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How to calculate the minimum cross-section of a cable in electrical installation

If you are thinking of doing the electrical installation of your van yourself or if you are already working on it, you must be clear that, in order to have a safe electrical installation, the correct dimensioning of the cables is a fundamental and very important part Read our post and minimize any risk or accident that may occur in your future home due to a poorly sized electrical installation.

First of all I want to point out that the dimension of the cables is calculated by the area of its section (π﹡r2) and not by its diameter. When we buy a 2.5 cable, it means that its section is 2.5mm2 and not that its diameter is 2.5mm.

If you already know all the theory and you want a simple way to calculate the cable cross section take a look at our cable cross section calculator that takes into account the maximum admissible voltage drop, you will find it at the end of this post.

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Generally used cable cross-section, depending on the application or maximum current

Ratio of cable cross-section and electrical device

If what you are looking for is a simple table that allows you to assign a section to each cable you need to install in your van, here are some that will work perfectly in most situations:

Cable sectionElectrical device
1.5 mm2Lighting
2.5 mm2Sockets, water pump
6 mm2Refrigerator* 25 mm2
25 mm2Batteries**
Table of general recommendations for cable cross-section

* Many people connect the refrigerator with a 2.5 mm2 cable. After reading in forums and asking people who understand the subject, we prefer to install and recommend a 6mm2 cable to reduce the voltage drop and the risks of heating.

** We use 16mm2 cable because this is the maximum cross section that the Orion TR-Smart converter can handle. If we could have used 25 mm2, we would have done so.

For devices such as inverters, solar panels, regulators, etc. it is better that you perform the calculations that we will explain below to know which section is the most appropriate according to the power of the device, as they can vary a lot.

If you do not have much idea, you can ask for advice directly where you buy the electrical components, that is what we did in many cases, because if sometimes we can have the knowledge of professionals why not use it? 😉

Cable section ratio, maximum current and maximum power at 12V and 24V

This type of tables that relate a cable section with the maximum intensity that it supports abound in the network. There are many different ones depending on the criteria they use when setting the maximum temperature that the cable could reach and how “prudent” they want to be.

Cable cross-sectionMaximum currentMaximum power (12V)Maximum power (24V)
1.5 mm211 A132 W264 W
2.5 mm215 A180 W360 W
4 mm220 A240 W480 W
6 mm226 A312 W624 W
10 mm236 A432 W864 W
16 mm248 A576 W1152 W
25 mm263 A756 W1512 W

Ratio of cable cross-section, maximum current and maximum power at 12V and 24V for loose copper cables

Looking for information about the installation of the auxiliary battery we found a post in the furgovw forum in which it was discussed, among other things, which were the appropriate cable cross-sections. In particular we liked a table that the user jallacru published in one of his posts and that, as he explained, is designed and tested with real tests that are more similar to our use case. We have “completed” it with the maximum power at 12V and 24V.

Cable cross-sectionMaximum currentMaximum power (12V)Maximum power (24V)Maximum fuse
1.5 mm215 A180 W360 W15 A
2.5 mm222 A264 W528 W20 A
4 mm232 A384 W768 W35 A
6 mm242 A504 W1008 W40 A
10 mm260 A720 W1440 W63 A
16 mm280 A960 W1920 W80 A
25 mm2100 A1200 W2400 W100 A
35 mm2123 A1476 W2952 W125 A

This is an approximate table, if in any case you are hesitating between two sections, read on and calculate the section or install the thicker one. Also keep in mind that if the cable is going to be very long you may need to increase its section.

Cable cross-section calculation: Maximum current and voltage drop

If, on the other hand, you are one of those who wonder why things happen and you like to understand what you are doing, we will try to explain how to calculate the minimum section of the cable you need to install easily.

