Almost every motorhome and caravan has electricity in the body or living area. For most mobile homes or trailers, 12 volts is the standard. However, 24 volts is also occasionally used in motorhomes, as is common in trucks.
It is always important to use the correct cable cross-section. This article will show you everything you need to know. At the beginning, I wrote down some important basics that you should know. If you already know these, click here to go straight to the cross-section calculation.
Inhaltsverzeichnis
- Why the cable cross-section is important
- Basics
- The right cable
- Calculate cable cross-section – formula
- Cable cross-section calculator 12 Volt
- Individual cable cross-section calculator
- Standardized cable cross-sections according to VDE 0295 IEC 60228 as a table
- Table of cable cross-sections 12 Volt
- Standardized cable cross-sections
- Regulations for 12 volt cable installation
Why the cable cross-section is important
If you send too much current through a cable that is too thin, it will be converted into heat. To a certain extent, this is perfectly acceptable. Every conductor (cable) has a resistance. This converts electricity into heat.
If the cable is much too thin, so much heat can be generated that the insulation melts and fires occur. You should therefore use the correct cable cross-section for your own safety.
Caution: This effect can also occur with poorly implemented transitions between two cables or plugs or cable lugs etc.. Often only after a while. Fires are then also possible. Also note that vibrations can occur in camping vehicles when driving and things can become loose.
With the relatively low voltage of 12 volts, the voltage drops are also particularly important. The longer a line and the lower the average, the more the voltage drops. Let me explain it with an example:
If your TV is connected via a cable that is too thin and too long, the TV may only receive 10 volts instead of 12 volts and may no longer work.
The problem can also occur with USB chargers
I just had a similar problem with my new “big” laptop. I bought four different 12V chargers, but just couldn’t get it to charge. Until a friend gave me the idea of measuring the power of the cables under load.
And lo and behold: under load, i.e. when the new laptop was connected for charging, the voltage drop was so great that the power supply unit could no longer get enough voltage and the laptop would not charge. The charger also became dangerously hot.
I simply installed a new 12V socket with a shorter and thicker cable and returned three out of four chargers. Because from then on, they all worked.
But let’s take a look at why the cable cross-section is so important:
Basics
The aim is always to keep voltage losses as low as possible. These losses are caused by the ohmic resistance of the cables. The electricity generates heat. My aim is to keep losses below 1 percent. Electricity is a valuable resource, especially when free-standing, and I want to waste as little of it as possible. Furthermore, there are critical devices that cannot tolerate low voltages (see my new laptop).
A maximum of 0.25 volts from the charger/solar regulator to the battery is a good value. For critical devices such as televisions and some other electrical devices, 0.5 volts is a sensible value for the maximum voltage drop. That is 4 percent at 12 volts. For loads such as LEDs, lamps or similar, a voltage drop of 0.75 to 1 volt is not normally a problem. I use these values as a guide.
The right cable
You should always use stranded copper wire. This is actually standard. But especially in self-build projects, many people use the rigid cables from house construction. These are not permitted.
Make sure that you do not use cables with rigid cores. These can break due to vibrations when driving. Always buy cables that are intended for motor vehicles. These are known as stranded or flexible cables. Here, the core is made up of many small wires.
Cable length and thickness
Always keep your cables as short as possible. This is the first step in avoiding losses. Keep the cables as short as possible where the highest currents flow. For example, between the battery and solar controller, between the battery and inverter, between the solar controller and solar panels or to power-hungry devices (like my new laptop).
A cable can never be too thick, only too thin. However, thick cables weigh more and are more expensive. It therefore makes sense to calculate the cable cross-section.
info
If you are looking for information on calculating the correct size of a solar system on a motorhome, take a look at this article on Solar system on the motorhome on. Also on the subject of We have a suitable article for batteries in motorhomes.
Cable cross-section and cable diameter
This is a nasty trap that is easy to fall into. I always talk about the cross-section in my article. This is actually also standard. The cross-section is usually specified in mm2 in the automotive and motorhome sector. Nevertheless, cables are sometimes labeled with the diameter instead of the cross-section.
Both are different values. The cross-section indicates the area of a core (area measurement) and the diameter indicates the diameter (length measurement). The cross-section of the copper wire is measured without the insulation.
Calculate cable cross-section – formula
The cable cross-section can be calculated using the following formula:
tip
A (in mm²) = (2 x length x current) / (conductivity of the cable x voltage drop x voltage)
or with abbreviations:
A= ( 2 x L x I) / (58 x fk x U)
- I is the maximum current in amperes
- 58.58 MS/m is the conductivity of copper. For the sake of simplicity, I am calculating without the decimal places. So only with 58.
