Electrical installation in the motorhome – it’s not magic

Reading time 8 minutes

Updated - June 21, 2025

An electrical installation in a motorhome differs from conventional house installations with 230 V AC (alternating current) only in the additional component of the DC battery supply with 12 V or, in trucks and buses, with 24 V DC (direct current).

This article aims to show general basic information and differences. Although work on 230 V AC installations is legally reserved for appropriately trained specialist personnel, it does not hurt to familiarize yourself with certain basics, especially as a layperson in this area, in order to develop an understanding of the connections.

Anyone planning to expand a vehicle, be it a motorhome, sprinter, etc., should lay a cable harness with a larger cross-section on both (!) sides in order to be able to supply consumers with larger power requirements, no matter where.
DCDC converters, for example, may require 30 A on the input side, which with a 6 m cable length and only 2 % voltage drops already has a cross section of 35 mm² conditional.

Basic Notes

If you don't know what to look out for, you're bound to make mistakes sooner or later. And since in both areas, whether direct or alternating voltage, there can certainly be circumstances that can be critical or even life-threatening, knowledge definitely helps to avoid these risks.

The general rule is: Always work in a voltage-free state, i.e. – disconnect the MINUS battery in the vehicle and remove the fuse/FI (residual current protection) switch in the 230 V installation!
For 230 V installations in a property, attach a sign to the fuse box stating that the FI switch has been switched OFF due to work on the electrical installation and must NOT be switched on!

The following applies to tools: ONLY use fully insulated tools in accordance with VDE. In my opinion, the common “voltage testers” in the form of a screwdriver with a transparent handle and a glow lamp inside are not included. Here, due to bright ambient light, the glow may be overlooked and the action on the - but - live installation becomes a game of Russian roulette.

Please note with screw connections: most cable fires occur, for example, in distribution boxes in which a number of cable conductors come together using “cluster terminals”. The originally tightened screw clamps loosen over time as the copper wire yields to the screw pressure. Therefore, every screw connection should be tightened TIGHTLY and then tightened again. Because contact resistance caused by loose connections increases the current flow considerably and leads to heating of this “loose contact” connection, ultimately leading to a fire!

And the final recommendation is: ALWAYS take it easy! Be careful and attentive to what you are doing and avoid outside interference!
Clothing should be tight-fitting, shoes should have rubber soles, hands should be dry, ideally protected with gloves, long hair should be braided and positioned so that it does not enter the field of vision and impair it.

Here we go!

Tool

Which tools are needed in order to be able to work qualitatively and professionally? The company is, for example, a quality tool manufacturer according to VDE Knipex. Positions marked “optional” are for more professional use.

• Phillips and long slot screwdriver set
• Side cutters
• Combination pliers
• Flat (round) pliers (straight, offset)

Cable assembly

• Stripping tool
• crimping pliers
• Ferrules
• Tubular cable lugs
• Crimp connectors / flat receptacles

measuring technology

Both devices listed below and their handling are here described in more detail.

• Digital multimeter
• Clamp multimeter (optional)

Soldering technique

Whether a soldering iron or a better station depends on your wallet: a soldering station with adjustable soldering temperature is more flexible and therefore suitable for both thin and thicker cables. Both the soldering iron and the soldering station work with 230 V AC, so they require power from the socket!

The company, for example, offers good products Weller.

• Soldering iron
• Soldering station

Photovoltaic installation

There are separate, in-depth articles on this: planning, MPPT controller, Inverter, Assembly.

• PV modules
• PV module rails, holders, screws
• MPPT controller
• PV connector, Y branch
• PV cable 6 mm²
• Roof cable glands
• LiFePo4 batteries 280 Ah or more
• Sine wave (!) inverter (for 230 V supply) – 1-phase
• FI switch 230 V 1-phase
• Circuit breakers (10 A, 16 A)
• Fuse box (for accommodating protruding components)

Other materials

• Cables in different cross sections, lengths and number of cores
• WAGO terminals
• Fuse holder 12/24 V DC
• Fuses (suitable for fuse holder)

12V/24V DC – Automotive installation

In a vehicle, the electronics, lamps, indicators, horn, windshield wiper motors, engine control, starter motor, etc. all get their required energy from the battery. This in turn is charged by the alternator while driving. The battery is usually a lead battery and is correspondingly heavy. Previously it contained battery acid, which had to be checked and refilled regularly. Today's batteries contain thickened battery acid that no longer evaporates and therefore does not need to be refilled, meaning it is maintenance-free.