To correctly size the cross-section of each of the cables that you will lay along the camperization, you will have to do it according to two criteria:

1. Calculation of the cable cross-section according to the maximum intensity: Avoid overheating

You should know that each cable is manufactured to withstand a maximum temperature, safely, before its insulating material runs the risk of degrading. The temperature that a cable reaches will depend on the intensity of the current flowing through it. Many people believe that the current at 12 V is harmless, since at this voltage a discharge will not cause us any damage. What we do not usually think about is the high intensity that can be transmitted at this voltage, and the danger of overheating that exists.

Insulated conductors in a conduit in a thermally insulated wall

2 loose wires inside corrugated pipe
Cross sectionCurrent
1.5 mm212,5 A
2.5 mm217 A
4 mm222 A
6 mm229 A
10 mm240 A
16 mm253 A
25 mm269 A
3 loose wires inside corrugated tube
Cross sectionCurrent
1.5 mm211,5 A
2.5 mm215,5 A
4 mm220 A
6 mm226 A
10 mm236 A
16 mm248 A
25 mm263 A

Multicore cable in a conduit in a thermally insulated wall

Hose 2 wires inside corrugated tube
1.5 mm211,5 A
2.5 mm215,5 A
4 mm220 A
6 mm226 A
10 mm236 A
16 mm248 A
25 mm263 A
Hose 3 wires inside corrugated tube
Cross sectionCurrent
1.5 mm211 A
2.5 mm215 A
4 mm220 A
6 mm225 A
10 mm233 A
16 mm245 A
25 mm259 A

Single-pole or multi-pole cables on a wooden or masonry wall

2 wires

1.5 mm217 A
2.5 mm223 A
4 mm231 A
6 mm240 A
10 mm254 A
16 mm273 A
25 mm295 A

3 wires

Cross sectionCurrent
1.5 mm214,5 A
2.5 mm220 A
4 mm226 A
6 mm234 A
10 mm246 A
16 mm263 A
25 mm282 A

Single-pole cables in contact in open air. Distance to the wall not less than the cable diameter

2 wires

1.5 mm220 A
2.5 mm226 A
4 mm236 A
6 mm246 A
10 mm265 A
16 mm287 A
25 mm2110 A

3 wires

Cross sectionCurrent
1.5 mm217 A
2.5 mm223 A
4 mm231 A
6 mm240 A
10 mm254 A
16 mm273 A
25 mm295 A

You may have already noticed that our vans do not fit 100% in any of the assumptions. We made a balance between the tables and other advice we found on the net and chose the section that seemed to us the most appropriate. If you finally want to know what section of cable we use in each of our electrical appliances, at the end of the entry you will find a table that collects this data.

2. Calculation of the cable cross-section to control the voltage drop

Another issue to be taken into account when calculating the appropriate cable cross-section is the voltage drop that can be assumed in each case.

Although in the electrical installation of a van the cables are not usually very long, there is also voltage loss in them. Depending on the consumable to be connected, this drop must be taken into account, especially in the case of electronic devices.

What does it mean if there is a voltage drop?

Although a cable is an electrical conductor it will oppose a certain resistance, depending on the material it is made of. Therefore the voltage applied at one end of the cable will not be the same as the voltage at the other end, there will always be a loss. The longer the cable the greater the voltage drop.

A high voltage drop could cause many electronic devices to not work properly, so it is important to reduce voltage drops below the allowable limit. Now, what is the maximum allowable in our case? 🤔

Maximum admissible voltage drops according to the REBT (Low Voltage Electrotechnical Regulation)

This is a list of the maximum drops that can be expected depending on the type of installation. Obviously it is best that the drop is as small as possible, so we can always increase the minimum dimension of the cable section to reduce the drop a little.

  • General power supply line: 0.5%
  • Individual branch (single user): 1.5%
  • Individual branch (several users): 1%
  • Indoor housing circuit: 3%
  • Lighting circuit (non-housing): 3% Power circuit (non-housing): 3%
  • Power circuit (non-dwelling): 5%

How is the cable section calculated by limiting the voltage drop?

