Table of Contents

 • Calculation
 • Base Conditions
 • Energy Conscious Solution
 • Example


The CableApp uses the correction factors defined in the tables of Appendix 4 of BS7671. This allows the user to tailor a circuit rating for their given prescribed installation. These correction factors cover the following parameters: ambient temperature (air, and ground where appropriate), soil resistivity, depth, proximity of multiple circuits for ladder, tray, direct in ground and in ducts in the ground.

Rating Factor Type Correction Table Reference in BS7671 Applicable Reference Method & Ratings Table(s)
Rating factors (Ca) for ambient air temperatures other than 30°CAmbient Table 4B1 Ref Method C, E & F
Tables: All
Rating factors (Ca) for ambient ground temperatures other than 20°CAmbient Table 4B2 Ref Method D,
Table 4E4A
Rating factors (Cs) for cable buried direct in the ground or in an underground conduit system for soil resistivitites (for Ref Method D) Table 4B3 Ref Method D,
Table 4E4A
Rating Factors (Cd) for depths of laying other than 0.7m for direct buried cables and cables In buried ducts Table 4B4 Ref Method D,
Table 4E4A
Rating factors for one circuit or one multicore cable or for a group of circuits of multicore cables, to be used with current-carrying capacities of Tables 4D1A to 4J4A Table 4C1 Ref Method A, B, C, E or F (appropriately)
Tables 4D1A, 4D2A, 4D5; 4E1A, 4E3A & 4E4A
Rating factors for more than one circuit, cables buried directly in the ground (for Ref Method D, Table 4E4A) Table 4C2 Ref Method D,
Table 4E4A
Rating factors for more than one circuit, cables in ducts buried directly in the ground (for Ref Method D, Table 4E4A) Table 4C3 Ref Method D,
Table 4E4A
Rating Factors for groups of more than one multicore cable, to be applied to reference current-carrying capacities for multicore cables in free air (Ref Method E, for Tables 4D2A, 4D5 & 4E4A) Table 4C4 Ref Method E,
Tables 4D2A, 4D5 & 4E4
Rating Factors for groups of more than one single-core cable, to be applied to reference current-carrying capacities of single-core cables in free air (Ref Method F, for Tables 4D1A, 4E1A & 4E3A) Table 4C5 Ref Method F,
Tables 4D1A, 4E1A & 4E3A

The base conditions for the published current ratings in BS7671 are as follows Top

Base Cable Installation Conditions for Cable Ratings

Parameter Condition
Ambient Air temperature 30°C
Ambient Ground temperature 20°C
Base installation depth (for cables installed in the ground) 0.7m
Base soil resistivity (for cables installed in the ground) 2.5 K.m/W

This CableApp executes voltage drop calculations using the pre-defined values for voltage drop, published in Appendix 4 of BS7671 for the appropriate Cable Type Table.

Cable Type Current Rating Table Reference Volt Drop Table Reference
Single-core 70°C thermoplastic insulated cables, non-armoured, with or without sheath 4D1A 4D1B
Multicore 70°C thermoplastic insulated and thermoplastic sheathed cables, non-armoured 4D2A 4D2B
70°C thermoplastic insulated and sheath flat cable with protective conductor 4D5 4D5
Single-core 90°C thermoplastic insulated cables, non-armoured, with or without sheath 4E1A 4E1B
Single-core armoured 90°C thermosetting insulated cables (non-magnetic armour) 4E3A 4E4B
Multi-core armoured 90°C thermosetting insulated cables 4E4A 4E4B

Energy Conscious Solution Top

The following information provides guidance in energy efficiency, and the calculation method used to provide the Energy Conscious Solution in the CableApp.

The calculation requires the end user to define some of the parameters used in the calculation, within the "settings" menu of the CableApp.

The potential savings should be considered as guidance only.

According to Joules Law, whenever a conductor carry’s current, it will generate heat (thermal energy).

It can be demonstrated that the thermal energy of a cable corresponds to the following general expression:

Ep = n/c • R • L • I² • t/1000


Ep energy generated (lost on the line) [kWh]
n number of loaded conductors (2 for single-phase/dc or 3 for three-phase)
c number of cables per phase
R conductor resistance [Ω / km]
L cable length [km]
I line current [A]
t time [h]

If the cross-sectional area (S) of a cable is increased, there will be a corresponding reduction in the resistance (R). When carrying the same current I, there will be a reduction in the energy lost (EP). This energy saving can be quantified both as a cost saving in electricity bills and a reduction in CO2 emissions.