What all batteries have in common is that deep discharge damages them, possibly even irreversibly. Likewise, cold is their enemy. The lower the temperature, the less likely the battery is to start. Some batteries, such as LiFePo4 types, cannot be charged at temperatures below zero (which is why they are equipped with a heater and a battery management system - BMS).

The starter motor draws very high currents during the starting process, which is why it is connected directly to the battery without a fuse using very thick and short cables. All other consumers run via so-called back-up fuses, sometimes over 100 A (Ampère), which are then followed by the well-known small plug-in fuses in the fuse box in the range of 1 ... 40 A.

The cable cross-sections depend on the current to be transported (consumers) and the cable length. The electrical resistance (measured in ohms – Ω) of a cable increases as the length and diameter decreases; conversely, a thick cable of the same length has a lower resistance. Calculating the required diameter for a given cable length is easy to do using this online tool:

Cable lugs

With tubular or compression cable lugs at the latest, the question arises as to which one is suitable for which cable cross-section and which type of conductor?

The first difference is that compression cable lugs have a longer and thicker cable receptacle than tubular cable lugs.

Hexagonal crimping tools are generally suitable (for cable classes 1 - solid, 2 - stranded, 5 - fine stranded and 6 - superfine stranded, e.g. HiFlex), for larger cross-sections and correspondingly stable cable lugs in the hydraulic version of the crimping tool.

The compression determines the lowest possible contact resistance, low heating and secure hold of the cable in the cable lug.

Now we usually know the cable cross-section, as it is the result of the current carrying capacity. But what internal diameter must a cable lug have in order to guarantee a clean connection between the cable and cable lug?
The following table provides information:

Fuses

Flat plug-in fuses have become established in automotive technology and are available in two sizes, namely: Mini and default gives.

For standard fuses have Fuse holder with LED indicators proven, which are available in different sizes, up to 12 fuses. In addition to the actual fuse holder, they also contain a set of standard fuses.

A defective fuse is signaled by the associated LED lighting up, which puts an end to the tiresome search for the defective fuse.

The size of the fuse is roughly calculated to be 1.35 times the consumer's power requirements. Assuming the load is specified as 3.5 A, the fuse is determined to be 3.5 * 1.35 = 4.995 => 5A.

230 V AC automotive installation

When using a (sine wave) inverter and an existing external socket for 230 V shore power, a so-called Mains priority circuit be installed, which SEPARATES the inverter from the vehicle installation and the shore power feed! It must be avoided that the voltage from the connected shore power AND the inverter voltage meet, which would trigger the fuse on the shore power side and, in the worst case, destroy the inverter.
When using mains priority switching, the load capacity of the switching contacts (relays) and the switching time must be taken into account. The load capacity is usually 16 A, the switching time varies between a few seconds and just 20 ms.
For example, anyone who operates life support systems (e.g. a ventilator) or a computer on an alternating voltage network must use the 20 ms variant, since computer systems (a ventilator also contains a computer) cannot tolerate network interruptions of more than 20 ms and therefore switch off in an uncontrolled manner.

Another word about the inverter: why sine wave inverters? Inexpensive “cheap” inverters try to imitate the alternating voltage by supplying 230 V positive for 1/50th of a second and negative immediately afterwards (50 Hz oscillation = 50 changes per second between plus and minus 230 V).
However, the 230 V 50 Hz mains voltage does not form a square wave signal with alternating 460 V jumps, but rather a continuously rising and falling sine curve.

Electronic devices, especially the so-called switching power supplies of laptops, etc., do not like this volatility and, in the best case, simply fail to function; in the worst case, they suffer permanent damage.

If you want to purchase good PV components, such as MPPT controllers, inverters, etc. from a well-known manufacturer, the products from Victron recommended.

