Cable Size Calculator

Final Circuit Type

Cable Type

70°C Thermoplastic

Cable Type

70°C Thermoplastic

Installation Reference Method

Select Circuit Breaker Type

Is This Motor Calculation

Air Temperature

Cable Grouping

Thermal Insulation

Total Required Load (Ib)

Cable Length In Meters

Required Circuit Braker

Min Cable Size (mm)

Voltage Drop (Volts)

Load in Amps

How To Calculate Cable Size

One of the doubting tasks with cable conductors is selection; it can be quite challenging. However, the current carrying capacity limit must be appropriately calculated while selecting the cable conductors.

If it’s over the limit, then you might have issues due to the operating temperature of the conductor under a specific load.

For instance, if we start putting current through a cable or conductor, it will heat up. The higher the current, the hotter it gets to a certain point when the insulation around the conductor will begin to get damaged and eventually get softer and start to break.

At this point, if you don’t interrupt that current, the insulation will ignite the PVC, which can cause fires.

Therefore, there are different insulations rated for different temperatures, so we have 60 ℃ thermosetting or 70 ℃ thermoplastic PVC and so on and so forth; these are all in Wiring Regs BS7671.

Precautions while installing cables:

In fact, while installing the circuit, we must avoid damaging the cable insulation and operating temperatures, which can melt the conductors themselves.

Usage of Circuit Protective Conductor (CPC) and Circuit Breaker:

For instance, if we are using Twin & Earth Cable, the CPC is the weakest link in the circuit. However, the Circuit Breaker or Fuse protects the circuit, which must be maintained and regularly tested.

However, if it’s not constantly maintained and gets faulty or has any other issue like selecting the wrong size or type in the consumer unit. In that case, We can have a situation where the cable will overheat to a point where the next weakest link is the CPC.

In the event of an earth fault, a larger current will flow down the line and return through the circuit protective conductor (CPC). This can cause the CPC to melt, which is problematic because the conductive parts of the equipment will remain energised due to a broken safety circuit.

How the current flow:

Line and neutral conductors normally carry the current in a circuit.
Under certain fault conditions, the CPC and earth conductor will carry current.

What is Design Current (Ib)?

Under normal use, the current flowing in a circuit can be calculated from the connected loads and is known as the design current(Ib). The design current can be calculated by using the given formula:

Ib=PV

For instance, if you are installing a 7kw shower and would like to calculate the design current (I). First, calculate the basic units of power(P) by 7×1000, which is 7000, then divide it by 230 volts, and we come up with a design.

Ib=P ÷ V

Ib=7000 ÷ 230

Ib= 30.43

Choosing a Circuit Protective Device and Cable size

Circuit protective devices can be a circuit breakers (MCB) or rewireable fuse. Once we know what the design current(Ib) is, we can select the protective device and size of the cable required by choosing certain parameters for the circuit installation methods. There are the following variables.

What to consider for cable selection?

To select the correct cable, we must consider some of the other variables:

It is the rating of an overcurrent protective device for the circuit, for example, MCB.

It is the tabulated current-carrying capacity of the cable, which can be retrieved from Appendix 4 of Wiring Regs BS7176.
The current carrying capacity of the cable for continuous service is installed with correct reference methods and correction factors applied.

Finally, while selecting a cable, we must satisfy the following equation:

Ib ≤ In ≤ Iz

When considering the variables that are mostly related to current In, the rating of overcurrent protective devices is considered. Now, it should be greater or equal to Ib if we got a 30.43 amp electric shower.

Installing a 20-amp breaker would be pointless as it is likely to trip frequently, causing inconvenience. In order to avoid this issue, we should use a breaker with a higher rating of at least 32 amps and, preferably, slightly higher. For an electric shower circuit, a 40-amp breaker would usually be the appropriate choice.

The tabulated current carrying capacity of a cable is an important factor to consider. These values are provided in Appendix 4 of the Wiring Regulations (BS 7671). There are numerous tables in Appendix 4, starting from 4d1, which relate to various types of cables with different insulation and conductors, such as copper and aluminium. It is crucial to select the right table for the cable you’re working with to ensure safe and accurate usage.

The current carrying capacity of a cable for continuous service is known as Iz. It includes all installation conditions and correction factors. In certain situations, the cable may heat up to a higher temperature than normal due to the current it carries, which depends on ambient temperature or whether it is surrounded by lots of lagging or grouping factors that prevent the dissipation of heat. When considering these factors, it is important to ensure that the nominal rating of the device is less than or equal to the rating of the cable.

Factors for cable temperatures:

Ca = the ambient temperature, its header to lose heat in a hot location

Cc = cable buried in the ground; it’s harder to dissipate heat when buried.

Cd = depth of burial; heat dissipation varies with depth of burial.

Cf = semi-enclosed fuses; when used to protect against overload, they are slow!

Cg = grouping. It’s harder to lose heat in a grouping.

Ci = Thermal insulation, heat dissipation restricted by an overcoat.

Volt drop calculation:

The voltage drop is the difference in the voltage measured at the consumer unit, and it is measured at the point of use, meaning at load.

The maximum permitted voltage drop depends on the type of circuit, for instance:

For the lighting circuit, the maximum permitted voltage drop is 6.9V, which is 3% of the supply 230V AC
For all other circuits, the maximum permitted voltage drop is 11.5V, which is 5% of the supply 230V AC

Therefore, while selecting the cable, we must satisfy the permitted voltage drop as well. Which can be calculated by following the formula:

Vd=((mV/A/m) × Ib × L) ÷ 1000

Example: If one wishes to install a 7 KW electric shower by wiring the circuit in Flat Twin & Earth 70 ℃ thermoplastic PVC cable. The total length of the circuit is 23 metres, and the cable is going to run on the top of 100 mm of thermal insulation. The operating temperature can reach up to 38 ℃. Finally, the circuit is protected by a BS EN 60898 Type B Circuit Breaker.

In this example, we got the required variable to calculate the correct cable and protective device. Go ahead and enter the variable in the cable selection calculator to find the result instantly.

Conclusion

When sizing cables, you first need to calculate the expected design current for the circuit based on the connected load. Then, make sure to select an overcurrent protection device that has a slightly higher amp rating to avoid nuisance tripping. Don’t forget to look up the correction factors for your specific cable installation method in the standard tables from Appendix 4 of Wiring Regs BS7176 that adjust the continuous current capacity of your chosen cable. You’ll use this capacity rating to verify that your cable can handle the design current without overheating.

Also, check that the voltage drop along the length of the cable is within the 3-5% acceptable limit. Follow these key steps using the reference standards, and you’ll be able to size and select cables for your electrical installations safely and efficiently.