When is comes to cable sizing, things can get a bit technical.. so rather than risk transcending our readers back to those awful school science classes, we have kept this guide simple and to-the-point.
Now, should you be 'that' way inclined and wish to discuss voltage drop etc in a little more detail, please let us know and one of our engineers can write a more complicated discussion surrounding the subject (we may even start referencing BS7671:2018).
But for the rest of us, let's get started!
We are talking specifically about 230V circuits (for 12V circuits, this guide does not apply).
We are talking about artic rated cable (which is sold in our shop).
We only cover the two most commonly required cable sizes in this guide (1.5mm2 and 2.5mm2).
We are assuming you have 'designed' your circuit, such that you understand what consumer unit you will be using and what size RCD & MCB's will be installed (If we just lost you.. Take a look at our 230v guide "230V From Start to Finish").
Establishing Maximum Allowable Current
Firstly, for the circuit, you need to write down the maximum allowable current. You will get this from the rating of your MCB.
For example (1): If you have an MCB with a 10A rating. The maximum allowable current will be 10 Amps. If the current draw exceeds this, the MCB will 'trip' and cut the power to the circuit.
For Example (2): If you have an MCB with a 16A rating. The maximum allowable current will be 16 Amps. If the current draw exceeds this, the MCB will 'trip' and cut the power to the circuit.
Now that you have established the maximum allowable current for the circuit, we need to establish the current rating of different cable sizes.
Establishing Cable Current Rating
NOTE: In accordance with BS7671:2018, the smallest cable cross sectional area that should be used in Motorhomes and Caravans is 1.5mm2.
We have recorded current rating for the cable cross sectional area in the table below (Note: This table is accurate for the cable that we sell in our online shop. Should you wish to use any other cable, please check the current rating of the cable that you have):
Cable Cross Sectional Area (mm2)
Current Rating (Amps)
Selecting A Cable
NOTE: The cable current rating should always be larger than the maximum allowable current (NOT with an equal current rating).
For Example (3): If your MCB is rated at 16A, the maximum allowable current for the circuit would be 16A. The 1.5mm2 cable would not be appropriate as the current rating is not larger than the maximum allowable current. So, in this case a 2.5mm2 cable would be appropriate as the current rating is larger than the maximum allowable current for the circuit.
Work out your maximum allowable current for the circuit.
Compare the maximum allowable current with the current rating for different size cables.
Select the cable with a current rating larger than the maximum allowable current.
Q) What happens if I undersize my cable?
A) This should not be taken lightly as it has the potential to be extremely dangerous. Using undersized cables has the potential to cause fires. Considering the campervan is a confined space full of combustible materials (including gas cylinders etc), this should be avoided at all costs.
Q) What happens if I oversize my cables?
A) Oversized cables in most cases will not cause any issues with your electrical circuit. Larger cables have lower resistance and will result in more efficient circuits.
All electrical work should be carried out by a competent person and in line with BS7671:2018.
All electrical work should be inspected and tested by a suitably qualified person in line with BS7671:2018 prior to use.
We hope you find this simple cable guide useful!
The Home Built Campers Team.