Otherwise, a 230 V AC installation in a vehicle is no different from a stationary installation in a property, apart from the grounding, because no vehicle remains a "vehicle" if you ram a grounding rod into the ground 😉 - (which you certainly do If you can do this temporarily, please use a VDE-compliant cross-earth electrode that is at least 1 meter long and corresponding to 16 mm² Ground cable connection to rod and body, as well as connected lightning rods to all four corners of the roof! – (for long-term inpatient stay)).

Fuse box installation location

Where to put all the technology? – In vehicles, fuse boxes are usually found in different, poorly accessible locations. Learning from this makes everyday life a lot easier! Anyone who has looked for a broken fuse at night during rain and storms with the side flap open knows what this means.

Fuse boxes therefore belong in a place that can be reached at any time (!)! You can hang a picture or whatever in front of it later...

And so that you don't have to search through dozens of fuses to find the defective ones Fuse holder with LED display proven, which immediately and clearly identify the defective fuse by lighting up next to it.

In order to keep cable routes to the battery short, installation should be carried out close to the batteries involved (vehicle/body battery(s)).

Cable laying

230 V AC and 12/24 V DC lines, as well as network cables if applicable, should not run directly parallel to each other. At least separate cable ducts for AC and DC lines must be provided. If you need LAN or antenna (HF) cables, you can use them separately in a third channel - taking into account the permissible ones Bending radii - embarrassed.

Cable assembly

As a rule, cable ends should be stripped to suit the connection method and connected with the appropriate accessories (cable lug, etc.).

For stripping single-core cables, use the stripping tool mentioned above, which is adjusted and used to the length of the cable end to be stripped.

The stripped end is then fitted with the desired connector and the same using Crimping pliers (with five inserts for different applications) firmly pressed into the bare wires. For connections to screw terminals (e.g. MPPT controllers). Ferrules, for fuse holder connections Ring cable lugs suitable diameter for battery connection Tubular cable lugs used.

The above-mentioned universal crimping pliers are suitable for the following connections and therefore cover all connection cases that arise in this context:

Cable, cable, cable…

As already mentioned, different types of cables have to be laid. The links to sources of supply listed here are only examples and have been chosen without regard to the respective pricing.

• 230V AC - 3-wire flexible hose type cable H05VV-F3Gx (x stands for the cross section)
• 12 / 24V – 2-wire, flexible flat cable (red / black) FLRyy in different cross sections
• Solar cablel 6mm²
• Network cable (LAN) CAT7
• Antenna cable (for LTE, radio, etc.)

As soon as you have an overview of the final consumers, their position in the vehicle (make a scale puzzle like when furnishing your home / moving, take into account that cables also take up space) and the required cable lengths and types, you can start measuring Create a shopping list of the required lengths, including the cable ducts and required connection parts (cable lugs, terminals, etc.).

Experience has shown that it is always a good idea to round up cable lengths generously (! – my wife just said “it’s better to double it…”), because you always need more than you “think”! Ten meters are a bit of a hassle when converting a Sprinter...

Protecting consumers

Every consumer should have their own fuse. Why? If a consumer causes a short circuit, the fuse disconnects the circuit for all consumers connected to it. If you imagine that you have connected all the lights in the vehicle to a fuse, you are immediately in the dark.

Even if it seems exaggerated, by the time a fuse is activated for the first time, you can estimate that everything else continues to work and that you can, if necessary, look for the fault at a later, more convenient time and then replace the fuse.
Troubleshooting should take place BEFORE replacing the fuse. Of course, you can simply use a stronger one, but at some point you risk that the cable takes over the task of securing instead of the fuse, which then becomes a “hot” affair in the truest sense of the word!

Finally, it should be noted: even if it is a tiresome topic, it is extremely helpful to document which consumer is connected to which fuse with how many amps and also where the associated cables run.
It is also advisable to lay reserve cables at the same time, so that you save yourself the hassle of dismantling the panels later so that you can pull in cables later.

Notice

All electrical installation work must be carried out by qualified personnel in compliance with applicable regulations. When working on electrical installations, they must always be kept safely disconnected from the power supply!
To inform uninvolved third parties, appropriate information signs must be attached to the shutdown devices during the work.
The above information is intended as a suggestion and makes no claim to completeness, compliance with relevant regulations, standards, etc.. Any liability is excluded.

ps If you need personal support in the implementation for a fee, you are welcome to Ticketing make!