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The Ultimate Guide to Campervan 12V Fuse Boxes

Dare I need say it? Knowing how to fuse your campervan electrical system is IMPERATIVE. If you don't size your wires and fuses correctly your newly converted campervan could end up in a smokey, smouldering heap at the back of the campsite. In this guide you will learn about the differences between fuses and circuit breakers for AC and DC systems, how to size your wires and fuses, how to choose the right fuse type, and how to wire up a campervan fuse box or distribution panel. By the end, you will be ready to hop into (and feel confident in) your campervan electrical system!


I'm Shane, I've been teaching people to convert campervans for years; I'm the author of Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan, writer of The Van Conversion Newsletter, instructor of The Van Conversion Course over at Udemy. And full-time vanlifer for 4 years!


Now let's jump in and have a look at fuses and circuit breakers for campervans!


This is Everything you Need to Know About Fuses and Breakers for Campervans

This post contains affiliate links, which means that if you click a product link and buy anything from the merchant (Amazon, eBay, etc.) we will receive a commission fee. The price you pay remains the same, affiliate link or not. By using these links, you are helping me to continue writing free educational content!


Index

 

Note: Before we hop in, you will definitely want to grab yourself a wiring diagram which you can get for free by signing up to The Van Conversion Newsletter

 
campervan electrics guide

Supplies list 🛒

selection of fuses

Terminal fuse block

​Terminal fuse block

For fusing the leisure batteries

ANL fuse

ANL fuse block holder

​ANL fuse block holder

For holding ANL fuses

Mega fuse

Mega fuse block holder

consumer unit

​Consumer unit

Protect your campervan electrical system in case of fault

inline circuit breaker

​Inline circuit breakers

very useful inline breakers that can double as a switch

What is a fuse and why do you need them?

A fuse is an electrical safety device that provides overcurrent protection to an electrical circuit. At the most basic level, it is a metal wire that melts when too much current flows through, thus stopping the flow of electricity. The point at which a fuse blows is known as its breaking capacity.

Fuses are critical to electrical systems in order to protect appliances and wires from being destroyed (and perhaps going on fire).

An unfused wire is a serious fire hazard!
An unfused wire is a serious fire hazard!

What is the difference between a fuse and a circuit breaker?

Fuses and circuit breakers both carry out the same function: to break electrical circuits in the event of a fault.

When a fuse blows (the wire melts) it needs to be thrown out and replaced; it is single use. In contrast, a circuit breaker is a switch which can be reset after it has been flipped.

Fuses are very cheap and can quickly be replaced by simply pulling them out (make sure the power is off). Breakers are more expensive and more difficult to remove. However the ability for a circuit breaker to act like a switch makes them very attractive for van conversions.

What is the difference between AC and DC fuses/breakers?

For a recap on AC and DC electrics, you can read this guide.


AC Fuses/breakers

AC fuses/breakers are designed to be used ONLY with alternating current circuits. AC fuses usually handle higher voltages. They are more resistant to electrical arcing when the fuse blows compared to DC fuses. Electrical arcing is when electricity jumps from one connection to another.

DC Fuses/breakers

DC fuses should only be used with direct current circuits. Quoted voltage ratings for these fuses are maximums that should not be exceeded.

Fuse performance: Terminology and ratings

Rated voltage

This is the maximum voltage the fuse is rated to handle. The typical ratings are 32, 60, 125, 300, 500, 600, and 750 Volts.

Minimum fusing current

This is the current (amperage) at which the fuse reaches a temperature where it will melt. The minimum fusing current is significantly higher (1.25 to 2 times more) than the rated amperage of the fuse.

Rated amperage

This is the amperage printed on the body of the fuse. It is less than the minimum fusing current; in other words, fuses can carry more (1.25 to 2 times) than their rated current before blowing.

Melting time

This is the amount of time it takes for a fuse to blow after the current has crossed the minimum fusing current threshold. There are ultra-fast, fast, and slow blow fuses. Fast blow fuses are generally used to protect sensitive electronics.

