Watch this step-by-step example of sizing an electrical cable using myCableEngineering. Follow the workflow from entering design information and selecting a cable through to reviewing the suggested size and calculation results.
Duration: 9 minutes 24 seconds. Uploaded to YouTube on 10 September 2026.
What the tutorial covers
- Entering the cable design information.
- Selecting a cable and BS 7671 installation method.
- Reviewing the suggested cable size.
- Checking voltage drop, fault withstand and earth fault loop impedance.
Follow along in myCableEngineering
Open your project and add or select a cable to follow the example. For explanations of the individual inputs and controls, see the Cables software guide. For help setting up a project, see the Projects guide.
New to the application? Explore the cable sizing software or start with myCableEngineering Software Basics.
Video chapters
Select a chapter to watch from that point on YouTube.
- 0:03 — Introduction
- 0:10 — Adding and describing a cable
- 0:30 — Supply system
- 1:19 — Load details
- 1:50 — Protection
- 2:51 — Cable selection
- 3:52 — Installation and current capacity
- 5:48 — Reviewing the suggested cable size
- 6:27 — Voltage drop
- 6:52 — Fault calculations and withstand
- 8:18 — Earth fault loop impedance
- 8:35 — Comments and documentation
- 8:49 — Saving and reporting
Transcript
Transcript based on the recorded voiceover, with punctuation and engineering notation edited for readability.
Adding and describing a cable
Welcome to this tutorial on how to size a cable with the myCableEngineering.com software. To begin, click the Add Cable button. The first part of the edit form lets you describe your cable. Here, you can enter the cable number, its start and end points, and a brief description. In this example, I’ve entered a sample cable that supplies a submain distribution board from the main DB.
Supply system
In the supply system section, you set the voltage and earthing details. It’s important to note that the edit form updates as you make changes. For example, if you switch to DC, the frequency field disappears and the earthing options change. As you go further, the available loads, protective devices, cable types, and sizing standards will also update. This setup guides you through the choices you need to make and helps ensure your cable is designed correctly. You only need to enter the information required for the calculation.
The app uses soil resistivity to calculate symmetrical components and fault levels. This value has a minor effect, so you can leave it at 100 or enter the actual value if you know it.
Load details and protection
Next, fill in the cable load details. The first two options—distribution circuits and final circuits—let you enter the load current for any cable circuit you want to size. If you need to be more specific, you can pick from the available load types. Enter the load current and power factor. The app will calculate the real and apparent power for you. If you only know the real or apparent power, click the edit icon next to the calculated value to enter it, and the app will work out the current.
For cable protection, you can choose from several device types, or select no protective device if needed. A general protective device works for any type, letting you enter the right overcurrent setting. There’s also a section for generic devices, where you can specify the type and possibly enable earth fault loop calculations. For this cable, we’ll choose an MCCB and the right trip frame size. The trip unit is calculated automatically, but you can pick it yourself if you want.
The “Simple Design Current” checkbox lets you size the cable based only on load current, without considering protective-device overcurrent. The calculated information box shows the cable load current, protective device overcurrent, and the resulting design current. It also displays the overcurrent design margin. To reduce this margin, I use an Ir multiplier. You can also add an RCD if needed.
Cable selection
Now it’s time to select your cable. There’s a wide range of generic cables and options from different manufacturers. For this example, I’ll use a generic XLPE armoured cable since I plan to bury it, and I’ll choose a four-core cable. When it comes to cross-sectional area, there are two options. If you already know the cable size and just want to check it, you can select the right CSA. If you need to find the correct cross-sectional area, as we do here, just pick any value for now. We’ll come back to it later. I’ll leave it at 25 mm² for now, even though it’s too small.
You can also choose to bond the armour at both ends or add an external CPC. Your previous choices filter the available cables, including by voltage. If you ever need a cable with a higher voltage rating, check the “Disable Voltage Filter” box to see all cables rated above your selected voltage.
Installation and current capacity
The Installation and Current Capacity section lets you check your selected cable or figure out the right cross-sectional area. By default, User Defined Rating is selected. This allows you to enter the cable’s sustained current rating if you know it. For example, you might have an existing cable with a known rating, or a special installation like a cable in liquefied natural gas, where the rating comes from the manufacturer. Entering the known rating here helps you document the cable with your other cables. The number in parallel and reference temperature are linked to this rating and used in later calculations.
Most of the time, you’ll either check your chosen cable or work out the cross-sectional area. To do this, pick the right sizing standard from the dropdown. In this example, I’ll use BS 7671. For the primary installation method, I’ll choose direct in-ground. For the secondary method, I’ll select armoured with mechanical protection. The circuit length is the distance from the cable’s source to its destination. In our example, that’s from the MDB to the SMDB.
The number of circuits means how many similar circuits run together in the same cable containment. This is used for group derating. For example, you might select 3 circuits, with all cables touching. When you set up the project, you entered default values for air temperature, ground temperature, and soil thermal resistivity. These values are already filled in for your cable. If your cable’s conditions are different, you can update them. You can also enter a User Defined Factor if you want to add extra safety margin or adjust for special environmental conditions.
Reviewing the suggested size
The calculation shows the results of applying the sizing standard to your cable. In this case, the result is red because the selected cross-section is too small. The suggested cable size is 95 mm², so I’ll update my selection to 95. Now the calculation passes. It includes references to the tables used, the calculated sustained current, derating factors, and the estimated conductor temperature.
Voltage drop
The voltage drop calculation displays the absolute and percentage voltage drop, cable resistance and reactance, and the power loss in the cable. The maximum percentage is the voltage drop limit you set based on your design needs. For this submain feeder, I’ll set the limit at 2%. Our cable is well within this limit.
Fault calculations and withstand
For cable fault and withstand calculations, enter the fault source impedance for both short-circuit and earth-fault conditions. Once you enter the resistance and reactance, the software calculates the short-circuit kA, power factor, source impedance Ze, and X/R ratio. Resistance and reactance are the primary inputs, as the software saves these values to the database when you save the cable. More commonly, you will know the fault kA or source impedance Ze. You can enter these by clicking the edit icon, and the software will calculate the resistance and reactance.
For example, I’ll enter a 9.6 kA short-circuit fault with a power factor of 0.6. The calculation displays the cable-end short-circuit and earth-fault levels, showing both the minimum and maximum values using the IEC 60909 C factor. The software also calculates I²t for both short-circuit and earth-fault, and compares them with the cable’s withstand capacity. The calculated 1-second and 3-second cable kA ratings are also provided for reference.
Earth fault loop impedance
The earth-fault loop calculation appears only if needed. For our cable, it’s required, and the calculation shows compliance. If there’s a problem, you might need to use an external CPC or add an RCD. You can also override the default values if necessary.
Comments, saving and reporting
At the bottom of the edit form, you can add comments to give extra documentation for your cable. You can include formatted text, images, tables, and equations. After you’ve sized the cable, remember to save it. You can come back later to update it, try different scenarios, or print reports for your client or end user. That’s the end of the tutorial on how to size a cable using our software app. If you haven’t used myCableEngineering before, or if it’s been a while, visit the site, run the app, and sign in to get free credits to try cable sizing yourself. When you’re ready, you can subscribe and pay only for what you use.
Related guidance
- Cables software guide — detailed help with the cable inputs and controls.
- Voltage drop — the engineering background to the voltage-drop check.
- Fault calculations — supporting calculation guidance.
- Earth fault loop impedance — supporting engineering guidance.