizing model executes a series of mandatory technical checks; a cable is only considered properly sized if it passes every step in this sequence. If any criterion fails, the process must be reset by increasing the conductor cross-section or the number of parallel cables.

The technical criteria are:

- [Current carrying capacity (ICCC)](/v1/docs/cable-sizing-model#current-carrying-capacity-iccc)
- [Voltage drop](/v1/docs/cable-sizing-model#voltage-drop)
- [Short circuit (withstand capacity)](/v1/docs/cable-sizing-model#short-circuit-withstand-capacity)
- [Power loss](/v1/docs/cable-sizing-model#power-loss)

### Current carrying capacity (ICCC)

The first step is to ensure the cable can handle the load. The derated capacity ($I_{ccc,derated}$), which is the capacity adapted to actual technical conditions using correction factors, must be greater than the nominal current ($I_{n}$).

$100 \cdot \frac{I_{ccc,derated}}{I_{n}} > 100$

where:

- $I_{ccc,derated}$: The current carrying capacity of a power cable recalculated (adapted) to actual technical conditions (derated with correction factors).
- $I_{n}$: The nominal current: the current flowing via cable (s) at reference conditions.

### Voltage drop

Next, the system calculates the phase-to-phase voltage drop ($\Delta V_{\%}$) must be less than the maximum allowed percentage voltage drop ($\Delta V_{\%max}$) set by the user.

$\Delta V_{\%} < \Delta V_{\%,max}$

where:

- $\Delta V_{\%}$: The calculated phase-to-phase voltage drop in percentage.
- $\Delta V_{\%max}$ : The maximum allowed phase-to-phase voltage drop in percentage.

### Short circuit (withstand capacity)

The third check verifies safety during faults. The short-circuit current rating ($I_{ccc}$,) of the cables must exceed the expected maximum thermal short-circuit current for a 1-second fault ($I_{th, 1sec, max}$).

$100 \cdot \frac{I_{sccr}}{I_{th,1sec,max}} > 100$

where:

- $I_{sccr}$: The short circuit current rating of power cable (s).
- $I_{th, sec, max}$: The expected maximum thermal short circuit current for 1sec fault.

### Power loss

The final verification step ensures efficiency by checking that the calculated active power loss ($\Delta P_{\%}$) caused by the nominal current is lower than the maximum power loss percentage ($\Delta P_{\%max}$) defined by the user.

$\Delta P_{\%} < \Delta P_{\%,max}$

where:

- $\Delta P_{\%}$: The calculated active power loss in percentage.
- $\Delta P_{\%,max}$: The maximum allowed active power loss in percentage.

## Mathematic methods

The underlying algorithm of the Solargis cable sizing model translates these criteria into results by applying mathematical models derived from the **IEC 60287** and **IEC 60502** series.

### Current derating and correction factors

The calculation of the actual current capacity ($I_{ccc, derated}$,) is achieved by taking the base capacity ($I_{ccc}$,) and sequentially applying correction factors for ground temperature ($K$1,) , depth of laying ($K$2,), soil thermal resistivity ($K$3,), and cable proximity ($K$4,).

$I_{ccc,derated} = K_{1} \cdot K_{2} \cdot K_{3} \cdot K_{4} \cdot I_{ccc}$

where:

- $K$1: Ground temperature.
- $K$2: Depth of laying.
- $K$3: Soil thermal resistivity.
- $K$4: Groups of circuits in proximity (thermal interaction).

### Voltage drop calculation

The absolute voltage drop for $n$ parallel cables over length $L$ is calculated using the complex impedance $Z$ :

$ΔV = \frac{L \cdot |\sqrt{3} \cdot (R \cdot I \cdot \cos(\phi) + X_{L} \cdot I \cdot \sin(\phi) + i(X_{L} \cdot I \cdot \cos(\phi) - R \cdot I \cdot \sin(\phi)))|}{n}$

where:

- $\Delta V$: The total line-to-line voltage loss from the source to the load.
- $L$: The one-way distance of the cable (km).
- $R$: The AC resistance of the conductor ($\Omega/km$ ).
- $I$: The load current (A).
- $X$L: Inductive reactance; The opposition to current flow caused by the magnetic field around the AC conductor.
- $n$: The number of parallel conductors per phase. If you have two cables per phase, $n=2$, which effectively halves the total impedance.