- L is the cable length in one direction (therefore it is multiplied by 2)
- fk is the loss factor, example: 1 % is 0.01
- U is the voltage
info
What loss factor is sensible?
– For LEDs and normal lamps, 10 % is ok
– For refrigerators, televisions, chargers etc. 4 %
– For solar systems, I recommend a maximum of 1 %
Let’s take the cable between the solar regulator and the battery in the motorhome as an example. I have installed a Steca PR2020. A maximum of 20A comes out of this. The cable to the battery is 2 m long. I would like to have a maximum loss of 1 %. The formula is then:
(2 x 2 x 20) / (58 x 0.01 x 12) = 11.49 mm²
I therefore need a cross-section of 11.49 mm2. The cable cross-sections are standardized to certain sizes. I would therefore have to use a 16 mm2cross-section , as this is the next largest cross-section. You can find the table of standard cable cross-sections here.
Cable cross-section calculator 12 Volt
Simply fill in the fields and the result will be displayed automatically below. We calculate here with 12 volts, which is the standard voltage in motorhomes and cars, and a voltage drop of 2 %, which works best for all devices.
For the power in watts, simply look at the devices that you want to connect to the cable and add the watts to them.
You can now use the following table to find the right cable with the right cross-section. I would generally round up to the next available cross-section.
Individual cable cross-section calculator
The following calculator lets you decide for yourself how much voltage drop you want and what voltage is available in your vehicle. Fill in all the fields and the result will be displayed automatically below the calculator.
You can now use the following table to find the right cable with the right cross-section. I would generally round up to the next available cross-section.
Standardized cable cross-sections according to VDE 0295 IEC 60228 as a table
Here you will find the standardized cable cross-sections according to the above-mentioned standards. In the first column are the cross-sections and in the second column the diameters of the conductors. The third column contains a link to Amazon for a suitable cable.
| IEC 60228 Klasse 5, VDE 0295 | IEC 60228 Klasse 6, VDE 0295 | |||
|---|---|---|---|---|
| Querschnitt | Anzahl der Drähte x | max. Draht-Ø mm | Anzahl der Drähte x | max. Draht-Ø mm |
| 0.14 mm²* | ≈ 18 x 0.11 mm | |||
| 0.25 mm²* | ≈ 14 x 0.16 mm | ≈ 32 x 0.11 mm | ||
| 0.34 mm²* | ≈ 19 x 0.16 mm | ≈ 42 x 0.11 mm | ||
| 0,50 mm² | ≈ 15/17 x 0.21 mm | ≈ 28 x 0.16 mm | ||
| 0,75 mm² | ≈ 23 x 0.21 mm | ≈ 42 x 0.16 mm | ||
| 1,00 mm² | ≈ 30 x 0.21 mm | ≈ 56 x 0.16 mm | ||
| 1,50 mm² | ≈ 27-29 x 0.26 mm | ≈ 84 x 0.16 mm | ||
| 2,50 mm² | ≈ 46 x 0.26 mm | ≈ 140 x 0.16 mm | ||
| 4,00 mm² | ≈ 52 x 0.31 mm | ≈ 224 x 0.16 mm | ||
| 6,00 mm² | ≈ 78 x 0.31 mm | ≈ 186 x 0.21 mm | ||
| 10,00 mm² | ≈ 77 x 0.41 mm | ≈ 320 x 0.21 mm | ||
| 16,00 mm² | ≈ 122 x 0.41 mm | ≈ 504 x 0.21 mm | ||
| 25,00 mm² | ≈ 190 x 0.41 mm | ≈ 760 x 0.21 mm | ||
| 35,00 mm² | ≈ 272 x 0.41 mm | ≈ 1083 x 0.21 mm | ||
| 50,00 mm² | ≈ 400 x 0.41 mm | ≈ 703 x 0.31 mm | ||
| 70,00 mm² | ≈ 543 x 0.41 mm | ≈ 988 x 0.31 mm | ||
| 95,00 mm² | ≈ 484 x 0.51 mm | ≈ 1340 x 0.31 mm | ||