The cable itself will be more expensive because it will have a higher cross-sectional area (S) but the installer will benefit from the following:

  - Lower running costs, reduced energy bills.
  - Reduced CO2 emissions, therefore an environmentally better proposition.
  - Extended design life for the cable because it is operating at a lower temperature.
    Standard design life is based on the cable being at its maximum load (maximum operating temperature) for every hour of that defined life in years.
  - Improved short circuit capability - larger cross-sectional areas will carry higher currents in a fault condition.
  - Potential to uprate the cable to carry higher loads in the future.

As a rule, cables do not carry the same current (I) continuously. For this reason, it is advisable to consider the mean square value of the current over time or at least to make an estimate.

The CableApp will offer by default the average usage of load (I ') equal to 75% of I, but other values can be selected or defined by the user in the "Settings" of the CableApp

  100% I
  40% I (residential)
  60% I (public place)
  75% I (industrial)
  Other %


Thus, the energy saved (EA) by installing conductors of lower resistance (R2) than (R1) will be:

EA = n/c • (R1-R2) • L • (%U • I)² • t/1000

Having calculated the saved energy, the economic savings can be calculated (A£) and the savings in CO2 emissions since we have defined the electricity tariffs (Energy Price) in £/kWh (in the "settings") and the approximate values of CO2 emissions (ACO2) kg per kWh generated taking account of the country’s energy mix is defined by the CableApp (note, this value cannot be modified by the user and is taken from information published in the UK by the Department for Energy and Climate Change).

Entering the value of the electricity rate and the value of CO2 emissions per kWh will therefore give the savings achieved by installing cross-section conductors with a larger section.

Energy Price 0.14 £ / kWh (this value is defined by the user in "Settings")
CO2 Emissions 0.40 kg CO2 / kWh (UK default value, not possible to edit)


Example Top

Let’s assume we want to carry out an economic and ecological calculation as follows:

System - three-phase; 130 m; 268A
Cable Type - 6491X (Table 4D1)
App Proposed Technical Size - 95mm² copper conductor

To calculate the savings, we must consider increasing the cross-sectional area to a larger size.
The next largest standard cross-section to the 95mm² would be 120mm².

Size Resistance (Ω/km)   Size Resistance (Ω/km)
1.5 14.478  120.0 0.183
2.5 8.866  150.0 0.148
4.0 5.516  185.0 0.119
6.0 3.685  240.0 0.090
10.0 2.190  300.0 0.072
16.0 1.376  400.0 0.056
25.0 0.870  500.0 0.044
35.0 0.627  630.0 0.034
50.0 0.463  800.0 0.026
70.0 0.321  1000.0 0.021
95.0 0.231 

Reference is made in the background of the App to the electrical resistance table (above), calculated at a given average operating temperature. The R value for the next size is selected, in this case 120mm². For the simplicity of the calculation, both conductors are assumed to be operating at the same temperature. In reality, the resistance for the larger conductor will be lower than that given in the table when carrying the same load, because the larger conductor will be operating at a lower temperature. The subsequent savings will be a conservative estimate.

If the calculation is undertaken for an annual usage, then the time (t) will be 365d x 24h = 8760 h.

We will assume the average usage is 75% (default value in the App, but this can be changed in the “settings” menu)

Now you can calculate the energy that can be saved in a year using 120 mm² instead of 95 mm².

EA = (n/c x (R150 - R185) x L x (%U x I')² x t) / 1,000
= (3/1 x (0.231 - 0.183) x 0.13 x (0.75 x 268) ² x 8760) / 1000
= 6625 kWh

In the example, we have defined the chosen installation as a property with a tariff rate of £0.14 / kWh (remember the user should define their Tariff in the "settings").

The App uses a default value for CO2 emission equal to 0.30 kg CO2 / kWh.

Energy Price 0.14 € / kWh (value can be changed in "settings" menu)
CO2 emissions 0.3 kg CO2 / kWh (value proposed by default, not user)
= 6625kWh x £ 0.14 / kWh = £927.54
ACO2 = 4002 kWh x 0.3 kg CO2 / kWh = 1987.6 kg CO2