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 many years, I'm the author of The Van Conversion Newsletter, the van conversion instructor at Udemy, and the proud owner of a beautiful self-build campervan called Beans. So let's jump in and have a look at fuses and circuit breakers for campervans!
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What is the difference between a fuse and a circuit breaker?
How to choose the correct fuse / circuit breaker for a DC circuit
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 (the wiring for an entire van conversion is included in the diagram 🙂 - wiring diagram gets sent out to you straight away).
Supplies list 🛒
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).
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 it out. 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 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 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
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 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.
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.
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 blow, fast blow 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.
This is the maximum voltage the breaker is rated to handle. AC is usually a lot higher than DC.
This is the amperage printed on the body of the breaker.
This is the amperage at which the circuit breaker will actually trip. It is normally about 130% the rated amperage.
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
1. Calculate the appropriate wire size
Before choosing a fuse, it is important that you first size the wire you are fusing correctly.
To calculate the size (diameter) of wire needed in a system we need two variables:
The length of the wire (distance to the appliance AND back)
The amps the wire will be carrying (ie. the amp rating of the appliance)
Is the circuit critical or non-critical? (voltage drop discussed below)
When we have these two pieces of information we can plug the variables into the wire size calculator over at http://circuitwizard.bluesea.com/ or consult the BlueSea diagram below (the formula for wire sizing is quite complex, so use these tools instead!)
Note: AWG (American Wire Gauge) and mm² (cross sectional area) are the units of measurement used to describe wire size. AWG is used in North America, mm² is used everywhere else.
What is voltage drop and why is it important?
Voltage drop occurs when the voltage at the end of a section of cable is lower than at the beginning. Voltage drop normally occurs when there is resistance in current flow usually due to cables, contacts or connectors.
We can only allow a 3% voltage drop on sensitive/critical circuits, whereas we can allow up to a 10% voltage drop on non-critical appliances (eg. LED lights).
For a complete guide to wires for campervans, including cutting, crimping, connecting, sizing, and more, you can check out this guide.
2. Select a DC fuse
In this article Bluesea mentions 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 X 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. Here is an excellent guide to fuse sizing from Bluesea:
AGC and MDL fuses
AGC and MDL fuses are slow blow fuses. They are constructed of glass tubing and brass endcaps. The glass tubing provides a visible indication when the fuse blows. They are cheap fuses and usually for small appliances.
Blade fuses are the most common fuses found in vehicles. They are cheap, color 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.
Terminal (MRBF) fuses
Terminal fuses are pretty much exclusively used for fusing leisure batteries. They are rated to handle a large current.
MIDI / MEGA / ANL / Class T Fuses
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.
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.
Do you need an inline fuse holder or block 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 (eg. 12v campervan fuse box)
You can learn how to connect the fuse holder to a wire in this information-packed guide on all the ways one can connect wires.
An important note on inline fuses vs. inline circuit breakers
While you can use inline fuses for your appliances, most van converters tend to use inline circuit breakers instead.
Why? The appliances we use an inline fuse/breaker for in a van conversion are the solar panels, solar charge controller, and overall 12v DC system; to name a few. I find that I semi-frequently (for one reason or another) need to turn off my solar system or entire 12v DC system. An inline breaker is a switch that I can manually switch if I want to. In my opinion, having this ability is non-negotiable for van conversions. You can get The Van Conversion wiring diagram (including where to put the breakers) by signing up to The Van Conversion Newsletter.
Campervan Fuse box
Most van converters will install a simple 12v DC blade fuse box in their campervan. This is a central point from which we fuse up our 12v DC appliances. 12v campervan fuse boxes typically take ATC blade fuses.
If we are running solar power in our campervan, we will typically wire the campervan 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 12v campervan fuse box directly to the bus bars (which are connected to the leisure batteries).
We will typically install a switch panel between the campervan fuse box and the 12v appliances; 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.
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 used a small consumer unit if we are installing shore power (ie. the ability to plug in and charge up at a campsite). Related guide: Shore power installation
The consumer unit contains an RCD (residual current device) and several MCBs (miniature circuit breakers). The RCD will ensure that the power supply is automatically cut to all of the circuits protected by the device if ground leakage is detected. In North America, these devices are known as ground fault circuit interrupters (GFCI).
Essentially the RCD will break the circuit if it detects that the electric current is unbalanced between the supply and return. Any difference between the currents indicates leakage, which presents a shock hazard. In a nutshell, RCDs protect you from electrocuting yourself. It also has the added benefit of cutting the circuit if it detects a short circuit (the live and neutral wires touch each other).
The MCBs in the consumer unit protect the devices / plug sockets they connect to.
A quick note on DC consumer units
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 use DC MCBs in a DC consumer unit.
Combined AC and DC Distribution panels
If you are running an AC and DC system in your campervan (more than likely), 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.
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!
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 sign up).
Be sure to check out the rest of the Electrics Guides. Related articles include: campervan wiring, campervan solar panels,