> **Note**: This formula represents the highest voltage drop that can occur at any power factor.

### Short circuit rating (Adiabatic method)

The model calculates the withstand capacity $90°C$ based on the conductor material, cross-section, and the duration of the fault (assumed as 1 second):

$I_{sccr} = n \cdot \left( \frac{K \cdot S}{\sqrt{T}} \cdot \sqrt{\ln\left(\frac{\theta_{1} + \beta}{\theta_{0} + \beta}\right)} \right)$

where:

- $I_{sccr}$: The maximum permissible RMS thermal current (in Amperes).
- $K$: Constant (226 for copper, 148 for aluminum).
- $S$: Conductor cross-sectional area.
- $T$: The length of time (in seconds) the fault persists before a breaker or fuse trips.
- $\theta_1$: The maximum allowed temperature during a fault (e.g., $250°C$ for XLPE insulation).
- $\theta_0$: The operating temperature of the cable before the fault (e.g., $90°C$ for XLPE ).
- $\beta$: A constant related to the zero-resistance temperature of the material (for Copper, $\beta = 234.5$; for Aluminum, $\beta = 228$).
- $ln$: Accounts for the non-linear way resistance increases as the wire gets hotter during the explosion of energy.

### Default acceptance criteria for voltage selection

The following tables detail the default acceptance criteria and voltage selection logic used by the model.

| Category | Max voltage drop | Max thermal short-circuit current | Max active power loss |
| --- | --- | --- | --- |
| **MV Cables** | 1.0% | 25 kA / 1 s | 0.5% |
| **HV Cables** | 0.8% | 40 kA / 1 s | 0.3% |

> **Note**: The model automatically selects the rated voltage ($U_{m}$) based on the electrical network's nominal voltage ($U_{n}$) in compliance with IEC 60038.

## Resistance and impedance

Calculations also involve determining DC resistance ($R_{DC20}$) and AC resistance at $90°C$ ($R_{AC90}$) , factoring in skin and proximity effects per **IEC 60287-1-1**. Inductance ($L$) is then derived from the conductor formation and axial spacing to complete the impedance profile.

**Reference values and default settings**

View Rated Voltages, Default Criteria, and Correction Factor Tables

**Voltage Selection (based on IEC 60038)**

- The Solargis cable sizing model algorithm selects $U_{m}$based on the network $U_{n}$. For example, a $13.8 kV$ network is assigned an $U_{b}$of $17.5 kV$ .

**Default acceptance criteria**

- **MV**: Max voltage drop $1.0 %$ %; Max short-circuit $25 kA/1s$ ; Max power loss $0.5$in % (V).
- **HV**: Max voltage drop $0.8$in % (V); Max short-circuit $40 kA/1s$ ; Max power loss $0.3$in % (V).

**Correction Factors (K1 - Ground Temperature)**

- At $30°C$ ground temperature, the factor for both **MV**and **HV**cables is 0.93.

## Usage in Solargis platform

The Solargis cable sizing model is used in [Energy system designer](/v1/docs/energy-system-configurator), an integrated component of the [Solargis Evaluate](/v1/docs/evaluate-overview) application.

## Further reading

- ["Electric cables - Calculation of the current rating - Part 1-1"](https://webstore.iec.ch/en/publication/68118): IEC 60287-1-1
- ["Electric cables - Calculation of the current rating - Part 3-1"](https://webstore.iec.ch/en/publication/33164): IEC 60287-3-1
- ["Power cables with extruded insulation - Part 1"](https://webstore.iec.ch/en/publication/64510): IEC 60502-1
- ["Power cables with extruded insulation - Part 2" (6 kV to 30 kV)"](https://webstore.iec.ch/en/publication/64511): IEC 60502-2
- ["IEC standard voltages"](https://webstore.iec.ch/en/publication/153): IEC 60038
- ["XLPE Land Cable System User's Guide, Rev 5"](https://www.scribd.com/doc/209629085/XLPE-Cable-Systems-Users-Guide-ABB): ABB
- ["Electric Cables Handbook, 3rd Edition"](https://www.wiley.com/en-us/Electric+Cables+Handbook%2C+3rd+Edition-p-9780632040759): G.G. Moore, BICC Cables