Welcome to cable calculator.


Within this site you will be able to check your designs against our simple cable calculator .
This program will be able to design a circuit from start to finish meeting all the criteria of the IET and BS 7671 the 18th edition. Knowledge base There are some articles in the knowledge base that will help the budding designer and some downloadable technical sheets from the download section.  

Design criteria

An installation design must start with the supply characteristics including the earthing arrangements. In the UK and most parts of Europe this information is available from the Electricity distributor.All other details are to be provided by the designer. Earthing arrangements fall broadly into 3 types: TN-S Supply has a separate earth (typically the cable sheathing). TN-C-S (Protective Multiple Earthing) the supply has a combined neutral and earth. TT. No earth provided by distributor, earthing achieved with earth rods locally. Declared supply characteristics in the UK (More or less universal in 50 Hz regions) Earth fault loop impedances:- 0.35Ω for PME supplies TN-C-S systems. 0.8Ω for separate earth supplies TN-S systems. 21Ω where no earth is provided TT systems. Prospective Fault Current I pf Will normally be given at 16kA and is sometimes higher in inner city areas. 

The changes in ENA ER P23/2:2018
ENA ER P23/2:2018, contains a set of values based on PD IEC/TR 60725:2012
Consideration of reference impedances and public supply network impedances for use in determining disturbance characteristics of electrical equipment having a rated current ≤ 75 A per phase.
The quoted values for existing supplies up to 100 A are shown in Table 2.
Table 2: Values of Ze for existing 230 V single-phase supplies not exceeding 100 A according
to EA ER P23/2:2018
Ze Comment
(0.25+j0.23) 90% of premises will have an EFLI below this value.
(0.46+j0.45) 98% of premises will have an EFLI below this value.
Note 1: These are typical maximum values and the measured value of Ze will change depending on the network configuration due to alterations, faults, maintenance and any embedded generation.

cable calc graphic

Note 2: Distribution network operators (DNOs) are under no obligation to design or maintain networks to provide a particular maximum value of Ze.

Many DNOs do, however, have their own internal quality and design standards,which often align with the Ze values in Table 1.
Note 3: The values for new networks are available from the DNO.
Note 4: The typical maximum Ze -S service
with a capacity of up to 100 A.
What to use for calculations in installations with single phase supplies up to 100 A
The IET recommends that designers continue to use the values in Table 1 of this article, for public TN supplies up to 100 A, unless alternative recommendations are provided on enquiry with the DNO. It is recognised that this will now, as previously, provide an over-estimate for many installations, but this helps safeguard the design against future changes in
transmission and distribution systems, and when embedded generation is added.
The IET also recommends using a calculation value for maximum prospective fault current of 16 kA for single phase supplies up to 100 A where the service cable exceeds 2 m in length, unless the DNO recommends an alternative value.
BS 7671:2018 requires that Ze and Ipsc are ascertained by measurement or another valid method during initial and periodic verification, to validate that disconnection times can be achieved for automatic disconnection of supply.

These recommendations will be reflected in the suite of IET guidance publications that accompany the 18th Edition of the IET Wiring Regulations, including:

The On-Site Guide (IET Wiring Regulations 18th Edition)

Student's Guide to the IET Wiring Regulations, 2nd Edition 2018
 Guidance Note 5:
Protection against electric shock, 8th Edition 2018

Guidance Note 6:
Protection against over current, 18th Edition 2018

Electrical Installation Design Guide, 4th Edition 2018

Further information can be found in the relevant guidance publications.

Why is Ze important for designers?

In design practice, the circuit length for a given cable cross-sectional area (csa) is limited by
one of the following constraints:

1. The maximum Zs for the circuit protective device used for fault protection to achieve
disconnection times proscribed by Table 41.1 of BS 7671.

2. The adiabatic criterion in relation to protection against damage due to over current faults in accordance with Regulations 434.5.2 and 543.1.3

3. Voltage drop.

The first of these constraints requires some consideration of Ze., and the last relates to
prospective fault current Ipsc.

The earth fault loop impedance Zs for a radial final circuit is given by:

𝑍𝑆 = 𝑍𝑒 + (𝑅1 + 𝑅2) , where

(𝑅1 + 𝑅2) = 𝐿𝑆 𝐶𝑟("𝑅1 + 𝑅2" ), and
Ls is the maximum length of the radial circuit in m to achieve automatic disconnection in accordance with Table 41.1 of BS 7671, and
("𝑅1 + 𝑅2" ) 𝑖𝑠 the “volt-drop” resistance in mW/m of the cable from Appendix 4 of BS 7671.

Zs is the maximum earth fault loop impedance Z41 from the relevant Table 41.2, 41.3,
41.4, or, where Regulation 411.4.9 applies, Table 41.5.

Substituting these values, we see that for radial final circuits 𝐿𝑠 = 𝑍41−𝑍𝑒

Similarly, for ring final circuits, 𝐿𝑠 = 4(𝑍41−𝑍𝑒)
("𝑅1 +𝑅2′′)𝐶𝑅

Note: See Section 2.6 of the IET’s
Electrical Installation Design Guide for a more in-depth discussion on these equations, and examples of how they are used.

With the exception of Ze all of these values are available for the designer to adjust by selection, and using data from the relevant tables in BS 7671. We can see that, as Ze increases, the maximum length of the circuit that will achieve the required disconnection time decreases.

The distribution system itself undergoes changes over time. There are two key issues that should be considered:

1. When some TN-S distribution systems are repaired, this must be done with combined neutral and earth (CNE) cable. In this case, additional earth electrodes are installed in the distribution system, and the distribution becomes, at least in part, TN-C-S. For this reason, it might be recommended to treat TN-S installations in the same manner as TN-C-S installations; however, they may still have a Ze > 0.35 Ω

2. Other network changes may mean that values Ze and prospective fault current Ipsc measured at the time of the previous verification and validation have changed over time. ©IET.

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