Circuit breaker performance: Terminology and ratings

There are two categories of circuit breakers: thermal and magnetic. Thermal breakers trip once a certain temperature is exceeded. In contrast, magnetic breakers trip when the magnitude of the current is exceeded. Magnetic breakers are generally preferred for van conversions.

Rated voltage

This is the maximum voltage the breaker is rated to handle. AC is usually a lot higher than DC.

Rated amperage

This is the amperage printed on the body of the breaker.

Tripping amperage

This is the amperage at which the circuit breaker will actually trip. It is normally about 130% the rated amperage.

Trip time

This is the amount of time it takes for a breaker to trip after the current has crossed the tripping amperage threshold.

How to choose the correct fuse / circuit breaker for a DC circuit

Step #1: Calculate the appropriate wire size

Before choosing a fuse, it is important that you first size the wire you are fusing correctly. Learn how to size wires correctly in this guide. Or you can head over to http://circuitwizard.bluesea.com/ and use their nifty wire-sizing calculator.

Step #2: Select a DC fuse

The rule of thumb in fuse sizing is that you should 'always select a fuse size to protect the wire according to its rating.' However, most amateur campervan builders should always size wires larger than they think they need. Most of us are not looking for maximum cost/weight efficiency, we are looking for maximum safety.

So, to calculate the fuse size, we should calculate the total amp rating of the appliance the wire is feeding and add on a 25% buffer. When we talk about the 'size' of a fuse, we mean its 'rated amperage' as mentioned above.

Example: Let's say we have eight 12V puck lights which are 3W each. The amp of each light would be 3W ÷ 12V = 0.25A. We have eight of them so: 0.25A × 8 = 2A. We want to give ourselves a 25% buffer, so we will choose a 2.5A fuse for this system.

Once we know the fuse size we can go ahead and choose an appropriate fuse. Let’s look at some of the most common fuses;

AGC and MDL fuses [0.25A - 30A]: AGC and MDL fuses are slow blow fuses. They are constructed of glass tubing and brass end caps. The glass tubing provides a visible indication when the fuse blows. They are cheap fuses and usually for small appliances.

AGC / MDL fuse
AGC fuse

Blade fuses [1A - 80A]: Blade fuses are the most common fuses found in vehicles. They are cheap, colour coded, and easy to replace. When the fuse blows, it is visible inside the plastic casing. Van converters will tend to use ATO (ATC) blade fuses in their 12V campervan fuse box. They are used with small to medium sized appliances. ATO fuses are designed to handle 1 to 30 Amps, MAXI fuses are designed to handle 30 to 80 Amps.

Types of blade fuses

Terminal (MRBF) fuses [30A - 300A]: Terminal fuses are pretty much exclusively used for fusing leisure batteries. They are rated to handle a large current.

Terminal fuse block

MIDI / MEGA / ANL / Class T Fuses [30A - 400A]: These fuses are all designed to take large current. Most of them are inline fuses, meaning they are installed on the wire. In van conversions, we use ANL and MEGA fuses to fuse the likes of our inverter.

MIDI fuses are designed to handle 30 to 200 Amps, MEGA fuses are designed to handle 100 to 300 Amps, ANL fuses are designed to handle 35 to 400 Amps, Class T fuses are designed to handle 110 to 400 Amps.

Mega fuse and holder

Step #3: Select the appropriate fuse holder

After you have chosen the appropriate fuse, you will need to grab a fuse holder for it. The choice of fuse holder really comes down to the purpose of the fuse and environmental factors.

There are two types of fuse holder: inline and block.

Inline blade fuse holder
Inline blade fuse holder

Inline fuse holders are compact and hold a single low-amperage fuse.

Block fuse holders mount to a solid surface and may hold a single fuse or multiple fuses. The 12V campervan fuse box is a block fuse holder.

Note: Whilst you can use inline fuses for your appliances, most van converters tend to use inline circuit breakers instead. An inline breaker acts as a manual switch. It is very useful to be able to turn off the solar panel feed, charge controller, or other parts of the 12V system. Especially during the build process.

