Adiabatic Equation CalculatorThe Adiabatic Equation.
✔ Calculate Protective Conductor Size mm²
✔ Calculates la
✔ Calculates I²
✔ Calculates k value
✔ Calculates t (s)
Download the Adiabatic Equation Calculator for iPhone, iPad or Android
The Adiabatic Equation Calculator in an app. Calculate the minimum permissible cpc or earth size using the Adiabatic Equation.
Sizing of protective conductors can be achieved by looking up the relevant table or where required by calculation using the Adiabatic Equation.
Adiabatic Equation Symbols
- S = Protective Conductor Size (mm²)
- Ia = Fault Current (A)
- I² = la Squared
- t = Time (s)
- k = Conductor k Value
Conductor k Value Tables
BS 7671 Values of k for Protective Conductors:
- Table 54.2 – Conductor not incorporated in a cable and not bunched with cables.
- Table 54.3 – Conductor incorporated in a cable or bunched with cables.
- Table 54.4 – Sheath or armour of a cable.
- Table 54.5 – Steel conduit or trunking.
Adiabatic Equation Parameters
- Uo Voltage
- Measured Zs Value
- Protective Device Type & Rating
- Conductor Type
- Conductor Material
Disconnection Time Tables
BS 7671 Maximum Operating Time / Current Characteristics Tables:
- Fig 3A2 (a,b) – BS 3036 Semi-enclosed Fuse
- Fig 3A3 – (a,b) – BS 88-2.2
- Fig 3A4 – BS EN 60898 / 61009-1 MCB/RCBO Type B
- Fig 3A5 – BS EN 60898 / 61009-1 MCB/RCBO Type C
- Fig 3A6 – BS EN 60898 / 61009-1 MCB/RCBO Type D
- BS 1361 Fuse
A great app for electricians and inspectors carrying out electrical testing & inspecting for verifying existing protective conductor sizes.
App demoWatch the app in action!
App ReviewsRead what Our Happy Customers Are Saying
All your applications are very useful, quick to operate.
Awesome app so easy to use with correct results..
Saves messing with the maths,
very handy of your Z’s are low.
Adiabatic Equation Calculator
Seem to work spot on,
comes up with the same answer as i calculate on paper.
Minimum csa of cpc
The minimum csa of a cpc can be selected from the appropriate table in BS 7671. In certain circumstances the cpc csa can be calculated using the adiabatic equation for a specific circuit, this will determine the actual minimum csa of a cpc required for the specific circuit in question.
Earthing Cable Size Calculation
The calculation to calculate an earthing cable size (circuit protective conductor size) is the Adiabatic Equation. Earthing cable size selection charts can also be found in BS7671.
What is the Adiabatic Equation?
The Adiabatic Equation is the mathematical equation / formula used to calculate the minimum cpc size, if a non standard cable size is calculated then the next available larger size shall be used.
The preferred method for selecting a circuit protective conductor size is by selecting from the appropriate table in BS 7671.
The Adiabatic Equation might be used to calculate the absolute minimum size of a cpc for an existing installed cpc should that be smaller than that required of the appropriate table.
Adiabatic Equation Example
Here we demonstrate an adiabatic equation example, we will explain how the adiabatic equation calculator works.
1. Enter a measured voltage value Uo = 230V.
The voltage value needs to be a measured value in-order to carry out an accurate calculation. Uo is the voltage to earth.
2. Enter a measured Zs value = 0.26 Ohms.
The Zs value needs to be a measured value in-order to carry out an accurate calculation. Zs is the earth (fault) loop impedance.
3. Select a protective device type = BS EN 60898 Type B.
4. Select a protective device rating = 10 Amps.
5. Select a conductor type = Single cable not bunched.
6. Select a conductor material = Copper – 70°C Thermoplastic.
Adiabatic Calculation Results:
- la (PEFC) = 884.62
- I² = 782552.54
- k = 143
- t (s) = 0.1
- Conductor csa = 1.956 mm²
As 1.956 mm is not a standard cable size the next available larger cable size shall be used.
la is the prospective earth fault current, I² is the fault current squared, k is the conductor k value, t is the disconnection time in seconds and conductor csa is the cpc earthing conductor size in millimetres.
Adiabatic Equation Online Calculator
Our Adiabatic Equation Calculator works online and offline, so it is always available when you need it.
Adiabatic Equation Formula
This Adiabatic Equation calculator uses the Adiabatic Equation formula from BS 7671.
k Factor for Copper Cables
The k factor for copper cables tables can be found in the wiring regulations. The actual k factor table to use depends on the cable conductor type,
- Single cable not bunched
- Incorporated in a cable or run with cables
Other Conductor Types:
- Sheath or armour of cable
- Steel conduit or trunking
Then there is the specification of the conductor material:
- Copper 70°C Thermoplastic
- Copper 90°C Thermoplastic
- Copper 90°C Thermosetting
Other Conductor Materials:
- Aluminium 70°C Thermoplastic
- Aluminium 90°C Thermoplastic
- Aluminium 90°C Thermosetting
- Steel 70°C Thermoplastic
- Steel 90°C Thermoplastic
- Steel 90°C Thermosetting
- Lead 70ºC Thermoplastic
- Lead 90ºC Thermoplastic
- Lead 90ºC Thermosetting
How to Use the Adiabatic Equation Calculator
To calculate the cpc csa size using the Adiabatic Equation Calculator is fairly simple.
The required tables, times and k factors are already programmed and will auto calculate the values depending on your selections.
- Input a measured voltage value
- Input a measured Zs value
- Select a protective device
- Select a protective device rating
- Select a conductor type
- Select a conductor material
- Tap Calculate
The Adiabatic Equation Calculator will then calculate fault current, conductor size mm, k value and disconnection time (s).
Get the AppThe easy way to calculate cpc sizes.
✔ Calculate Protective Conductor Size
✔ Uses BS 7671 Time Tables
✔ Uses BS 7671 k Value Tables
✔ BS 7671 Adiabatic Equation Formula
✔ Works Online and Offline
Download the Adiabatic Equation Calculator for iOS and Android