Wire Size Calculator including Switches, Relays & Fuses

Graham Bogie

This wire size calculator does NOT supersede manufacturers recommendations. Always follow the manufacturers recommended wire and fuse sizes.

Disclaimer: Mowgli Adventures has made every effort to ensure that the results generated by the wire size calculator are correct and valid.

However, Mowgli Adventures warrants nor accepts any liability for the its use. The user is cautioned to seek competent electrician’s assistance in the use of the calculator and application of results generated.

The user is further cautioned that the results generated are based upon certain simplifying assumptions,  which may or may not be valid in specific cases.

Sizing the major components of a campervan electrical system is a crucial part of your build. 

Installing enough solar panels and an appropriately sized solar charge controller will help you maximise how much solar electricity you can generate. 

An optimum-sized battery-to-battery charger or split charge relay will help maximise how much and how fast you can charge your batteries while you drive. 

The same applies to your battery charger (or converter) when you’re hooked up to a mains supply.

And a big enough battery bank will help ensure you can store that electricity and even have a little in reserve for rainy days.

Amongst all this effort to calculate the major components, make sure you calculate the wire gauge needed too.

This wire size calculator is ideal if you want to quickly check the wire size needed for a specific part of your build.

It’s simple and straightforward to use. We’ve included a section below to answer some questions you may have.

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The Anatomy of a Wire

A wire is made up of insulation – the plastic sheath – and the conductor – the metal strands inside the insulation.

Copper is an excellent conductor, better than steel, so most wires have copper strands. The wire’s conductor allows current to flow.

The insulation protects the conductor inside the wire. If this is damaged, the copper wire becomes exposed, presenting a risk of electric shocks and circuit faults.

Cables should have a minimum rating of 600 volts and 105°C, moisture, and flame retardant.

In the UK and Europe, wires are sized by their cross-sectional area in mm². 

In North America, they’re sized by the conductor’s diameter and expressed using the American Wire Gauge (AWG).

A diagram showing the components of a wire

Why is it Important to Use the Proper Wire Size?

Electrical wire has a certain amount of resistance, resulting in an amount of voltage drop from one end to another.

Your electrical components need a certain amount of voltage to operate, depending on their specifications.

In your 12v electrical system, most of your appliances need 12 volts, especially those with voltage sensors like diesel cookers, fridges, and heaters. 

But voltage drops affect how much of the battery voltage can reach the appliance.

Wire Length

Electrical current transmits through the wire, but the longer the wire, the greater the resistance and the greater the voltage drop.

Therefore, installing wires too small for the current draw of the appliance could result in so much voltage drop that it won’t operate.

We’re gone to a lot of trouble to generate electricity for an off-grid lifestyle, so wasting energy is best avoided. Our wire size calculator assumes only a 3% voltage loss at most.


As current flows through a wire, the resistance generates heat. If the wire is too small for the current passing through it, the wire may get so hot it causes a fire. 

Using larger wires minimises voltage drops. 

While you don’t usually need specialised wiring, like armoured cabling, you need to be aware of other factors resulting in voltage drops in a campervan.

Ambient temperature affects the wire ampacity. In temperatures above 50°C or 122°F, you can expect to reduce its rating by maybe as much as 15%. 

If your wires are installed in the ceiling or walls of a van on a hot summer day, the ambient temperature around them could easily hit this level.

If you think this applies to you, we recommend going up one wire size.

High Current Components

We recommend going up one wire size for heavy current components like inverters, heaters, and water heaters. The calculator already accounts for this in the inverter calculation.

When working out your battery cables, again, we recommend going up one wire size. The cable must be able to cope if the battery bank tries to dump its entire current in one go. 

Unless the cables are oversized, at worst, this could cause the wires to melt, cause a fire or even result in loss of life.

Circuit Components

You can read more information about circuits and components in our post on campervan wiring, but in summary:


Switches break the circuit for operational purposes. So you flick a switch to turn the lights on, closing the circuit.

Some appliances have in-built switches like reading lights or a heater. Others don’t, like water pumps and LED lights. For the latter, you’ll need to add a switch to turn it on and off. 

You can add switches to appliances that already have inbuilt switches too.

As with all electrical components, switches have an amp rating indicating a safe current at which they can operate.

You can buy switches to handle massive currents, but they get physically bigger and more clunky with a higher rating. 

We recommend keeping all onboard switch sizes to a standard 20 amps, except for the battery isolator switches.


Fuses protect the power supply and wires

Fuses act as a layer of protection. When they receive an unexpected current, they melt, snap, and break the circuit. The resulting open circuit can’t do any harm, so it’s effectively a safety device.

For any appliance rayed over 40 amps, you’ll need to install an ANL fuse instead of a blade fuse because the larger sizes don’t fit in the fuse blocks. You could alternatively use a breaker, but we don’t think the price justifies their use.


relays for DC circuits need to be sized to the components they operate

Relays (sometimes called a solenoid) are electrically controlled switches.

Wherever an appliance is rated over 20 amps, install a relay to keep the switch size to 20 amps.

The calculator works out the relays you need. We’ve sized them to be twice the amp rating of the appliance because, in our experience, if they’re too close to the same size, they fail more often. 

For every relay, you need some 14 AWG wire and a small blade fuse too. About 5 amps will do the job because the relay only draws milliamps.

Component manufacturers often specify fuse and wire sizes. This calculator does not override their advice. They know their product better than we do, so follow their guidance.

Use this tool for reference only.

Wire Size Chart | AWG To Metric Cable Size Conversion

The cable size calculator determines a complete parts list for your 12v DC circuits. 

The individual components may all need a different minimum size wire.

Rather than buy every wire gauge calculated, we encourage you to buy only three or at most four cable sizes. 

It will standardise the wiring throughout your conversion, keep costs down, and help minimise the amount of spare cable you need to carry. 

The wire size calculator has figured this out for you, too.

Wire gauge calculator table

If you need a complete shopping list of all the tools and parts needed for the major components on your circuits, use out camper van electrics kit & parts list.

Graham Bogie

Graham Bogie - Expert in RV & Campervan Electrics - Mowgli Adventures

Graham is a seasoned marine electrical engineer with two decades of experience designing customized electrical systems for plant machinery and converting campers and overland vehicles. His expertise has led him to author the reputable Campervan Electrics Handbook and become the chief designer of our Campervan Electrical Design Service. As a knowledgeable figure in the field, his YouTube channel, blog, Facebook group, and newsletter, offering electrical advice and product reviews, reach more than a million users each year.

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