| 120.00 mm² | ≈ 589 x 0.51 mm | ≈ 1680 x 0.31 mm | ||
| 150.00 mm² | ≈ 740 x 0.51 mm | ≈ 2122 x 0.31 mm | ||
| 185.00 mm² | ≈ 902 x 0.51 mm | ≈ 1472 x 0.41 mm | ||
| 240.00 mm² | ≈ 1220 x 0.51 mm | ≈ 1910 x 0.41 mm | ||
| 300.00 mm² | ≈ 1525 x 0.51 mm |
Table of cable cross-sections 12 Volt
Below I have compiled tables for various lengths and cable cross-sections. I have rounded up the values to the standard cross-sections. You can find the table of standard cross-sections here. The cross-sections are given in mm2
12 volts, maximum 1 % voltage drop in the cable (0.12 volts)
| Strom (A) | Leistung (W) | 1 m | 3 m | 5 m | 7 m | 10 m | 20 m |
|---|---|---|---|---|---|---|---|
| 1 | 12 | 0,25 | 0,75 | 1,0 | 1,5 | 2,5 | 4 |
| 2 | 24 | 0,5 | 1,5 | 2,5 | 4 | 4 | 10 |
| 5 | 60 | 1 | 4 | 6 | 10 | 10 | 25 |
| 10 | 120 | 2,5 | 6 | 10 | 16 | 25 | 50 |
| 15 | 180 | 4 | 10 | 16 | 25 | 35 | 70 |
| 20 | 240 | 4 | 16 | 25 | 35 | 50 | 95 |
| 25 | 300 | 6 | 16 | 25 | 35 | 50 | 120 |
| 30 | 360 | 6 | 25 | 35 | 50 | 70 | 120 |
| 35 | 420 | 10 | 25 | 35 | 50 | 70 | 150 |
12 volts, maximum 2 % voltage drop in the cable (0.25 volts)
| Strom (A) | Leistung (W) | 1 m | 3 m | 5 m | 7 m | 10 m | 20 m |
|---|---|---|---|---|---|---|---|
| 0,8 | 10 | 0,75 | 0,75 | 0,75 | 1 | 1,5 | 2,5 |
| 1,66 | 20 | 0,75 | 0,75 | 1,5 | 2,5 | 2,5 | 6 |
| 4,1 | 50 | 0,75 | 2,5 | 4 | 6 | 6 | 16 |
| 8,3 | 100 | 1,5 | 4 | 10 | 10 | 16 | 25 |
| 12,5 | 150 | 2,5 | 6 | 10 | 16 | 25 | 50 |
| 16,66 | 200 | 2,5 | 10 | 16 | 25 | 25 | 50 |
| 25 | 300 | 4 | 16 | 25 | 35 | 50 | 95 |
| 41,66 | 500 | 10 | 25 | 35 | 50 | 70 | 150 |
| 83,33 | 1000 | 16 | 50 | 70 | 95 | 150 | 300 |
12 volts, maximum 4 % voltage drop in the cable (0.5 volts)
| WATT | A | 1 | 3 | 5 | 7 | 10 | 20 |
|---|---|---|---|---|---|---|---|
| 10 | 0,8 | 0,75 | 0,75 | 0,75 | 0,75 | 0,75 | 1,5 |
| 20 | 1,66 | 0,75 | 0,75 | 0,75 | 1 | 1,5 | 2,5 |
| 50 | 4,1 | 0,75 | 1 | 1,5 | 2,5 | 4 | 6 |
| 100 | 8,3 | 0,75 | 2,5 | 4 | 6 | 10 | 16 |
| 150 | 12,5 | 1 | 4 | 6 | 10 | 10 | 25 |
| 200 | 16,66 | 1,5 | 4 | 6 | 10 | 16 | 25 |
| 300 | 25 | 2,5 | 6 | 10 | 16 | 25 | 50 |
| 500 | 41,66 | 4 | 10 | 16 | 25 | 35 | 70 |
| 1000 | 83,33 | 10 | 25 | 35 | 50 | 70 | 150 |
12 volts, maximum 10 % voltage drop in the cable (1 volt)
| WATT | A | 1 | 3 | 5 | 7 | 10 | 20 |
|---|---|---|---|---|---|---|---|
| 10 | 0,8 | 0,75 | 0,75 | 0,75 | 0,75 | 0,75 | 0,75 |
| 20 | 1,66 | 0,75 | 0,75 | 0,75 | 0,75 | 0,75 | 1 |
| 50 | 4,1 | 0,75 | 0,75 | 0,75 | 1 | 1,5 | 2,5 |
| 100 | 8,3 | 0,75 | 0,75 | 1,5 | 2,5 | 2,5 | 6 |
| 150 | 12,5 | 0,75 | 1,5 | 2,5 | 4 | 4 | 10 |
| 200 | 16,66 | 0,75 | 1,5 | 2,5 | 4 | 6 | 10 |
| 300 | 25 | 0,75 | 2,5 | 4 | 6 | 10 | 16 |
| 500 | 41,66 | 1,5 | 4 | 10 | 10 | 16 | 25 |
| 1000 | 83,33 | 2,5 | 10 | 16 | 25 | 25 | 50 |
Standardized cable cross-sections
The IEC 60228 standard summarizes the cross-sections of insulated cables. In Germany, it is valid as DIN standard DIN EN 60228 or VDE 0295. Two classes are relevant here. Class 5 and Class 6, which differ in the number of individual cores. Class 6 has thinner individual cores and is therefore more flexible. Class 6 strands therefore contain more cores with the same cable diameter and can therefore be bent more.