To calculate the minimum cable section to be used according to the voltage drop allowed, the following formula should be used:

S = (2﹡P﹡L) / (γ﹡e﹡U)

  • S: Cable cross section in mm2
  • P: Appliance power in W
  • L: Cable length in m
  • γ: Conductivity in m / (Ω∙mm²)
  • e: Voltage drop in V
  • U: Voltage in V
Material/Temp20 ºC70 ºC90 ºC
Copper and aluminum conductivity table

The worst-case conductivity value should be used for the calculation. For cables sheathed with thermoplastic materials we will take the conductivity data at 70 ºC, since higher temperatures are not recommended, while for thermosetting materials at 90 ºC.

For example: We want to install a compressor refrigerator in our van, so we need to calculate the minimum section of the cable that will feed it. The refrigerator has the following power consumption characteristics: 40 W at 12 V and the power cable will be made of copper and PVC sheathing with a length of 2 m.

  • Power: 40 W
  • Cable length: 3 m
  • Copper conductivity at 70 ºC: 48 m / (Ω∙mm²)
  • Voltage drop for an indoor circuit: 3% ⟶ 12 V﹡0,03 = 0,36 V
  • Voltage: 12 V

S = (2﹡40 W﹡3 m) / (48﹡0.36 V﹡12 V) = 240 / 207.36 = 1.16 mm2

Therefore, the cable with the minimum cross-section to be laid, according to the admissible voltage drop, would be 1.5 mm2.

This is a theoretical calculation. Our recommendation is to be conservative and, depending on the element, give a little more section to the cables. For example, in this particular case we opted for a 6 mm2 cable to avoid overheating, since the refrigerator is an appliance that operates 24 hours a day.

Calculator for calculating the cable cross-section according to the permissible voltage drop

To make the calculations a little easier for you, we have prepared a calculator that will help you to solve this little problem in a very simple way.

This calculator uses and applies the formula and the data from the table of temperatures and materials that we explained in the previous step in a way that allows the calculation of the cross-section in an easy and practical way

Once you have entered all the data, the calculator will show the minimum cross-section necessary for the voltage drop produced to be equal to the percentage you have entered.

You will have to take, as a minimum, a cable whose cross-section is immediately higher than the result given by the calculator. Remember that the cross-sections of commercial cables are: 1.5 mm2, 2.5 mm2, 4 mm2, 6 mm2, 10 mm2, 16 mm2, 25 mm2, 35 mm2, etc.

Do not forget to check that the resulting cable cross-section also complies with the criteria for the maximum current and temperature that a cable of this cross-section can withstand, as explained in step 1.

Cable cross-sections used in our van

Here is a table showing the cable cross-sections that we finally installed in our van, for each electrical device or consumable.

Electrical consumableIntensityPowerVoltageSection of cable
Gel battery 316 Ah12V16 mm2
200V socket for boiler230V2,5 mm2
Water pump3,9A12V1,5 mm2
Heating62W12V6 mm2
Battery charger16 mm2
Skylight3A36W12V2,5 mm2
DC converter16 mm2
Light + Bathroom exhaust fan0,7A5W + 3W12V1,5 mm2
Refrigerator3,33A40W12V6 mm2
Inverter12V25 mm2
LED lights0,21A2,5 W12V1.5 mm2
LED lights0,42A5W12V1,5 mm2
Reading lights0,5A6W12V1,5 mm2
Exterior light0,37A4,5W12V1,5 mm2
Battery level panel1,5mm2
Water level panel1.5mm2
Solar panel19A455W24V6 mm2
Solar controller30A12V10 mm2
Router0 – 1A7W1.5 mm2
LED strips0,21A2,5W12V1,5 mm2
12V sockets12V1,5 mm2 | 2,5 mm2
Sockets 220220V2,5 mm2
USB sockets12V1,5 mm2 | 2,5 mm2

In no case do we claim that our choices are the only valid ones, not even the best ones, they are simply ours. Maybe in some cases we have been too conservative or maybe in other cases we have been too short… We accept suggestions 😉