Note: Amazon, Ebay, and Aliexpress seem to be flooded with fake inline breakers that are indistinguishable from genuine ones. These knock offs have no value as current sensing trips even on steady flow DC circuits. They might not trip at the current they should, they might trip at a lower current than rated for, or they might run very hot near trip current. They are very high on the suspect hit list if seen in conjunction with electrical system faults. The only brands you should trust are BlueSea or Eaton-Bussmann.

Renogy electrics

Campervan fuse box (12V fuse box)

Most van converters will install a simple 12V fuse box in their campervan. This is a central point from which we fuse up our 12V DC appliances.

12V fuse boxes typically take ATC blade fuses. Buying an "assortment pack" of blade fuses is probably a good idea - it's important to have them on hand for when you need them.

If we are running solar power in our campervan, we will typically wire the fuse box directly to the solar charge controller; this allows us to get some pretty nifty monitoring of our DC system.

If you do not have solar power in your campervan, you wire the fuse box directly to the bus bars (which are connected to the leisure batteries).

Wiring fuse box to 12v DC appliances

We will typically install a switch panel between the campervan fuse box and the 12v appliances (eg. lights); this allows us to individually switch the appliances on and off.


Note: While fuse boxes do exist for AC systems, they are not very common and have mostly been replaced by distribution panels, which we will discuss next.

AC fuse box
AC fuse box

Blade fuse sizes

Blade fuse sizing is a crucial aspect of electrical systems in campervans, ensuring the safety and functionality of various components. There are several types of blade fuses, each designed for specific applications and rated with different amperages. The most common blade fuse sizes include mini, standard, and maxi. Blade fuses are often color-coded for easy identification.

  1. Mini Blade Fuses: They typically come in amp ratings ranging from 2 to 30 amps.

  2. Standard Blade Fuses: The standard-sized blade fuses are the most widespread and versatile. They are available in amp ratings from 2 to 40 amps.

  3. Maxi Blade Fuses: Maxi blade fuses are larger and designed for heavy-duty applications. They usually have amp ratings from 20 to 100 amps.

assortment of blade fuses

Distribution panels

Distribution panels (also known as panelboards or electric panels) are found in just about every home or office building around the world. It is a plastic box that divides an electrical power feed into subsidiary circuits, while providing protective circuit breakers for each circuit. Distribution panels can house AC and DC systems.

AC Consumer unit

A consumer unit (breaker box) is a type of distribution panel.

In a van conversion, we use a small consumer unit if we are installing shore power (ie. the ability to plug in and charge up at a campsite).

Shore power to consumer unit

The consumer unit contains an RCD (residual current device) and several double pole MCBs (miniature circuit breakers). We will discuss consumer units at length in the chapter on shore power.

We may also need to install a consumer unit between the inverter and our plug sockets.

Note: While we normally use 12V fuse boxes with our DC system, we can use a DC consumer unit if we so choose. This has the benefit of using switchable circuit breakers rather than fuses. However they are a lot rarer. We must use DC MCBs in a DC consumer unit.

DC consumer unit
DC consumer unit

Combined AC and DC distribution panels

If you are running an AC and DC system in your campervan, a sleek way to house your fuses / circuit breakers is by using a combined distribution panel. It combines a 12V campervan fuse box with an AC consumer unit, all in one neat container.

Note: You must be extra careful if you are installing the AC and DC systems in the same cabinet. Under no circumstances should the wiring touch.

Combined AC and DC distribution panel
Combined AC and DC distribution panel

Explorist Life put together a really excellent video on how to wire up a combined distribution panel which you can check out here:


If you want to go even deeper down the distribution panel rabbit hole, Greg Virgoe showcases a thing of beauty in this video. It's certainly on the complex end though!


Conclusion

And there we have it! Everything you need to know (and possibly more) about 12v fuse boxes and circuit breakers for your campervan! I truly hope you found this article useful!


Don't forget to subscribe to The Van Conversion Newsletter for everything you need to get started with your own van conversion (I'll send you a free wiring diagram when you join).


If you're looking for some guidance with your van conversion, you might be interested in Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan. In the 380-page book (or ebook), you'll learn directly from me how to convert a van into your dream home - no prior experience needed!

van conversion book

Until next time,


Shane ✌️