The following cross-sections are included in the standard:
| IEC 60228 Klasse 5, VDE 0295 | IEC 60228 Klasse 6, VDE 0295 | |||
|---|---|---|---|---|
| Querschnitt | Anzahl der Drähte x | max. Draht-Ø mm | Anzahl der Drähte x | max. Draht-Ø mm |
| 0.14 mm²* | ≈ 18 x 0.11 mm | |||
| 0.25 mm²* | ≈ 14 x 0.16 mm | ≈ 32 x 0.11 mm | ||
| 0.34 mm²* | ≈ 19 x 0.16 mm | ≈ 42 x 0.11 mm | ||
| 0,50 mm² | ≈ 15/17 x 0.21 mm | ≈ 28 x 0.16 mm | ||
| 0,75 mm² | ≈ 23 x 0.21 mm | ≈ 42 x 0.16 mm | ||
| 1,00 mm² | ≈ 30 x 0.21 mm | ≈ 56 x 0.16 mm | ||
| 1,50 mm² | ≈ 27-29 x 0.26 mm | ≈ 84 x 0.16 mm | ||
| 2,50 mm² | ≈ 46 x 0.26 mm | ≈ 140 x 0.16 mm | ||
| 4,00 mm² | ≈ 52 x 0.31 mm | ≈ 224 x 0.16 mm | ||
| 6,00 mm² | ≈ 78 x 0.31 mm | ≈ 186 x 0.21 mm | ||
| 10,00 mm² | ≈ 77 x 0.41 mm | ≈ 320 x 0.21 mm | ||
| 16,00 mm² | ≈ 122 x 0.41 mm | ≈ 504 x 0.21 mm | ||
| 25,00 mm² | ≈ 190 x 0.41 mm | ≈ 760 x 0.21 mm | ||
| 35,00 mm² | ≈ 272 x 0.41 mm | ≈ 1083 x 0.21 mm | ||
| 50,00 mm² | ≈ 400 x 0.41 mm | ≈ 703 x 0.31 mm | ||
| 70,00 mm² | ≈ 543 x 0.41 mm | ≈ 988 x 0.31 mm | ||
| 95,00 mm² | ≈ 484 x 0.51 mm | ≈ 1340 x 0.31 mm | ||
| 120.00 mm² | ≈ 589 x 0.51 mm | ≈ 1680 x 0.31 mm | ||
| 150.00 mm² | ≈ 740 x 0.51 mm | ≈ 2122 x 0.31 mm | ||
| 185.00 mm² | ≈ 902 x 0.51 mm | ≈ 1472 x 0.41 mm | ||
| 240.00 mm² | ≈ 1220 x 0.51 mm | ≈ 1910 x 0.41 mm | ||
| 300.00 mm² | ≈ 1525 x 0.51 mm |
Regulations for 12 volt cable installation
You should know and observe the following regulations. Even with 12 volts, cable fires can occur if you do not work correctly.
- 12 V and 230 V cables must not be laid in the same cable duct
- 12 V and 230 V cables must not be distributed in the same junction boxes
- Cables that pass through sheet metal or plastic bushings must be protected with a cable bushing. As there is always vibration and movement in vehicles, the cables would otherwise rub against the edges over time and damage the insulation, which can lead to short circuits and fires
- No cables may be routed through the gas box. Fire and explosion hazard!
Source(s):
Table on stranded wire structure: https://www.sab-kabel.de/kabel-konfektion-temperaturmesstechnik/technische-daten/kabel-leitungen/litzenaufbau.html
Conductivity of copper: https://kupfer.de/anwendungen/elektrotechnik-und-energie/elektrische-leiterwerkstoffe/
This might also interest you:
- Electricity in motorhomes and caravans – the basics
- Electricity in motorhomes and caravans: electricity consumption and saving electricity
- Electricity in motorhomes and caravans: shore power – but the right way
- The CEE plug provides power when camping
- The right and best camping (CEE) power cable
- Buying guide: Camping CEE cable reels – for power supply on the campsite
- CEE adapter – every camper actually needs one
Do you have any questions, is something missing from the article or have you found a mistake? Please write me a comment!
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